Solar Panel Calculator

Payback, savings & ROI — anywhere

Tell us about your electricity

Three or four answers is enough. Anything you're unsure about has a sensible default already filled in.

Where are you?

This sets your currency, measurement units and how much sun the roof gets.

What do you pay for electricity?

Your bill is what the savings are measured against.

That works out to about 600 kWh a month.

How big a system do you need?

Pick whichever you can answer most confidently.

Can you sell extra power back?

Solar makes most of its power at midday. What you don't use right then is either sold to the grid or wasted.

I have net metering ⓘ
Power you use as it's generated ⓘ 50%
Assumptions — change these only if you know your numbers ⌄

Free solar panel calculator — ROI, savings & roof planner

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 appliance-based sizing option turns your appliance list into a recommended system size in kW, plus inverter and battery-bank sizing for backup power during outages.

How the solar calculator works

  1. Your setup. Pick your country, enter your electricity bill and rate, and choose how to size the system — from your bill, your appliances, or a size you already know. Set your panel type, install cost and any rebates or net metering.
  2. Your roof. Search your address or tap “use my location”, then draw your roof outline on the satellite map — or type its area. We check the system fits and read local sun hours from NASA data.
  3. Your results. Instantly see your payback period, net savings, ROI, system size, year-1 production and lifetime CO₂ offset — adjust the key sliders live, compare cash vs. a loan, and download a PDF report.

Common questions

How many solar panels do I need?

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.

Is solar worth it?

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.

How much do solar panels cost?

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.

How is the payback period and ROI calculated?

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.

What size solar system and battery do I need?

Choose the From my appliances option 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.

Is it free, and does it work in my country?

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.