DIY Powerwall / Battery

[johnb80]

House Supply With Battery And Solar

I embarked on my solar and battery path when energy prices went ballistic in 2021, my contract had 6 months remaining my energy supplier informed me that my monthly payment of £178 would rise to around £530. There was no way I would (or could afford) such an increase, it would effectively take my state pension and more. I had looked at solar and battery before, but the figures didn’t make it very attractive with a break even point a long way in the future, now with the imminent price rise it would be a very different situation.

I embarked on the road to getting solar and battery installed, I ruled out DIY at this stage because an MCS certificate was needed if I wanted to export excess energy into the grid. Cutting a long story short, my system comprised of 7kWh battery, 6kW inverter and 6.7 kW of panels split across South facing and West facing roofs.

The system worked well but had serious shortcomings which I discovered after a short time, the 7kWh of battery storage wasn’t enough and the 6kW inverter was fine until it was running on battery only then the output was reduced to 3kW which was a constant frustration.

I looked at solutions, bigger inverter, additional battery storage to existing system etc, all were very expensive and didn’t tick all of the boxes. I came across the concept of an AC Coupled battery, a system that was connected to my house supply, had it’s own inverter and ‘x’ amount of battery storage. Would it work with my existing inverter? Nobody would commit and the systems were still expensive (£12,000 for 10 kWh AC coupled battery with 3kW inverter built in). I spent a lot of time on DIY forums and there are a lot of very knowledgeable people in this area of interest. There are real enthusiasts on Victron forums and the Victron equipment is amazing. So, after months of research I bit the bullet and started to make my own AC coupled battery (often termed powerwall).

How does it all work

The power to our homes usually comes from the grid via overhead or underground cables. The cables feed into your electricity meter and the meter then feeds your consumer unit (fusebox) where the supplies to circuits are individually protected to a suitable level eg 6 amp lighting, 32 amp sockets, 50 amp cooker etc. When we add solar and/or battery we add an inverter to this. The inverter is a clever box of tricks that essentially converts DC (Direct Current) from the batteries or solar into AC (Alternating current) suitable for distribution round our homes.

The AC mains in our homes is in fact a sinewave, the Live conductor goes from zero to around 335v, back down to zero and the to -335v, then returning to zero, it does this following a sinewave pattern and in the UK 50 times per second (50Hz).

The installation of this is very simple, Live, Neutral and Earth to the inverter, battery supply onto the inverter and the magic begins.

The Inverter and it’s magic

The inverter does all of the clever stuff, first of all it synchronises its output to match the AC 50 Hz waveform of the incoming supply, having done that it sets it’s output voltage to be exactly the same as the incoming supply i.e. +/- 335v sine wave . Then the clever operation begins, using information from the CT (Current Transformer) it can see if electricity is flowing into your property or out. If it sees energy being imported it will increase it’s voltage which in turn means it starts to supply energy to your house. It continues to increase it’s voltage until there is no current flow in or out (import or export) to/from your property, the whole system is in balance. As the situation changes, you switch on the kettle for example, the inverter sees an import happening so again increases it’s output voltage to reduce the import to zero. When the kettle switches off, the result will be an export starting to happen due to the kettles load being removed. The inverter once again adjusts it’s output voltage down until the export stops and the system is once again balanced. All of this takes place typically 50 times per second. The inverter can also be programmed that any excess energy you have can be exported, ignoring any battery at this stage, if your house load is being satisfied by the output from the inverter and it can see more energy is available from the panels, it will increase it’s output voltage and therefore start to push energy into the grid. Your meter will record this and you will receive payment / credit from whichever energy company you choose to export to (it doesn’t have to be the same company you import from).

Battery operation

A battery can be added to Inverters (more correctly termed Hybrid Inverters). The inverter works in just the same way as above using power from solar and/or battery. The additional functions needed or a battery charging system. The inverter can use solar or grid import to charge the battery and use the stored energy to supply your house or export to the grid. At the moment Octopus are charging 7p kWh for import during off peak and they pay 15p to export, there are additional export times during cold weather where the export amount can go as high as £4 per kWh. Money can be earned by charging off peak and selling peak. The inverters have settings that allow priorities eg charge the battery before exporting to grid, export to grid to a maximum of xx kW, charge the batteries between hh:mm and hh:mm. and so on, if it’s needed, it will be in there.

