this post was submitted on 17 Sep 2025
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O, god, it's going to be huge. You really can't do the off-grid thing unless you have enough power production to satiate you over any given 3-day moving window. Trying to store power from summer until winter is going to be too expensive, instead buy more panel.
This isn't even going into the fact batteries lose charge slowly. So any power generated in summer will be much diminished by winter, even if you have big enough batteries.
Seems to me his panel capacity is to small anyway.
We have 11 kWh panels, and yes in the summer we routinely produce 4 times more than we use, and we have a 7.5 kWh battery But November December and January it's not even close to enough.
In the Winter you can easily have a week with near zero production:
Our Import / export from grid last year:
November 215 / 59 kWh
December 300 15 kWh
January 268 / 34 kWh
Despite we have almost 3 times the capacity, and produce more than twice what we use per year, and we have a decent battery and believe it or not, even the shortest day we can produce enough power for a whole 24 hour day if it's a clear day! But we can also have clouds for 14 days!
But for those months we imported 783 kWh and exported 108 that could have been used with bigger battery. But the net import was still 675 kWh!! For those 3 months, and that's the minimum size battery we could have managed with, and then we even need 10% extra to compensate for charge/discharge losses.
TLDR:
Minimum 740 kWh battery in our case, and that's without heating, because we use wood pellets.
That means it would require at least the equivalent of 10 high end fully electric car batteries. But also a very hefty inverter, which AFAIK ads about 50% the price of the battery.
PS: Already in February we exported more than we imported.
(Author here) As I say in my post, our roof is full. We have 16x 320 Watt panels - 8 on each side of the roof.
OK I didn't see that, that's bigger than I expected, we make about 12.5 MWh per year on our 11.2 kWh panels = 1.1 MWh per kWh capacity.
Your system is 5.1 kWh but you only make 3.8 MWh per year = 0.75 MWh per kWh capacity.
Meaning we have 50% higher yield per kWh rated capacity!
So our production remains 3.3 times higher than yours, despite we only have twice the capacity.
But our panels are pretty optimally placed towards the south.
Considering you are further south compared to us, I'm surprised your yield is so low, despite London is infamous for being cloudy.
Damn, those winter numbers mean full off-grid is quite difficult with pure solar. A propane or diesel generator to occasionally top off the batteries would be required for winter.
It is not remotely close to economically viable to go off grid, and the exports of solar power to the grid pay for the connection anyway.
The reason to have a battery is that it lasts through the night, or even with a smaller system, it can handle dinner time, which is the most expensive time of day to buy electricity.
Now if you live in some remote area without a grid, a generator is a way better option than a huge battery.
Maybe if you live somewhere very sunny, like Spain and especially southern parts of USA you can probably do it with a modest battery that can handle a couple of days.
In the summer we can make enough electricity on by far the most cloudy days, but in the winter, the sun can't penetrate the clouds nearly as well.
Admittedly London is south of where I live, which is close to the most southern part of Denmark, but on the other hand London is infamous for grey weather with heavy clouds.
You could probably get by with a gas generator and only run it 2-3 times/year in many areas. It's not 100% green, but it could get you off grid for a fraction of the price.
Power the generator with vegetable oil. There are multi fuel generators that are designed to work well with that kind of fuel. You could also use them for heating which is very useful in Northern regions where you usually need heating and electricity during winter.
Diesel generatorsare significantly better on fuel consumption than a gas one and diesel takes alot longer to go bad than gasoline.
Diesel generators run fine on heating oil, which is cheaper since no fuel tax and has longer shelf life.
Stabil 360 additive to fuel.