Solar Payback Calculator
Model production, savings and payback over 25 years, including rebates, export prices and rising tariffs.
Home Battery Sizing
Turn your daily usage into the nominal capacity and module count you need, after depth-of-discharge and round-trip losses.
EV Charging Cost
Compare a year of charging versus refuelling in miles or kilometres, with charging losses counted in.
Free renewable energy calculators — private by design
These are the numbers that decide whether a clean-energy project makes sense: a
solar payback calculator, a home battery sizing tool, and an
EV charging cost comparison. Each one uses the same engineering assumptions a
installer would apply — performance ratio, panel degradation, depth of discharge, round-trip
efficiency and charging losses — so the answers hold up outside the spreadsheet. Everything runs
locally in your browser, with no sign-ups and no uploads.
Frequently asked questions
How do I calculate solar panel payback?
Payback is the point where cumulative savings equal what you paid. Enter your system size, installed cost, any rebate, local peak sun hours and your electricity rate. The calculator models 25 years using an 80% performance ratio, 0.5% annual panel degradation and your expected tariff rise, then reports the year your savings overtake the net cost.
How many peak sun hours does my area get?
Peak sun hours are the daily equivalent of full-strength sunshine, not daylight hours. Most of Europe sits around 2.5–4, the southern United States and Australia around 5–6, and desert regions can exceed 6. Your installer or a national irradiance map will give a local figure — enter that number for a realistic estimate.
What size home battery do I need?
Start from the energy you actually draw during the outage, not your whole daily use. The tool multiplies your daily consumption by the share of the day you want covered, then divides by depth of discharge and round-trip efficiency to give the nominal (rated) capacity to buy — always larger than the usable figure.
Is charging an EV cheaper than petrol?
Usually yes, but it depends on your electricity rate. The comparison divides your annual distance by the EV's efficiency to get kWh, adds charging losses you still pay for at the meter, and sets that against the fuel the petrol car would burn. Off-peak or solar-charged electricity widens the gap considerably.
Are these energy calculators free and private?
Yes. Every tool is 100% client-side JavaScript — no signup, nothing is uploaded, and your inputs never leave your device. They work on mobile and desktop, and keep working offline once the page has loaded.