Off-Grid Solar System Calculator
Panels, battery, charge controller and inverter from one number — sized for the worst month of the year, not the average one, which is where most off-grid calculators quietly under-build.
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In short: An off-grid array must be sized for winter, not the annual average. Covering 10 kWh a day at US-average sun needs about 5.0 kW sized for winter versus 3.3 kW sized on the annual average — four extra panels.
Solar array
5.0 kW
13 panels at 400W
Battery bank (nominal)
29.4 kWh
613 Ah at 48V
Charge controller
Parallel controllers
Inverter (continuous)
2.5 kW
Designed against 2.93 peak sun hours — your winter figure, not your annual average. Sized on the annual average instead, the array would be about 3.3 kW, which is the number that fails in December.
This array is past what a single charge controller handles — controllers stop at around 100A. That is normal on larger off-grid builds: split the array into two or more strings, each with its own controller, feeding the same battery bank.
Why winter decides everything
A grid-tied system can be sized on annual averages because the grid quietly covers every shortfall. Off-grid has nothing behind it. If the array cannot refill the battery during the darkest fortnight of the year, the bank drains a little further every day until the lights go out — and that happens in January, when it is cold and you least want to be rationing power.
So the design month is the worst month, not the mean one. In much of the United States December delivers something like two thirds of the annual daily average, and in the cloudier northern states considerably less. Sizing on the average is not a small optimism; it is an array roughly a third too small.
What the four numbers are for
The array replaces what you used, in the worst month, after inverter and round-trip losses. The battery carries you through the nights and the overcast days, sized on usable capacity after depth of discharge rather than on the figure printed on the box. The charge controller has to handle the array's full output with the margin the code requires. The inverter is sized by your largest simultaneous load, which is a different question from how much energy you use in a day.
People routinely get the last one backwards and buy a huge inverter to match a big array. An inverter does not store anything. An oversized one simply burns more power idling, which off-grid you pay for twice.
Where to get a real number for your location
The winter margin here is one honest assumption applied everywhere, and it is good enough to plan and budget with. It is not good enough to sign off a final build, because local winter cloud matters as much as latitude — Denver and Seattle sit four degrees apart and their December sun differs by roughly half.
For the real figure, run your exact address through NREL's PVWatts, take the December output, and put that straight into the sun hours this page uses. That is the number your system will actually live on.
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Questions
Why is this bigger than other off-grid calculators tell me?
Because most of them size the array on annual average sun and this one sizes it on winter. Off-grid has no grid to borrow from in December, so the worst month sets the size. At national-average sun, covering 10 kWh a day needs about 5.0 kW sized for winter against roughly 3.3 kW sized on the annual average — 4 extra panels. The smaller number is the one that leaves you in the dark in January.
What is the winter margin and can I change it?
We design against 65% of your annual average sun hours, which is a reasonable middle figure for the continental US. It is a deliberate single assumption rather than a per-state December number, because real December sun depends on local winter cloud as much as on latitude — Denver and Seattle are four degrees apart and their December figures differ by about half. For a final design, look up your own location in the NREL PVWatts tool and put its December figure straight into the sun hours box.
How many days of autonomy do I need?
Two is the usual answer and the default here. One day means a single overcast day empties the bank; three or more gets expensive fast and spends most of the year as capacity you are paying for and not using. If you have a generator for the genuinely bad weeks, two days is plenty. If you have no backup at all and winter storms are normal where you live, three is defensible.
Why is the inverter so much smaller than my array?
Because the inverter is sized on the largest load you run at once, not on how much energy you use in a day. A kettle, a microwave and a well pump starting together set that number; the array and battery are sized by the daily total. They are different questions and sizing one from the other is how people end up with an inverter that trips every time the pump starts.
Does this include wiring and a charge controller?
The controller is sized above, from the array and the battery voltage. Cable is a separate calculation that depends on how far apart things are, and it is the one most often got wrong — see the wire size calculator. Neither is optional: an undersized controller is a bottleneck and undersized cable is a fire.