Battery

The actual battery consists of 2 major parts, the cells and the management system. The cells can be of various chemistry’s such as Lead Acid, AGM, LiPo, LifeP04 and more. There are many people making up storage solutions from scrap computer and powertool batteries, search youtube for more info on this. I chose LifeP04, it was one of the later chemistry’s, wasn’t as likely to go thermally unstable and offered a very good power to physical size ratio. I chose EVE cells, they were considered the very best at the time, high quality, good performance and a suggested 8000 cycles before their capacity degrades to 80%. It should be noted that 8000 cycles is 22 years assuming one full cycle every day. It should be good for my remaining life LOL. Each cell has a voltage of 3.2v a capacity of 280A/h and weighs a hefty 5.5Kg. Each cell has a capacity of 0.896 kWh. 16 cells were used to give a nominal working voltage of 50v with a capacity of 14.3 kWh.

Physical Build

I decided to make up battery modules in groups of 4 to make them easier to handle and convenient to make. I designed a battery tray to hold 4 batteries, have an Anderson connector for the heavy current and a smaller multipin connector for balancing (see later). The batteries physically had to be held in compression too, this was achieved with treaded rod between the ends of the modules. All of the batteries are connected in series i.e. +ve of the first battery connected to –ve of the second battery, +ve of the second battery connected to negative of the third and so on. Great care should be taken whilst doing this, the batteries have a very low internal resistance and have no difficulty is putting 40kW into a short circuit. Definitely insulate spanners etc whilst assembling. The output from the assembled cells is taked to the Anderson connector on the outside of the module, -ve from battery 1 –ve and the +ve from battery 4 +ve. This makes up a module of 12.8v @ 280A/h. The balance connections, a lead has to be taken from the battery 1 negative and every battery +ve so that the management system can monitor battery voltages and to balance them all.

Whats all this balancing malarkey

Each cell in a battery has slightly different characteristics in terms of capacity (how much energy it can store), internal resistance (determi9nes how much heat is made during charging and discharging) and internal leakage or self discharge.

LifeP04 cells have a strictly controlled maximum voltage of 3.65v during charging and an absolute minimum of 2.5v when discharging, exceed these parameters and it will rapidly cost you money. So, during charging the inverter will output a maximum of 3.65v x 16 = 58.4v. So it’s all dead easy, apply 58.4v the the battery will charge to 100%, nice idea but you cant do that. All of the batteries are in series and therefore the charging current passes through all of them equally. The cells with the lower capacity will reach 100% first and then go on to overcharge causing damage to the cells and further reduction in capacity and during discharge, the ones with lowest capacity will be at 0% first with further discharging taking place again damaging the cells. The weakest cells in this situation die very quickly and can under the wrong circumstances become a fire hazard. This is where another piece of equipment comes into play, the BMS (Battery Management System). This device is wired to every cell so it can see the voltage, it opens and closes charging and discharging routes so it can prevent damage and it communicates to the inverter to increase or decrease charging current and/or voltage. It’s an electronic doorman or bouncer controlling the electrons in or out of the battery nightclub. It also keeps a watchful eye on temperature, any hint of a thermal runaway starting, the batteries will be disconnected, LifeP04 should not be charged when the temperature is below zero, I have placed some heater mats under mine because theyre located outside and winter can see inside temperatures where the batteries are of -5 deg C. The BMS will switch off the charge or discharge if the temperature parameters are exceeded. During operation of the battery, charging or discharging, any cell whose terminal voltage is higher than the others gets picked on by the BMS, it discharger that cell slightly and gives the charge to the cell that has the lowest voltage. Parameters can be set of how close you want the voltages to be, mine are all within 5mV (5000ths of a volt).

Commercial batteries at the time were very expensive, in the region of £1000 per kWh storage. Looking around at the cost of the cells, they were much cheaper and better quality items could be sourced

That’s the technobabble done, here's the construction.

This is the physical battery module with 4 cells mounted in it. The MDF and acrylic cover were laser cut, the threaded rod to keep the cells in compression is sleeved in silicone tube. The large grey connector at the front is a 150 amp Anderson Connector which will carry the main current in and out of the battery, theyre robust and very reliable. The small circular connector to the right of it is the balance connections, a wire goes to each battery cell so that the individual cell voltages can be read by the BMS.

This is a finished module, the busbars connecting the cells together, the thick red (+ve) and black (-ve) are the main supply cables to/from the module. The Thinner wires are connected from the first negative to all of the positive cells are the balance connections.

