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Off-grid solar system size calculator.
Daily kWh in, array kW, panel count, battery kWh and inverter watts out. Size for the worst month, not the average.
Inputs
System size
Enter your numbers.
How do you size an off-grid solar system?
Start with the energy you use per day, then work backwards through each loss. Array kW = daily kWh / (peak sun hours x (1 - system losses) x inverter efficiency x battery efficiency). Peak sun hours are the hours per day at 1 kW/m² that would give the same energy as the day's real sunlight, so use the lowest month you need to cover. Battery kWh = daily kWh x days of autonomy / usable depth of discharge / inverter efficiency. DOE notes off-grid battery banks are typically sized for 1 to 3 days.
Worked example. 5 kWh per day, 2.5 peak sun hours in the worst month, 14% losses, 95% inverter and 90% battery efficiency: 5 / (2.5 x 0.86 x 0.95 x 0.90) = 2.72 kW, which is 7 panels of 400 W (2.8 kW). Two days of autonomy at 80% depth of discharge: 5 x 2 / 0.8 / 0.95 = 13.2 kWh of battery.
Frequently asked questions
How many solar panels do I need to go off-grid?
Divide the array kW from this tool by your panel wattage and round up. With 5 kWh per day, 2.5 peak sun hours in the worst month and the default losses it is about 2.7 kW, or 7 panels of 400 W.
Which peak sun hours should I use off-grid?
The lowest month you have to run through without a backup generator. Using the annual average undersizes the array for winter.
How many days of battery autonomy do I need?
DOE's Small Wind Guidebook says off-grid battery banks are typically sized for 1 to 3 days of load. Your answer depends on how long poor weather lasts and whether you have a generator.
Does this include a generator or wind?
No. It sizes a solar-only system. A generator or wind turbine reduces the array and battery you need; see the wind tools in the related list.
Sources
Checked
- NREL PVWatts v5 manual (default 14% system losses)
- U.S. Department of Energy WINDExchange: Small Wind Guidebook
- Battery University: BU-501 Basics about Discharging
- U.S. DOE: Solar radiation basics
The 14% loss figure is the NREL PVWatts default and the 1 to 3 day autonomy range is from DOE. Inverter and battery efficiency, inverter headroom and the example loads are placeholders you should replace with datasheet values. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Planning estimate only. Off-grid systems involve high DC current and stored energy; have the design reviewed and installed to your local electrical code by a qualified person.