Off Grid Solar System Calculator
A home using 30 kWh per day needs roughly 8–9 kW of solar panels, 60+ kWh of battery storage for two cloudy days, and a 10 kW inverter. This calculator sizes all three — panels, batteries, and inverter — plus a budget estimate.
Off-grid arrays are typically 20–30% larger than grid-tied ones for the same load, because every winter kWh must come from the roof.
Estimates only. Real designs account for shading, roof angle, local code, and seasonal load swings. Get a professional system design before buying.
How off-grid solar sizing works
Three numbers define an off-grid system. The solar array must replace each day's use: array kW = daily kWh ÷ peak sun hours ÷ 0.75, where the 0.75 derate covers temperature, wiring, inverter, and soiling losses. The battery bank must carry you through sunless days: battery kWh = daily kWh × days of autonomy ÷ depth of discharge. The inverter must survive your worst moment: 1.25× peak simultaneous load.
Peak sun hours are not daylight hours — one peak sun hour equals 1 kWh/m² of solar energy. Most US locations average 4–5; your installer or NREL's maps give the local figure. Undersizing the array is the classic off-grid mistake: the batteries never fully recharge in winter and die young.
panels kW = daily kWh ÷ sun hours ÷ 0.75 · battery kWh = daily kWh × days ÷ 0.8 · inverter kW = peak kW × 1.25
Related tools
Keep exploring: fine-tune storage with the Solar Battery Size Calculator, or cover cloudy weeks with the What Size Generator Do I Need Calculator.
Off grid solar system calculator FAQ
How many solar panels do I need to go off-grid?
Divide daily kWh by your peak sun hours, derate by 0.75 for real-world losses, then divide by panel wattage. A 30 kWh/day home with 4.5 sun hours needs about 8.9 kW — roughly twenty-three 400 W panels.
How big a battery bank do I need for off-grid solar?
Multiply daily kWh by your days of autonomy and divide by depth of discharge. Two cloudy days at 30 kWh/day needs 30 × 2 ÷ 0.8 = 75 kWh of lithium storage. The Solar Battery Size Calculator explores this in depth.
What size inverter do I need for an off-grid system?
Size it to 1.25× your peak simultaneous load. If the well pump, fridge, and microwave can run together at 8 kW, choose a 10 kW inverter so motor starting surges don't trip it.
How much does an off-grid solar system cost?
Roughly $2.75–$3.75 per watt installed, including panels, batteries, inverter, and labor. A 9 kW system lands near $25,000–$34,000 before incentives — batteries are the biggest slice of that.
Can I go off-grid with a grid-tied solar system?
No. Standard grid-tied inverters are required to shut down during outages (anti-islanding). Going off-grid needs a hybrid or off-grid inverter plus batteries, and usually a redesign of the array wiring.
How many peak sun hours do I need for off-grid solar?
Most of the US gets 4–5 peak sun hours daily on average; the desert Southwest sees 6+. Fewer sun hours just means more panels — Alaska at 3 sun hours needs about 50% more array than Arizona for the same load.
Do off-grid systems need a backup generator?
Strongly recommended. Even a well-sized system meets a week of storms eventually. A small generator — see the What Size Generator Do I Need Calculator — covers the gap far cheaper than a battery bank sized for the worst week of the year.