Off-grid, sized for the month that runs short
An annual average hides the winter. Every month is checked on its own, and a year that balances while December does not is reported as the problem it is.
Design an off-grid system
Sized against the month that actually runs short — because on an annual average, December disappears.
1 · What it has to run
2 · Sun, month by month
Twelve figures, not one average. The winter months are the entire question here.
3 · Design target
4 · Battery bank
5 · Controller and inverter
Month by month
Battery bank
Equipment checks
How this is calculated
array kW = daily kWh ÷ (peak sun hours in the design month × retained fraction)
Each month is then checked on its own: generation = array × sun hours × days × retained against load = daily × days. The annual total is shown but never used to size anything, because summer surplus cannot be stored until winter.
usable battery = daily kWh × autonomy days, then divided by depth of discharge to get the nominal bank. Modules are arranged series = system voltage ÷ module voltage and enough strings in parallel to hold it, rounding up.
Series multiplies voltage and leaves amp-hours alone; parallel does the reverse. A higher system voltage means fewer parallel strings and lower currents — but it also means buying batteries in bigger lumps, so the bank usually overshoots more.
Losses are applied once, in the retained fraction. Off-grid systems retain less than grid-tied ones because the round trip through the battery costs energy that a grid-tied system never pays.
The annual average is the enemy
Take a system generating 20% more than it uses across a year. On a grid connection that is a good result. Off grid it can still be a house in darkness every evening from November to February, because the surplus arrives in June and there is nowhere to keep it. This tool computes every month separately and says so loudly when the year looks healthy but individual months do not — that combination is reported as an error, not a footnote.
Autonomy is not the same as resilience
Three days of autonomy means the battery carries three days with no generation at all. It does not mean the system survives three cloudy days, because cloudy days still generate something and a partly-charged battery starts each one lower than the last. A week of heavy overcast in the worst month will empty a bank sized for three days of autonomy, and that is normal rather than a design fault. It is the reason most off-grid systems keep a generator — not for the average, but for the fortnight a year when the weather refuses to cooperate.
Why a generator often beats more panels
Sizing an array for the darkest month means it is enormously oversized for the other eleven. Past a point, the extra panels and batteries needed to cover midwinter cost far more than a generator that runs a handful of days a year — and the generator also covers the cloudy fortnight that no array size fixes. The third design option above sizes for a shoulder month and tells you how much energy a generator would need to make up.
What this will not tell you
Wire sizes, fuse and breaker ratings, earthing arrangements, battery enclosure and ventilation requirements, or whether your chosen battery is safe in your climate. Lithium banks in particular must not be charged below freezing, and lead-acid needs ventilation for the hydrogen it produces. Those are the specific things that make off-grid installations dangerous when done from a web page, and they belong with a qualified installer working to your local requirements.