One family's solar story

[Deadeye]

We've had a 4kWp system on our east facing roof for 8 years now. We get feed in and generation payments at c. 5p and 17.5p respectively. The system cost £6k. It's on an east facing roof in Buckinghamshire and we're on track to break even this year in cash terms. We have a basic diverter box that spots export and heats the thermal store instead but I don't have a way to value that. There's a second tier outlet to a radiator in the cellar in winter.
Couple of chip-in comments:
Firstly, panels do better when cool so east better than west if that's your choice.
Secondly, the diverter box (£400 I think) is brilliant and has got us used to running the washing machine etc. More when it's sunny.
Thirdly, I was worried the efficiency would decline over time, but the payments year to year are steady.
We don't have an electric car yet (but we don't fly or eat red meat) so can't use PV to charge, but overall couldn't recommend it highly enough.
In a roundabout connected way, I'd also recommend loft stilts and an extra 170mm of rockwool, and just checking hot water pipe routing. Our shower is under the thermal store but used to go the length of the house and back!

 

[D_W]

I know of a chap that fitted a 100kWh battery, charges it between 00:00 & 05:00 on Octopus's economy 7 tariff, at 5am the house disconnects from the mains and powers his house till 00:00. Then rinse and repeat. His usage is around 11kWh\day but from what I've read, by not deep discharging the battery much, the lifespan is longer.

Found it very interesting especially as phase 2 for him is the addition of PV panels.

Different battery type, but eons ago now, the toyota prius drew a lot of fire for "how much will it cost to replace the battery when it no longer works".

The figure was something like $3k - the article writer interviewed a battery supplier who conveyed that most of their sales were for repairs, not end of life replacement. The batteries never cycle deep in the older prius and they never really wore out.

Repairing the charging system on them or any of the others in the US was a different story, though. Shorter life than the battery and more cost to fix.

 

[pe2dave]

How long are Octopus rates good for?

 

[Dibs-h]

How long are Octopus rates good for?

No idea Dave. What interested me was that he used the battery pack out of a BMW i3, whilst others had used the packs out of Teslas and Leafs (I think).

 

[1steven]

Very i

Thank you for this and it is always interesting to see detail.

Just one think have you considered the arbitrage potential using a battery and the Octopus Agile tariff?

Here is a screenshot of my sons installation:
View attachment 142640
Note the price difference between export and import. The arbitrage can be between 20p per KWH and 45p per KWH

Clearly he is charging his battery from solar or by importing it and exporting to maximise arbitrage. He is still refining his controls and timing for best effect but is already yielding impressive results.

Not sure how long this ‘situation’ will continue

Very interesting graphs have been looking for realtime information for a while

 

[point5clue]

Presumably the shift to variable renewable energy generation (both commercial and domestic) will incentivise energy companies to offer variable rates that encourage consumption when there’s an excess of available power and discourage it when it is scarce.

I’m wondering if the long-term economics of having a domestic solar PV system will be reduced by this trend, and I guess on the flipside the economics of having a battery or even just simple energy diverter to a tank or pile of bricks will improve.

This may be just me rationalising the fact that I don’t have spare cash to buy any of these systems at the moment.

I think in the future a sweet spot for many people will be an electric vehicle with V2G that can both arbitrage the energy system for the house and charge cheaply itself. It remains to be seen if any of this becomes a reality before the sea level rises take us back to the Stone Age

 

[Sideways]

Thank you for this and it is always interesting to see detail.

Just one think have you considered the arbitrage potential using a battery and the Octopus Agile tariff?

Here is a screenshot of my sons installation:
View attachment 142640
Note the price difference between export and import. The arbitrage can be between 20p per KWH and 45p per KWH

Clearly he is charging his battery from solar or by importing it and exporting to maximise arbitrage. He is still refining his controls and timing for best effect but is already yielding impressive results.

Not sure how long this ‘situation’ will continue.

This is a very appealing idea. When I first started seriously considering solar + battery and looking at tarrifs, I had a spell of considering a large battery and no solar. The issue is battery life. It very much feels like a roll of the dice just how long a battery will last (#cycles x depth of discharge, not years).
My battery (will be) a huge part of the financial investment.
I've researched as best I can about it but really don't know. This is much the biggest unknown in the whole project. It may mean I've made a superb long term investment or perhaps just "prepaid" my next 20 years electricity bill in one go.

If you calculate the economics based only on the warranted life of the battery to eliminate that uncertainty, this DID NOT look like a way to profit when I was looking at rates, but it is early days and tarrifs change.

Consider:
£10,000 battery
Will it achieve 10,000 cycles ? Maybe. At 1-2% capacity loss per 1,000 cycles, that would be tolerable. It's not unreasonable to hope for this.
So £1 per cycle, and some arbitrary average of 12kWh per cycle so as not to fully cycle it and extend the life
That's 8.3p per unit of your arbitrage needs to be set aside to pay for wear and tear on the battery (replacement after a nominal 10,000 cycles)

We just need to be careful, go into this with eyes open and pay a lot of attention to battery reliability and lifetime data. Household or V2G.

