Going semi off grid with a FIT

[mfred68]

Hi guys I live in the UK and I have recently purchased 28kwh of battery storage, and im going down the Victron route with a multiplus 2 inverter and more solar panels.
However I also plan to charge the batteries over night on an economy 7 tariff , so the inverter will be timed to charge during cheap rate hours. Then during day rate, the ac input of the inverter will be disconnected, and the house will only run from battery power. However, I currently have a solar array with micro inverters and a feed in tariff. The generation meter monitors how much energy my array is producing, and I get paid for this, regardless if I export it or use it all myself. So if I did the above, I should in theory still get paid for my solar generation. Or would a smart meter tell the power company (Scottish power)that I've stopped exporting? Although my FIT is paid to me by a different energy supplier (British gas), so they have no access to my smart meter?

 

[SeaGal]

Suggest you speak first with the FiT payment provider; British Gas. Minimal changes are permitted to retain FiT payments; will depend on what the provider allows and whether you have bi-directional export meter on the inverter - they don't want you importing when cheap and then exporting it to increase the export payments.

 

[Stu675]

Don't ask, don't tell. How would BG find out? What did you decide? how much were your batteries? sounds a lot of storage.

 

[hsync33]

They can measure and if payments change dramatically run an inspection and fine or revoke the contract. That's on the fine print signed. Don't mess with FiT unless you're replacing like for like.

But some early FiT weren't actually metering your self consumption (only what came out of the inverter), so I would start trying to use as much as possible from that, if that's the case.
With that our of the way, if should be possible for you to install a secondary system, provided the service can accomodate the aditional production energy from your instalation.

batteries... I would be wary. I've run calculations. Batteries are more of a comodity if you need aditional power or UPS backup. They are, at best a gamble in terms of payback.

For example, if you buy energy off-peak at 12p and use it to offset energy which you would otherwise buy at 35p, then the difference is 23p. 48V systems are inneficient (round trip no more than 75%), so your savings will be less even (400V systems are available, at a premium from LG and the likes, claiming round trip efficiencies of 95%).

23p/KW Example... Supose 100% Eff, 20KW every day - which would cycle your batteries daily to a healty DoD (71%). Also suppose your battery lifespam to be 5 years, because that's a typical warranty period. Then 20*365*5 = 36.500Kw and you would have cycled your batteries short of 2000 times. That would have saved you short of £8500.
You can try to go for 10 years and 4000cycles - If your batteries are really that good and if you really manage to use 20KW a day, in adition to what your panels are producing.

How much did these batteries, supporting inverter and aditional PV modules cost you? How long do you expect them to last? How does it compare with the figures above? And if electricity prices drop to pre covid numbers of <20p/KW - How long will it take you to pay the system off?

On contrast, adding aditional PV modules is always a winner, provided there is space. Even if the utility pays you a lousy 6p/KW - Your rate of self consumption will go up dramatically, so you save straight away. There are no roundtrip losses on inverter/battery and all those exported KW will make you some pocket money for as long as your system lives.

 

[SeaGal]

They are, at best a gamble in terms of payback.

I disagree, but YMMV depending on usage. Have 14.3kWh battery with 6.6kWh of PV and were self-sufficient 24/7 for 5 months on the year in sunny East Anglia. Outside of that period some economy 7 top up at night was required, but avoided all peak time electricity over the last 2 winters.

Payback will be < 4 years for us.

8V systems are inneficient (round trip no more than 75%)

IMHO this is incorrect, how did you measure that and with what inverter? We achieve 85% round trip with Solis EH1P inverter and our battery pack.

which would cycle your batteries daily to a healty DoD (71%). Also suppose your battery lifespam to be 5 years, because that's a typical warranty period

Do you have batteries? I work mine hard and have achieved cycle count of 345 over 20 months (including 2 winters). Lifespan should be 10 to 15 years due to calendar aging.

 

[hsync33]

I disagree, but YMMV depending on usage. Have 14.3kWh battery with 6.6kWh of PV and were self-sufficient 24/7 for 5 months on the year in sunny East Anglia. Outside of that period some economy 7 top up at night was required, but avoided all peak time electricity over the last 2 winters.

Payback will be < 4 years for us.


IMHO this is incorrect, how did you measure that and with what inverter? We achieve 85% round trip with Solis EH1P inverter and our battery pack.


Do you have batteries? I work mine hard and have achieved cycle count of 345 over 20 months (including 2 winters). Lifespan should be 10 to 15 years due to calendar aging.

It's wise advise to calculate the payback over a 5 year period, because after that time, very few systems will provide a working warranty. And the ones that do, will likelly have costed more. Sure, you may be lucky, but will you?

In many places the utility buys back unused energy. The prices aren't great, but the earnings push the payback of the battery even further.

You mentioned yourself your payback will be 4 years.
It's always a matter of maths and to some extent luck. The rest is opinion. In the UK, Germany, etc where the KW last year was 40 or 50p..., they can be worth it - But that will not be valid everywhere and certainly the price drop (or return no pre covid values) we have experienced so far is making the payback longer

Efficiency wise, generally is quoted at the maximun value. Partial load will be worse. Every conversion (panel to battery, battery to inverter, grid to battery and battery to grid) will add some losses and as the battery ages and it's internal resistance goes up, battery losses increase too. The point above is that a high voltage battery system will be more efficient than a low voltage system - everything being equal.

 

[SeaGal]

It's wise advise to calculate the payback over a 5 year period, because after that time, very few systems will provide a working warranty. And the ones that do, will likelly have costed more. Sure, you may be lucky, but will you?

I built it, so I hope so