See how many panels you need & the payback — anywhere in the world
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Draw your roof on the map, enter a system size, or estimate one from your bill or appliances — your payback and savings appear here.
List what you run — fans, ACs, fridge, lights… We total the daily energy and suggest the solar system size to cover it.
Suggested size uses 4.5 peak sun hours/day. Set your location in the ROI Calculator tab for a local figure.
Inverter carries the peak load with surge headroom. Battery is nameplate capacity to run everything listed for the backup hours (at the set depth of discharge) — trim the hours if only essentials stay on.
KnowMySolar is a free solar panel calculator that helps you decide whether solar is worth it — anywhere in the world, in any currency, with no sign-up. Draw your roof on a satellite map, enter your electricity rate and install cost, and instantly see how many solar panels you need, your payback period, net savings, return on investment and lifetime CO₂ offset. The System Sizing tab turns your appliance list into a recommended system size in kW, plus inverter and battery-bank sizing for backup power during outages.
It depends on your electricity use, roof space and local sun hours. Enter your monthly bill or draw your roof and we estimate the system size (kW) and panel count to cover your usage. Most homes need roughly 15–25 panels for full offset, but your exact number comes from your own inputs.
Solar is worth it when your lifetime bill savings comfortably exceed the net system cost. We show your payback period, 25-year net savings and ROI so you can decide for your own electricity rate, install cost and sun hours.
Cost is driven by system size and your local install price per watt. Enter your cost per watt and any rebates, and we calculate net system cost — then compare cash purchase vs. a solar loan with monthly payments and total outlay.
Payback is the year your cumulative electricity-bill savings equal the net cost of the system after rebates. We project production year by year (accounting for panel degradation) against a rising electricity price, then report the interpolated break-even point, net savings and return on investment.
Use the System Sizing tab to list your appliances, their wattage and daily run-hours. We total your daily energy use and suggest a system size in kW based on your location's peak sun hours, plus inverter and battery-bank sizing for a set number of backup hours during outages.
Yes — completely free, no sign-up, and it works anywhere in the world in any currency. Peak sun hours come from NASA POWER irradiance data, and you can draw your roof or use Google's Solar API where available.