Free Solar Energy Calculators
Brand-neutral solar tools for homeowners, installers, and off-grid enthusiasts.
No login. No ads. No product bias.
Solar Panel Sizing
How many panels do you need? Enter monthly kWh usage, peak sun hours, and panel wattage.
Battery Bank Sizing
Size a battery bank for off-grid or backup. LiFePO4, lead-acid, AGM, NMC — with days of autonomy.
Solar Payback Period
Calculate simple payback, 25-year ROI, and NPV with the 30% federal tax credit.
Off-Grid Load Calculator
Add appliances from a built-in library to calculate daily Wh and peak load for system sizing.
Solar String Sizing
Calculate panels per string to stay within inverter voltage limits. Accounts for temperature effects.
About These Calculators
Solar Panel System Sizing
The most common first question for anyone going solar: how many panels do I need? The answer depends on your electricity usage, local peak sun hours, panel wattage, and system efficiency. Our calculator handles all four variables and shows estimated annual production and roof space required. Start here if you're evaluating a quote or sizing a new system.
Battery Bank Sizing
Off-grid and backup systems need enough battery capacity to survive days without sun. Battery bank sizing depends on daily energy use, how many days of autonomy you need, the chemistry (lead-acid, AGM, LiFePO4, or NMC), and your bank voltage. Our calculator accounts for depth of discharge and round-trip efficiency — the variables most online calculators skip.
Solar Payback Period & ROI
Solar is a 25-year investment. Payback calculators should show the full picture: net cost after the 30% federal Investment Tax Credit, annual savings accounting for net metering policy, panel degradation over time, and cumulative savings. Our calculator produces a year-by-year table and NPV figure so you can evaluate solar the same way an investor would.
Off-Grid Load Calculator
Before you can size panels or batteries, you need to know your daily energy consumption. Our appliance accumulator includes 19 pre-loaded appliances with typical wattages. Add what you use, set hours per day and days per week, and get total daily Wh and peak load — exactly what you need to feed into battery bank and panel sizing calculators.
Solar String Sizing
Wiring solar panels in series creates a "string." The number of panels in a string must keep string voltage within inverter limits — both on the coldest day (when voltage is highest) and hottest day (when MPPT voltage drops). Overvoltage on a cold winter morning can damage your inverter. Insufficient voltage on a summer afternoon means no power. String sizing is safety-critical and often overlooked.