The 4 modules, 4 cells in each making 16 cells in total are housed outside in the lower section of a modified toolshed (bought from Amazon and is just about the perfect size.

The heater matts can be seen under the battery modules, the Anderson connectors can be seen connected placing all 16 batteries in series. You may have noticed another multipole connector hanging from the wires, this is for some extra monitoring added into Home Assistant. Note the insulation to the sides of the shed and on the door to keep the temperature up in winter time.

The 16 cells across the top, the heavy current cables down to BMS BATT + & - and the balance lines down to the battery inputs B1 to B16.

The Seplos BMS I used came as a bare PCB and display, I created a stand to hold it all and support the cables. I also made a front panel to dress it up a little.

This is the rear view of the BMS with one of the battery cables in place to test the design.

This is the front panel, I used laser engraving laminate to make a very acceptable outward facing cover to the electronics behind..

The little shed that was housing it all now had the 4 batteries in the lower half, the top half needed the inverter and a few ancillary parts.

Shown here is the Victron Multiplus II GX Inverter, A small consumer unit, A large fuse for the DC supply from the batteries and a battery isolator switch. The two units at the right hand bottom side are temperature controllers, the top one switches controls the cooling fan seen at the top of the photograph, the lower one controls the heating mats under the batteries.

This is the Multiplus II GX, the heart of the system, it’s not the cheapest of inverters but it is in my opinion one of the very best. The flexibility of the programming, the support worldwide from enthusiasts and Victron themselves is second to none. The weight is unbelievable. Only two wires for the battery supply (+ and -), three wires for the mains (L,N & E), a data cable plugged into the BMS and the CT plugged in and that’s the inverter done.

Apologies for any errors or spelling mistakes etc.

Part II to follow

 

[johnb80]

Part II

The isolator switch is in both +ve and –ve battery lines from the BMS going to the inverter, the fuse is also in the positive line between the switch and inverter. It’s worth mentioning that switching and fusing DC needs to be carefully specified. DC has the ability to maintain an arc and continue to burn. The fuse is designed to break up to 120,000 amps successfully, it’s rated at 250 amps which is around 12 kW load on the inverter, a little in hand to cover peaks and surges.

A local consumer unit installed to provide local circuits in the shed for 13 amp sockets (convenience), Lighting and the 50 amp supply to inverter.

All that remains is a fairly simple setup of the inverter via a laptop or PC plugged into it. It needs details of the batteries, maximum charging current, maximum depth of discharge (I stop at 10%) and the schedule for charging (23:30 to 05:30). It then just runs, supplying your electricity demand during the non charging times and charging at 7p kWh during the off peak times. In rough figures my energy costs are 26p peak, 7p off peak so I save 19p kWh, I save £2.40 per day from this battery system. If there is any energy left over in the evening I export that at 15p kWh, I average around 110 kWh per month of export in this way which earns a further 54p per day. This system cost me just under £4200 when I built it, prices have come down considerably now, the price would be nearer £3000. Taking my high price, the breakeven point will occur in just under 4 years which is an excellent return in my book.

Legalities

I’m a qualified ex steelworks electrician and thus I feel met the ‘competent person’ criteria for working on my own electrics. If youre not qualified, to stay legal you would need a ‘competent person’ to connect the L,N & E from the inverter into your supply, it should take less than 20 minutes to do if you’ve run the cables etc. All of the other work with batteries, wiring the cabinet etc you’re perfectly entitled to do. You do need permission from your DNO (Distribution Network Operator) the application is done using a G99 form if your inverter power is going to exceed 3.67 kW. Previously an MCS certificate was required if you wanted to export but some companies now don’t require this.

Help

If anyone wants any help with this please give me a yell. I can give you the files to laser cut the battery modules or indeed cut them for you for the cost of materials and postage.

John

 

[Sideways]

Excellent !
Who's your DNO and what energy tarriffs are you on please ?

As you get a few months of use in with this, I'd be very interested to see your graphs if you feel like adding an update to this thread a month in, 6 and 12 months on, etc.

 

[johnb80]

Excellent !
Who's your DNO and what energy tarriffs are you on please ?

As you get a few months of use in with this, I'd be very interested to see your graphs if you feel like adding an update to this thread a month in, 6 and 12 months on, etc.

