Methodology · Updated June 2026

How the numbers on Watt Guide are made.

Every cost figure on this site is either pulled from a public dataset or computed from one with a formula you can check. This page is the audit trail: where the data comes from, the assumptions baked into each calculator, and how the guides are written and reviewed. Written and maintained by Bruno de Madrazo.

1. Where the data comes from

No number gets published unless it traces to one of these. Each guide also cites them inline.

Source	Used for
US EIA	US state residential electricity & gas prices; fuel-price history.
NREL PVWatts	Solar PV annual production by location, tilt and orientation.
DSIRE	State, utility and local incentives (solar, heat pump, EV, storage).
EnergySage · LBNL	Quoted $/W medians and long-term residential PV price trend.
Ofgem · Energy Saving Trust	UK retail prices, price cap, appliance benchmarks.
EPA fueleconomy.gov	EV efficiency (kWh/mi) and gasoline MPG benchmarks.
ENERGY STAR	Heat pump HSPF/SEER, fridge kWh/yr, washer/dryer ratings.
Eurostat · CRU · AER	Residential electricity prices for the EU, Ireland and Australia.

2. What each calculator assumes

The tools are deliberately transparent. These are the core assumptions; you override the inputs that matter for your own case.

Electricity cost

Cost = power (kW) × hours of use × days × your rate ($/kWh). We default to the latest EIA US residential average but you should enter your real rate from your bill — it's the single biggest driver. Appliance wattages default to ENERGY STAR / nameplate figures.

Solar ROI

Annual production comes from NREL PVWatts for your location and system size. Savings value that production at your retail rate (or, where modelled, at NEM 3.0 export rates, which are well below retail in California). Payback = net install cost ÷ annual savings. Since 1 Jan 2026 the calculator assumes no 30% federal credit (Section 25D, ended by the OBBBA for systems placed in service after 31 Dec 2025); state incentives are added only where they exist. We do not assume utility-rate inflation in the headline payback — adding it shortens payback and is the most common way these numbers get inflated elsewhere.

Heat pump vs gas

Annual heating cost compares a heat pump (rated by HSPF, derated for your climate zone) against gas/oil/propane/resistance at local fuel prices. We use seasonal averages, not nameplate COP, because cold-climate performance is where most "heat pumps always win" claims break down — and we say when gas still wins.

EV charging & whole-home payback

EV cost per mile = vehicle efficiency (kWh/mi from EPA) × charging rate, split between home and DC-fast at the mix you set, with optional time-of-use pricing. The whole-home payback tool stacks solar + heat pump + EV into one cash-flow model so the interactions (e.g. solar offsetting EV charging) aren't double-counted.

3. How the guides are written — and our AI policy

Plainly: AI helps with the first draft; a human owns the result. Research and initial drafting are done with the assistance of AI tools. Then every figure is checked against the cited public source, every claim that can't be sourced is cut, and the piece is rewritten and edited by Bruno de Madrazo before publishing. We never publish unverified AI output, and we don't put a human byline on anything a human hasn't actually reviewed.

What that means in practice: if a guide states a cost, you can click through to the dataset behind it. If it ranks products, the order reflects published efficiency ratings and testing data — never payment. Where we rely on a third party's measurement rather than our own, we say so rather than implying we tested it ourselves.

4. Freshness & corrections

Country and state average rates are refreshed quarterly; each guide is reviewed at least every 6 months.
The date shown on a guide is the date it was last verified, not first written.
Found a number you think is wrong? Tell us via the contact form. Substantive corrections are made promptly and noted.