I've seen just one public domain data set on my specific model of household battery, it's encouraging, but no one is sharing data about tests out at 10,000 cycles so it's all just extrapolation from the first 1000, 2000, 3000 cycles.

 

[Inspector]

Until 4 years ago there was a magazine published in the US called Home Power. There downloadable copies of all the issues for free as long as it is for personal use. While it may not have the latest and greatest information there was a lot of good information. Home Power Magazine | Downloadable Archive | Home

I stumbled across a wind and solar forum in the US, there must be others. Another place for information that would be current. Northern Arizona Wind & Sun Solar Forum

Pete

 

[Sideways]

Have a look here:

https://batterytestcentre.com.au/reports/
This was a 3 year project in Australia
Report #12 was the last, it's now finished.

My own on-order battery the BYD HVM is among those. It was cycled 1100 times - once a day for almost 3 years. Degredation wasn't measurable as a firmware upgrade actually increased it's capacity along the way, but it's curves like the ones in this report that are needed to really judge how good a battery may be.

Notice how rather a lot of batteries had early failures or issues that needed the manufacturer to be called in.

Notice also - and not picking on them, merely using a well known name to make the example - the performance data collected in this trial suggested that their Tesla Powerwall 2 battery pack would be down to 60% capacity in less than 5,000 cycles.

I don't think it's safe to assume that batteries will have an infinite life. You need to consider that they may degrade and need replacement in as little as 10 years In my case that could be £1,000 / year, making the whole project more break even than profitable.

 

[Phil Pascoe]

It remains to be seen if any of this becomes a reality before the sea level rises take us back to the Stone Age

It's likely to be a long time - the ODN (Ordnance Datum Newlyn) hasn't changed in the last hundred years.

 

[Jameshow]

This is a very appealing idea. When I first started seriously considering solar + battery and looking at tarrifs, I had a spell of considering a large battery and no solar. The issue is battery life. It very much feels like a roll of the dice just how long a battery will last (#cycles x depth of discharge, not years).
My battery (will be) a huge part of the financial investment.
I've researched as best I can about it but really don't know. This is much the biggest unknown in the whole project. It may mean I've made a superb long term investment or perhaps just "prepaid" my next 20 years electricity bill in one go.

If you calculate the economics based only on the warranted life of the battery to eliminate that uncertainty, this DID NOT look like a way to profit when I was looking at rates, but it is early days and tarrifs change.

Consider:
£10,000 battery
Will it achieve 10,000 cycles ? Maybe. At 1-2% capacity loss per 1,000 cycles, that would be tolerable. It's not unreasonable to hope for this.
So £1 per cycle, and some arbitrary average of 12kWh per cycle so as not to fully cycle it and extend the life
That's 8.3p per unit of your arbitrage needs to be set aside to pay for wear and tear on the battery (replacement after a nominal 10,000 cycles)

We just need to be careful, go into this with eyes open and pay a lot of attention to battery reliability and lifetime data. Household or V2G.

I've seen just one public domain data set on my specific model of household battery, it's encouraging, but no one is sharing data about tests out at 10,000 cycles so it's all just extrapolation from the first 1000, 2000, 3000 cycles.

What size battery do you recommend 5x 10x daily usage?

 

[emlclcy]

Have a look here:

https://batterytestcentre.com.au/reports/
This was a 3 year project in Australia
Report #12 was the last, it's now finished.

My own on-order battery the BYD HVM is among those. It was cycled 1100 times - once a day for almost 3 years. Degredation wasn't measurable as a firmware upgrade actually increased it's capacity along the way, but it's curves like the ones in this report that are needed to really judge how good a battery may be.

Notice how rather a lot of batteries had early failures or issues that needed the manufacturer to be called in.

Notice also - and not picking on them, merely using a well known name to make the example - the performance data collected in this trial suggested that their Tesla Powerwall 2 battery pack would be down to 60% capacity in less than 5,000 cycles.

I don't think it's safe to assume that batteries will have an infinite life. You need to consider that they may degrade and need replacement in as little as 10 years In my case that could be £1,000 / year, making the whole project more break even than profitable.

hi, very interesting, can you elaborate on your system a little more pls?
regards
carl

 

[emlclcy]

hi, very interesting, can you elaborate on your system a little more pls?
regards
carl

 

[NormanB]

How long are Octopus rates good for?

They are half hour pricing intervals

 

[NormanB]

This is a very appealing idea. When I first started seriously considering solar + battery and looking at tarrifs, I had a spell of considering a large battery and no solar. The issue is battery life. It very much feels like a roll of the dice just how long a battery will last (#cycles x depth of discharge, not years).
My battery (will be) a huge part of the financial investment.
I've researched as best I can about it but really don't know. This is much the biggest unknown in the whole project. It may mean I've made a superb long term investment or perhaps just "prepaid" my next 20 years electricity bill in one go.

If you calculate the economics based only on the warranted life of the battery to eliminate that uncertainty, this DID NOT look like a way to profit when I was looking at rates, but it is early days and tarrifs change.