Yep, that’s no problem, my system is logged on half hourly usage. I’ll post the data when I get home. Meanwhile, here’s what Octopus tell me from their analysis this month so far (pretty typical)

My tariff is Intelligent Octopus Go.

 

[johnb80]

Excellent !
Who's your DNO and what energy tarriffs are you on please ?

As you get a few months of use in with this, I'd be very interested to see your graphs if you feel like adding an update to this thread a month in, 6 and 12 months on, etc.

 

[Sideways]

Cheers !
Last question for now :
If you took the capacity of your battery, scale it down a little to be prudent (80 or 90%) and multiply by the 8000 cycles expected lifespan. You'll get the total throughput of the system over it's expected life.
Divide the system cost by that and you'll have a pence per kWh cost for buffering the power through your battery. There's efficiency losses too but lets ignore those.
I'm curious what that cost is.

My own storage is a commercial solution : Fronius and BYD. It was expensive just as you said so it's not economic for me to do what you do. Mine just stores our own solar energy for our own use and I export surplus PV on the fixed 15p outgoing octopus tarriff. I never export from the battery.
I'm pleased with it, but DIY storage like yours has far better economics.

 

[johnb80]

The 8000 cycles is the predicted life to 80% capacity so it can still be used at that point for a lengthy time.
So, 8000 cycles at say 12 kWh = 96000 kWh. Dividing the cost 4200 by 96000 gives 4p kWh

 

[nickds1]

Really interesting writeup - nice project.

I like the detail in using the PID controllers & propagation mats, plus all the sensible (essential!) safety features - it's exactly how I like to do projects.

My current one is installing a 3ph ATS and auto-start generator for our house out in the sticks. We get a load of brown outs and some extended blackouts every year. Last year, we lost all power for 10 weeks and NPower (not my friends) had to place a 100KVA 3ph diesel generator outside out eco self-build for the full 10 weeks. 100KVA is the smallest 3ph they have - we are an old farm building site, so we had 3ph and used a 3ph GSHP in the project as it was a bit more efficient than the 1ph one...

For monitoring I use some homebrew stuff but mainly Emporia Vue to look at each phase, balance and distribution on the site, plus Emporia plugs to monitor individual circuits (network cabinet, AV, fridges, freezers, septic tank etc.)

I like the setup!

 

[Sideways]

The 8000 cycles is the predicted life to 80% capacity so it can still be used at that point for a lengthy time.
So, 8000 cycles at say 12 kWh = 96000 kWh. Dividing the cost 4200 by 96000 gives 4p kWh

Thanks !
Mine would be at least twice that. There's a real economic benefit from going homebrew

 

[johnb80]

Really interesting writeup - nice project.

Thank you, it's working better than I expected.

I like the detail in using the PID controllers & propagation mats, plus all the sensible (essential!) safety features - it's exactly how I like to do projects.

They were incredibly cheap and do the job well enough, £4.50 each in that well known auction site. I use them for other jobs too, the cooling water for my laser cutter the chiller is started and stopped by one of these too.

My current one is installing a 3ph ATS and auto-start generator for our house out in the sticks. We get a load of brown outs and some extended blackouts every year. Last year, we lost all power for 10 weeks and NPower (not my friends) had to place a 100KVA 3ph diesel generator outside out eco self-build for the full 10 weeks. 100KVA is the smallest 3ph they have - we are an old farm building site, so we had 3ph and used a 3ph GSHP in the project as it was a bit more efficient than the 1ph one...

In one of my former jobs I installed a 350 kVA Volvo generator to supply 6 factory units we had on an industrial estate. I modified the generator to have a heat exchanger on the exhaust and the water cooling and made it into a CHP plant. Allen Bradley SLC500 controlled all of the factor units changeover and decided when the generator should be used based on temperature, electrical load, cost of grid electricity and cost of diesel. It was one of the most interesting projects I have done and is still working today - 25 years on!

For monitoring I use some homebrew stuff but mainly Emporia Vue to look at each phase, balance and distribution on the site, plus Emporia plugs to monitor individual circuits (network cabinet, AV, fridges, freezers, septic tank etc.)

I like the setup!

