Solar, sized in seconds.
Work out how big a system you need, when it pays for itself, and how long a battery keeps the lights on — no installer sales pitch required.
How big a solar system?
Estimate the system size, panel count and roof space to cover your electricity use.
Assumptions & sources
System kW = annual use × offset ÷ (sun hours × 365 × 0.85 system derate). Panels assume 420 W each; roof area ~18 ft² per panel. "Sun exposure" maps to typical peak-sun-hours bands (NREL). A planning estimate — a site survey refines shading and orientation.
When does it pay for itself?
Estimate net cost after incentives, annual savings, payback time and 25-year return.
Assumptions & sources
Gross cost = kW × 1,000 × $/W. Net subtracts the incentive (e.g. the 30% U.S. federal credit). Production = kW × sun hours × 365 × 0.85; savings = production × your rate. Lifetime applies ~0.5%/yr panel degradation over 25 years. Excludes rate inflation, which would shorten payback.
How long will a battery last?
Pick what you want to keep running and see how long a home battery powers it during an outage.
Assumptions & sources
Usable energy = capacity × usable depth. Runtime = usable energy ÷ total selected load (continuous-watt averages for common appliances). Real runtime varies with duty cycles, surge loads and temperature; pumps and heaters draw far more when starting.
Solar & battery questions, answered
Direct answers to what people ask before going solar.
How many solar panels do I need?
A typical home using 900 kWh a month needs roughly a 6 to 8 kW system, or about 15 to 20 modern 400-watt panels, in an average-sun climate. The exact count depends on your usage, local sun hours and how much of your bill you want to offset.
What is the payback period for solar?
Most U.S. residential solar systems pay for themselves in roughly 7 to 10 years after incentives, then produce nearly free power for another 15-plus years. Payback is faster with high electricity rates, strong sun, and the 30% federal tax credit applied.
How big a battery do I need for backup?
For essentials only — fridge, lights, internet and phones — a single 13.5 kWh battery can last roughly a full day or more. Whole-home backup, including heating or well pumps, usually needs two or more batteries or careful load management.
Do solar panels work in cloudy climates?
Yes. Panels still generate on overcast days, just less — often 10 to 25% of peak. Cloudy regions simply need a slightly larger system to produce the same annual energy, and solar remains worthwhile where electricity prices are high.
Is home solar worth it?
For most homeowners with decent sun and ownership of their roof, yes: payback typically lands under a decade and lifetime savings often exceed the system cost several times over. It is least attractive with very low electricity rates or heavy shading.