Adding solar panels to an existing system.

[graduate_owner]

Hi all, looking for information on solar panels.

We have a 4kW solar panel system which was installed about 13 years ago and so qualifies for FIT payments. I would like to add more panels now, which will not qualify for FIT payments, plus battery storage. I don't want to jeopardise my entitlement to FITs as they are worth around £3000 a year overall now. Is it possible to have another 4kW system alongside the existing one without major issues? Could a battery be used to store energy from both sets of panels? Could both sets of panels be integraded into the existing house mains? I have contacted my DNO and they said I could expand my system without problems.

Any advice would be appreciated.

K

 

[Eshmiel]

Hello Grad,

Yes, you can do as you want - preserving your FiT arrangement whilst adding another array of solar panels, batteries etc. There are some limits and things to watch out for .....

The FiT system needs to remain distinct from any new solar and so forth so that it behaves just as it did before new stuff was added. This means that you shouldn't connect any batteries to it as this would allow you to use batteries to gather more solar from your new panels then export it via your FiT inverter. That would break and cancel your FiT agreement.

You can change your inverter that serves your FiT solar panels (they tend to have a life of only around 12 years) but you can't change or add to your solar generation equipment that is covered by the FiT agreement without breaking and losing it. In most Fit agreements, you can't even change the solar panels for different panels.

You'll probably also have excess solar from your new panels that you export to the grid if you can't use it in the house or get anymore into your batteries because they're full. You can't come to an arrangement to sell that excess solar without, again, breaking and losing your FiT agreement. You have to give it to the grid for nowt. Get an e-car to put it in instead.

Some areas have a grid infrastructure that isn't very capacious. Until recently that meant that proposals for additional solar panels that might also export to the grid (in addition to your FiT export) were sometimes refused on a "grid can't cope" basis. However, there's now a new inverter standard (G100) that allows the grid operator to prevent any excess export from that new inverter if the grid is being overloaded by exports. This means you can have more solar panels as long as they operate through such a G100 inverter.

Hope this helps.

 

[Sideways]

If you add extra panels with a hybrid inverter and a battery connected directly to it, the meter tails will still be point in common between the new system and the one with feed in tarrif. Your new installation can have a current sensor on the meter tails and that will give you a good deal of flexibility. Most of all, it will allow you to take surplus power generated by your original 4kW array and pull it into the new inverter to charge your battery. Whether this is a good thing depends on whether the FIT tarrif is more or less than the price per kWh that you are paying these days.

As to the point about the grid and exports, I was approved a G99 license by my DNO to export upto the 9kW peak of my own solar panels. In practice I can never use it. The grid wiring has highish resistance around here (1970's vintage, and much added to) and our average voltage has been set high - a legacy of the days when the power companies used to get mostly complaints about grid voltage dropping too low. There are brief times of day when I could export several kW, but for most of the day this would drive the voltage up too high and the system would repeatedly trip and reset. I found by trial and error a 3kW export limit is the best for reliability in my case. I'd recommend anyone to put a logging multimeter on their mains for a few days and see what the grid looks like - especially spring onwards when other local solar systems are doing their thing.

 

[John Brown]

If you add extra panels with a hybrid inverter and a battery connected directly to it, the meter tails will still be point in common between the new system and the one with feed in tarrif. Your new installation can have a current sensor on the meter tails and that will give you a good deal of flexibility. Most of all, it will allow you to take surplus power generated by your original 4kW array and pull it into the new inverter to charge your battery. Whether this is a good thing depends on whether the FIT tarrif is more or less than the price per kWh that you are paying these days.

As to the point about the grid and exports, I was approved a G99 license by my DNO to export upto the 9kW peak of my own solar panels. In practice I can never use it. The grid wiring has highish resistance around here (1970's vintage, and much added to) and our average voltage has been set high - a legacy of the days when the power companies used to get mostly complaints about grid voltage dropping too low. There are brief times of day when I could export several kW, but for most of the day this would drive the voltage up too high and the system would repeatedly trip and reset. I found by trial and error a 3kW export limit is the best for reliability in my case. I'd recommend anyone to put a logging multimeter on their mains for a few days and see what the grid looks like - especially spring onwards when other local solar systems are doing their thing.

If he gets £3000 a year, then I imagine the FIT is way higher than the domestic rare per kWh - we have a 3 to 4kW installation with a grandfather tariff of around 60p per generated unit, and we get slightly more than £2000 per annum.

 

[Lard]

Having read that some of you are getting £2k-£3k p,a I immediately thought 'Christ! they must be living on the equator! We're in south wales and get, on average just under £300 p.a

Then I realised that you obviously had your system fitted when the fit payments were much, much higher than our very late installation (near the end) of the fit scheme.

Having said that I bet your break-even period is still around 9-10yrs, as is ours?

 

[Agent_zed]

This is exactly what I've been thinking of doing and have the same scenario of 4kw of FIT, so didn't want to effect that by adding another set.

Something to think about is the term 'batteries'. A lot of people want to store electric in batteries but in reality how much electric do you use at night? The answer is very little, unless you are running your appliances at night. The trick is to run your appliances in the day when the sun shines and use that 'free' electric. Paying £5k+ for a battery, that lasts 10 years or less, so you can run your dishwasher at night makes no sense to me, when all it takes is to set the timer for it to come on when you are likely generating the most electric in the day.

So instead of thinking of a battery as a lithium battery pack, I think of a water tank as my battery. A solar diverter is very cheap and from what I can see is not against the fit rules. Any excess solar goes into heating water. You are storing energy just like a battery, just in a different form. Heating water is the most energy intensive part of your homes usage, so to me makes the most sense to put the energy into that first.

The solar diverter takes care of making sure you aren't heating water when you want to use the electric for running your appliances so no problem there, along with a few timers and you are good to go.

As an aside the other thing is rather than spending all that money on a battery is to put it into improving your house efficiency. A heat recovery system on your shower can save around 40% of the waste heat just going down the drain. That heat was very expensive to put into that water. Stopping the heat leaving your house with improved insulation is obviously the usual way of saving energy. Both of these things are thousands cheaper than a battery.