I use Home Assistant at the centre of my home stuff, I have some smart plugs around and can monitor consumption through these into HA. I run 13 air to air heatpumps (AC units), 1 Air - Water heatpump (13kW Mitsubishi ECODAN) and a Gas Boiler. The individual AC Units I designed a plug that resides in a 13A socket and I taught an arduino the Mitsubishi Infra Red commands. It links back to HA via WiFi so it can start, stop and change mode of any AC unit under control of HA. Logic is in HA to look at energy storage levels, energy cost and the COP of each unit, it then decides to run Gas, Air-water of Air-Air systems.

It keeps me out of mischief!

 

[johnb80]

Thanks !
Mine would be at least twice that. There's a real economic benefit from going homebrew

to be honest, I'd never considered that angle before until you mentioned it. In reality it will be even cheaper than I said, the batteries will go on well beyond the 8000 cycles but with a reduced capacity but the inverter, BMS and other associated equipment will continue to operate. In reality it should just be batteries that need replacing, assuming this to be the case at 8000 cycles, the current price is £99.99 each which would give a cost per KWh of 1.7p, much better!

 

[Sideways]

to be honest, I'd never considered that angle before until you mentioned it. In reality it will be even cheaper than I said, the batteries will go on well beyond the 8000 cycles but with a reduced capacity but the inverter, BMS and other associated equipment will continue to operate. In reality it should just be batteries that need replacing, assuming this to be the case at 8000 cycles, the current price is £99.99 each which would give a cost per KWh of 1.7p, much better!

I have a thread on here telling the story of my own installation. 16kWh BYD battery hanging off a 6kW Fronius hybrid inverter etc etc.
No EV for now so no access to the best overnight tariffs.
BYD have a good reputation but even if I get the same 8000 cycles to 80%, our battery stack cost me £10k so wearing them out trying to earn a few pence from the arbitrage doesn't make any sense to me.
Of course we hope for more but the performance of kit that we hope to last 20 odd years isn't proven, only predicted, extrapolated and modelled.
Upside is that we are self sufficient on electricity almost 10 months of the year thanks to the storage and 9kW of panels.
It is great to see more interest in energy storage and I'll be following your story.
Fronius are top quality but they aren't interested in giving owners as much control or information as they should. They're a bit "apple" to Victron's "linux".

I'll second the comment from @nickds1 . You've done a very good write up and the install shows professionalism and attention to detail. Spot on

 

[johnb80]

I have a thread on here telling the story of my own installation. 16kWh BYD battery hanging off a 6kW Fronius hybrid inverter etc etc.

I'll have a nosey at that. I needed more than the 6kW I had installed due to the heatpumps I use, careful logic inside HA keeps us within our inverters capacity (11 kW Total).

No EV for now so no access to the best overnight tariffs.

You should definitely go for EV UNLESS you have to drive 600 miles without stopping, tow a 3 tonne trailer and don't understand the concept of you can leave the EV charging whilst you sleep / watch a movie / dine out etc. I went from a 4.4 Litre TDV8 Range Rover to a BMW i3 in 2017, I feared I would miss the cathedral like surroundings inside the Range Rover wafting me along., but I neednt have feared it. I was completely sold on the i3 and I still have it today with 98,000 miles on the clock. The Range Rover used around £130 per week in diesel, the i3£120 per YEAR in electricity for the same use. Meg, would never go back.

BYD have a good reputation but even if I get the same 8000 cycles to 80%, our battery stack cost me £10k so wearing them out trying to earn a few pence from the arbitrage doesn't make any sense to me.

BYD are indeed very good batteries and I do see your point but I do it more because I can rather than for finacial gain, making the system work automatically is an interesting project and I find it rewarding.

Of course we hope for more but the performance of kit that we hope to last 20 odd years isn't proven, only predicted, extrapolated and modelled.

True but looking at the degradation on my house batteries and my i3 theyre not as bad as predicted.

Upside is that we are self sufficient on electricity almost 10 months of the year thanks to the storage and 9kW of panels.

|Nice, I have an east facing roof that I may do something with, I could get probably 6 kW on there.

It is great to see more interest in energy storage and I'll be following your story.

Necessity is the mother of invention.

Fronius are top quality but they aren't interested in giving owners as much control or information as they should. They're a bit "apple" to Victron's "linux".

Victron is amazingly open with programming and it's a joy to use.

I'll second the comment from @nickds1 . You've done a very good write up and the install shows professionalism and attention to detail. Spot on

Thank you for your kind words, it's good to chew the fat with like minded people.

