WattPayback
Neutral home-energy math. Every assumption editable.
Solar payback, batteries, heat pumps, EV charging and more — with formulas shown in plain numbers, not an installer's lead form.
Nine calculators
Live results. No submit buttons. Region defaults you can override.
Solar payback calculator
Estimate simple payback and 25-year net gain from a rooftop PV system with transparent assumptions.
Solar system size estimator
Size a PV array from annual consumption, target offset, and local specific yield.
Home battery ROI calculator
Work out whether a home battery pays back by capturing more of your own solar.
Heat pump vs. gas boiler
Compare annual running costs using SCOP, boiler efficiency, and fuel prices.
EV home charging cost
Calculate home EV charging cost with day/night tariffs and compare to petrol.
Appliance running cost
Build a list of household appliances and see annual electricity cost by device.
Insulation payback
Estimate payback for loft, cavity, draught-proofing and glazing with multiplicative savings.
Time-of-use tariff comparison
Split usage across peak, off-peak and night bands and rank up to three tariffs.
kWh ↔ cost ↔ CO₂ converter
Convert freely between kilowatt-hours, cost and CO₂ using editable grid intensity.
Latest from the blog
How the numbers work — and where installer quotes diverge.
2026-03-26
Ten questions to ask before signing a solar quote
A practical checklist for anyone about to spend four or five figures on rooftop PV.
2026-03-19
Time-of-use tariffs: who they suit and who they punish
Cheap night rates help flexible loads. Immobile evening peaks can make smart tariffs more expensive than flat rates.
2026-03-12
Reading your electricity bill: standing charges, unit rates, and where the money goes
Separate fixed daily charges from variable unit rates before you trust any calculator default.