Consider:
£10,000 battery
Will it achieve 10,000 cycles ? Maybe. At 1-2% capacity loss per 1,000 cycles, that would be tolerable. It's not unreasonable to hope for this.
So £1 per cycle, and some arbitrary average of 12kWh per cycle so as not to fully cycle it and extend the life
That's 8.3p per unit of your arbitrage needs to be set aside to pay for wear and tear on the battery (replacement after a nominal 10,000 cycles)

We just need to be careful, go into this with eyes open and pay a lot of attention to battery reliability and lifetime data. Household or V2G.

I've seen just one public domain data set on my specific model of household battery, it's encouraging, but no one is sharing data about tests out at 10,000 cycles so it's all just extrapolation from the first 1000, 2000, 3000 cycles.

Yes I agree on the life of the batteries. For modern battery technology, using the battery to support the normal domestic load is a gentle affair and charging from solar is similarly gentle. However importing to charge up the battery from a low state of charge to maximum and then discharging it ‘rapidly’ to the grid is probably more detrimental to battery life.

I also cannot get my head round the ‘huge’ arbitrage available at the moment so cannot figure out if this in an aberration or sustainable in the long term. I guess for ordinary mortals they will not be investing in significant battery capacity and most will not invest at all, so in grid terms the % of homes with an excess of 10KW/hr to pump back into the grid is probably less than 1%. However, post 2030 when EVs gain a significant share of personal transportation AND it is all V2G capable (including the DNO infrastructure) with the potential for individual 50KW/hr inputs to Grid at peak times - this could be a paradigm shift for the Grid and will influence price (lower the available arbitrage - I think)

To be honest - ‘it does my head in’ - getting too old for this.

 

[Cooper]

It's likely to be a long time - the ODN (Ordnance Datum Newlyn) hasn't changed in the last hundred years.

Not according to the OS. From their website:
While ODN was a measurement of mean sea level in 1915-1921, it's important to know that mean sea level has changed since then – it even varies throughout Britain depending on where you are measuring! So, it's best to think about ODN as a height reference point, a measurement of sea level at a specific point in time at a precise location.

I am fascinated by the reasoning involved in this discussion. I'm not sure what the average value of the homes of contributors to this column is but lets say that a current valuation is £500,000. In very recent times there has been rise in value of 10 or 11% so a home worth £450000 is now worth £500000, just by being a house. A sum considerably more than the cost quite a lot of solar panels etc, external cladding and possibly a contribution to a heat pump. In the same period a very nice motor car bought about a year ago that cost the same as the increase in the value of the house, is now worth several thousand pounds less.
It seems that an investment that is contributing to reducing carbon emissions, reducing our dependence on fluctuating energy prices and making the home considerably more comfortable has to show a monetary profit, irrespective that the value of the home increasing. While the flash car can merrily sit on the drive and lose a few thousand a year (in some cases a sum equivalent to the energy costs of a home) but it is considered normal economic activity.

I realise that most of us here aren't in a position to splash out 50K on a flash motor but looking at the number of newish SUVs I see around, lots of people are. Those cars are harming the future of our children and grandchildren, investing in clean energy and reducing dependence on fossil fuels makes a contribution to reducing the harm.

Funny old world.

 

[Phil Pascoe]

While ODN was a measurement of mean sea level in 1915-1921, it's important to know that mean sea level has changed since then – it even varies throughout Britain depending on where you are measuring!

The height is still measured in Newlyn, and has been for over 100 years ......... in which time it hasn't changed (according to another of their sites).

 

[bansobaby]

It's likely to be a long time - the ODN (Ordnance Datum Newlyn) hasn't changed in the last hundred years.

The ODN datum level hasn't changed because it is the average of a large number of sea level readings taken over a period of several years up to about one hundred years ago.

It is not related to current sea levels, which have changed since the ODN was established....

 

[bansobaby]

The height is still measured in Newlyn, and has been for over 100 years ......... in which time it hasn't changed (according to another of their sites).

It's a fixed datum, which is now underwater at high tide.....

 

[moosepig]

LiFePo4
They haven't landed yet but it will be a BYD HVM battery stack.
7 modules of 2.x kWh apiece making 16kWh
It looks like a stack of shoeboxes and stands about 1.6M tall x a couple of feet wide.

Thank you for this thread, following with great interest. Especially keen to learn how your battery performs when it finally gets added.

Are there limitations in where the inverters, battery, isolators etc can be installed? For example could they be be put in a (fully accessible and boarded) loft space? I'm looking for a solar solution, similar to yours with a dc-coupled battery, but I have no garage and realistically loft space is the only part of the house that has room. The panels would be on the roof anyway...

Also will any SMETS2 smart meter work with these setups, or is this something that the installer should be recommending? I'm very keen to end up with a system that will continue to power the house during a power cut BUT be intelligent enough not to try to export while the grid is off (I'd rather not fry the grid repair team!); is the choice of meter part of this aspect of the design?

Thanks again, brilliant thread!