 

[t0ne]

@johnb80 thanks so much for this excellent write up, it has given me lots to think about

 

[gmgmgm]

Thanks for the write-up johnb80, very professional job. May I ask what laser cutter you use to get those smart boards?

Did you look at diverting energy into hot water? I've held a belief that it's more cost-efficient to hold energy in hot water than in batteries, perhaps this is out of date now. Do you think you'll add more batteries in the future to make it "better"?

I'm also fascinated by "some energy companies don't require MCS now". Octopus energy would like to know my MCS for export... could you suggest some companies which don't require such paperwork for export?

 

[Sideways]

I'm also fascinated by "some energy companies don't require MCS now". Octopus energy would like to know my MCS for export... could you suggest some companies which don't require such paperwork for export?

I didn't bother signing up for an export tariff when we first installed. They didn't pay much so I simply didn't bother. Once Octopus started to pay much better rates I signed up with them too and yes, they asked my MCS number as part of the export sign up. They also made the approach to my DNO to have them issue me the second meter number used for export.

MCS is only a scheme. There should be an alternative route for people who choose not to use it, but may involve jumping through more hoops.

 

[johnb80]

Thanks for the write-up johnb80, very professional job. May I ask what laser cutter you use to get those smart boards?

It's a 40w CO2 laser from HPC

Did you look at diverting energy into hot water?

Yes I have MyEnergi EDDI installed but it's more cost effective to export electricity at 15p kWh and import gas at 4.2p kWh so EDDI is currently switched off.

I've held a belief that it's more cost-efficient to hold energy in hot water than in batteries, perhaps this is out of date now.

Cost efficient in terms of equipment maybe but in terms of saving energy I wouldnt have thought so. I have really good insulation on my tank with very little loss, the airing cuboard doesnt get warm! I run the hot water tank at 80 degC and have a thermostatic blending valve to reduce it to 50 deg C around the house. That effectively gives me a bigger hot water storage tank.

Do you think you'll add more batteries in the future to make it "better"?

I dont think I need to, I can supplement the batterys with V2L from the car so we should be OK. I may add some more PV panels on my East facing roof though.

I'm also fascinated by "some energy companies don't require MCS now". Octopus energy would like to know my MCS for export... could you suggest some companies which don't require such paperwork for export?

Octopus.

See No MCS Needed

It doesn't really affect me, I have an MCS cert from my Solar install so I haven't fully looked at T&Cs, there was quite an outcry from MCS guys on another forum when it was announced.

 

[Ollie78]

This is fascinating and timely for me. I have been looking into getting solar and a battery myself and am quite dissapointed in the quality of the quotations I have had. Many are nonsense and not designed well (obvious even to a non electrician like me) many are using tigo optimisers inapropriately as a crutch for not having many 3 mppt inverters (we have 3 small roofs) and some just want absolutely silly money.
This has lead me to discover Victron stuff which immediately makes sense to me as you can configure the components as you need, control it how you want and upgrade it later.

I am considering doing the grunt work myself and then getting an electrician to connect it all and certify it. Could save considerable money this way.

Ollie

 

[johnb80]

This is fascinating and timely for me. I have been looking into getting solar and a battery myself and am quite dissapointed in the quality of the quotations I have had. Many are nonsense and not designed well (obvious even to a non electrician like me) many are using tigo optimisers inapropriately as a crutch for not having many 3 mppt inverters (we have 3 small roofs) and some just want absolutely silly money.
This has lead me to discover Victron stuff which immediately makes sense to me as you can configure the components as you need, control it how you want and upgrade it later.

I am considering doing the grunt work myself and then getting an electrician to connect it all and certify it. Could save considerable money this way.

Ollie

Thats not a bad way to go, just make sure you can find a willing MCS guy to sign the cert for you first, sometimes theyre reluctant to do it. Victron is a great choice, completely modular, you can mix and match, all units talking to each other. Absolutely brilliant, I have in my thoughts a plan to change the Growatt Hybrid in my system to Victron equipment to get better co-operation between the two systems.

 

[woodieallen]

This is an excellent thread, John. Food for thought. Plus Sideways excellent thread, I'm seriously thinking about doing something similar. I have the Emporia energy monitor and have access to a wealth of data over the year(s) which should give me a good idea as to what size 'battery' /system is needed.

One thing I don't understand is right up there at the start, your first system, you said your available power dropped to 3KW. I don't understand that bit....why the drop ?