How the numbers are calculated
makemesustainable provides free calculators that estimate what appliances, habits, and homes cost to run, and what changing them saves. Every result is computed from published data sources and the figures you enter, using the formulas below. This page lists each source, each formula, how often the data refreshes, and where the estimates stop being useful.
Data sources
| Source | What we use it for | Updates |
|---|---|---|
| US Energy Information Administration (EIA) | Average residential retail electricity price for each state and the US average. Current snapshot: 2026-03 (snapshot retrieved 2026-06-13). | Monthly, by an automated refresh job |
| US Department of Energy (energy.gov) and ENERGY STAR | Typical appliance wattages and duty cycles for the electricity cost presets, and the home energy audit recommendations. Each preset and each audit item cites its specific DOE or ENERGY STAR page. | Checked when the sources update |
| US EPA eGRID and EPA equivalencies references | Carbon factors for electricity, natural gas, and gasoline in the carbon footprint calculator. | Checked when the sources update |
| UK DESNZ conversion factors | Per-passenger flight emission factors, from which the per-round-trip flight figures are derived. | Checked when the sources update |
| Our World in Data | Published research on food-system emissions that informs the per-person diet estimates. | Checked when the source updates |
| Our World in Data (Global Carbon Budget) | US per-capita fossil CO2 emissions (14.2 tonnes per person for 2024), shown as a comparison line on the carbon footprint results. This is fossil CO2only, a narrower measure than the calculator's CO2e estimate. | Checked when the source updates |
| Ofgem energy price cap | Reserved for UK versions of the calculators. UK tools coming later. | Quarterly, once UK tools launch |
Electricity rates
Both electricity calculators price your usage with the US Energy Information Administration's average residential retail electricity price for your state, currently the 2026-03 snapshot. An automated job refreshes these rates from the EIA data service every month, so the period shown updates as new figures are published. You can always override the average with the exact rate from your own bill.
The raw rates are EIA data. The results the calculators produce are this site's own calculations, estimated using those EIA average residential rates; they are not EIA figures, and EIA does not endorse this site.
The formulas
Electricity cost calculator
Wattages and duty cycles in the appliance presets are typical values from the DOE and ENERGY STAR pages cited on each preset. The duty cycle is the fraction of the time an appliance actually draws power (a refrigerator's compressor, for example, cycles on and off).
- kWh per day = (watts / 1000) x hours per day x duty cycle
- cost per hour = (watts / 1000) x duty cycle x (cents per kWh / 100)
- cost per day = kWh per day x (cents per kWh / 100)
- cost per month = cost per day x (365 / 12)
- cost per year = cost per day x 365
Electric bill calculator
- monthly bill = kWh per month x (cents per kWh / 100) + fixed monthly fee
Carbon footprint calculator
All figures are tonnes of CO2e per household per year. The electricity factor of 0.3497 kg CO2e per kWh is derived from the EPA eGRID 2023 national average output emission rate of 770.884 lb CO2e per MWh. The natural gas factor of 5.3 kg CO2 per therm and the gasoline factor of 8.887 kg CO2 per gallon come from the EPA greenhouse gas equivalencies references.
- electricity = kWh per month x 12 x 0.3497 / 1000
- gas = therms per month x 12 x 5.3 / 1000
- driving = (cars x miles per car per year / MPG) x 8.887 / 1000
- flights = short round trips x 0.25 + long round trips x 1.1
- diet = household size x diet factor
- total = electricity + gas + driving + flights + diet
- per person = total / household size
The flight figures are derived from the UK DESNZ per-passenger-km conversion factors, assuming average return trip distances for the short-haul and long-haul categories, with radiative forcing included. The long-haul figure is conservative: a very long route will exceed it. The diet factors (1.5 vegan, 1.7 vegetarian, 2.5 average, 3.3 meat-heavy, in tonnes CO2e per person per year) are estimates informed by published research summarized by Our World in Data; they are category averages, not direct source line items.
The results also show a comparison against the US average of 14.2 tonnes CO2 per person per year (Our World in Data, Global Carbon Budget, 2024 figure). That average measures per-person fossil CO2 from fuel burning and industrial processes, a narrower measure than the household CO2e estimate this calculator produces, so treat it as a rough benchmark rather than a like-for-like comparison.
DIY home energy audit
The audit is a checklist, not a formula. Each answer of “fair” or “poor” flags that check, and the flagged items are ordered by severity (poor before fair) and then by impact, so the biggest likely savings appear first. Every recommendation and savings note cites the specific energy.gov or ENERGY STAR page it comes from, shown next to each item in your results.
Update cadence
- Electricity rates: refreshed monthly by an automated job that pulls the latest EIA residential rates. The period in use is always shown on the calculators and at the top of this page's table.
- Emission factors, wattages, and audit guidance: checked against their published sources when those sources update, and corrected whenever an error is reported.
Limits
- Rates are state averages. Your utility's actual price, fees, and taxes will differ, and tiered tariffs, time-of-use pricing, and taxes are not modeled.
- Appliance presets use typical wattages and duty cycles. Your model, settings, and climate change the real figure.
- Carbon factors are national averages. A cleaner or dirtier local grid, your car's real fuel economy, and your actual flight routes all shift the result.
- Your usage varies, and the estimates cannot see it.
For decisions that involve real money, treat the results as a guide to where to look first, then confirm prices with your utility or a qualified professional. See also the terms of use.