A free set of home-energy calculators for UK and US households, built on official data, that shows its working and says plainly when something doesn't pay.
- Power station sizer: enter your appliances' watts and hours, and it checks 58 portable power stations from 4 brands against both running and start-up (surge) watts. Models that don't fit are listed too, with the reason. The power station figures come from makers' published spec pages, each linked.
- Charge at night, use by day: works out whether charging a battery on a cheap night rate saves money once battery wear is counted. UK day rates come from Ofgem's regional cap on standard variable tariffs; US visitors see their state's average.
- Heat pump vs gas: running cost and payback against your own last-year gas use, with and without keeping the gas connection. The UK side includes Boiler Upgrade Scheme grants (England and Wales), with visitors typing their own SCOP. The US side covers 192,446 certified heat pump ratings, plus a cold-weather capacity check.
- Appliance running costs: cost per hour, load, day and year from typed watts, a plug-in meter reading, or, in the US, one of 8,110 models listed as ENERGY STAR certified.
- Every figure links to its source and the date it was checked, and no 'typical' figures are invented. Datasets are checked against a second official source. Counts as of 27 Oct 2026.
- A static site: every calculator runs in the visitor's browser, with no accounts, cookies or tracking. The calculations sit in a small, separately tested core of several hundred tests, each named with real figures so it can be checked by hand.
Data sources: Ofgem's regional tariff cap levels and Met Office HadUK-Grid regional temperatures, both under the Open Government Licence v3.0; U.S. Energy Information Administration electricity figures; U.S. EPA ENERGY STAR certified product lists (data.energystar.gov); NOAA NCEI statewide heating degree days; GOV.UK and IRS.gov for grants and tax credits. Watts & Hours is independent and is not affiliated with, or endorsed by, Ofgem, the EIA, the U.S. EPA, ENERGY STAR, the Met Office, NOAA, GOV.UK, the IRS or any manufacturer.