Off-Grid System Sizing Calculator
Daily watt-hours into the three numbers that decide what to buy: array wattage, battery bank capacity, and charge controller current.
From the appliance power budget.
Not the annual average. NREL publishes this by location and month.
Days the bank must carry the load with no meaningful sun.
Solar array
833 W
Round up to whole panels. More is rarely wasted on an off-grid system.
- Battery bank
- 260 Ah at 24 V
- Bank energy
- 6250 Wh nameplate
- Usable energy needed
- 4000 Wh
- Charge controller
- 43 A minimum
The bank figure includes a 25% margin for inverter losses, cold weather, and the capacity a battery loses as it ages. The controller figure includes 25% for cold-weather overproduction.
The input that decides everything
Peak sun hours matters more than any other figure here, and it is the one most often entered wrongly.
It is not hours of daylight. It is the number of hours equivalent to full-strength sun once weaker morning, evening, and overcast light is accounted for. A site averaging 4 peak sun hours in December might have nine hours of daylight, most of it weak.
Use the figure for the worst month you intend to use the system, not the annual average. The seasonal swing is large: a location averaging 5.5 across the year might see 6.5 in June and 2.2 in December. Size on the average and you build something generous in summer that fails in the month you most need it. NREL publishes monthly irradiance data for locations across the United States.
How each number is worked out
Array wattage is daily watt-hours divided by peak sun hours, then divided by a system efficiency factor of 0.8. That factor covers cell temperature, angle, soiling, wiring, and controller losses. Panels reliably produce 75 to 85% of their rated output on a good day, so sizing on nameplate figures leaves you short.
Battery bank is daily watt-hours multiplied by days of autonomy, divided by the usable depth of discharge for your chemistry, plus a 25% margin for inverter losses, cold weather, and the capacity a battery loses as it ages.
Charge controller current is array wattage divided by battery voltage, plus 25%, because panels can briefly exceed their rated output in cold, bright conditions.
Days of autonomy is a budget decision
Each additional day multiplies the largest single cost in the system. Two to three days suits a cabin with a generator as backup; three to five suits a full-time home in a cloudy climate with none.
A generator is very often cheaper than the two extra days of storage it replaces, which is why so many long-running off-grid homes have one. That is a genuine engineering trade-off rather than a compromise.
Check the array can charge the bank
A bank the array cannot refill is worse than no bank at all, especially for lead-acid, where chronic undercharging causes sulfation and kills it early. Lead-acid generally wants a charge current of at least 10% of its capacity to reach a proper full charge. The calculator warns when the array falls short of that.
The full method, with a worked example, is onhow to size an off-grid battery bank.