Method

Everything here is a simplified estimate. Change any input in the check to fit your home.

Prices

If you enter a bill amount and its kWh (or therms), we divide them to get your real rate. Otherwise we use your state's average from the U.S. EIA (see sources).

Heating and cooling

Heat loss = floor area × a loss factor (0.35, 0.25 or 0.15 Btu/h per ft² per °F for leaky, average or tight homes; our assumption) × degree days × 24 h. Degree days use NASA POWER's 18.3°C (65°F) base. Gas furnaces are taken at 80% efficiency, electric resistance at 100%, existing heat pumps at HSPF2 7.5, existing air conditioners at SEER2 13.4.

Fixes

• Setback: 10% of heating and cooling × the share of 8 hours you don't set back yet (DOE).
• Sealing: 15% of heating and cooling unless you mark the home as tight (ENERGY STAR).
• Heat pump: your current heating and cooling cost minus the same load at SEER2 15.2 / HSPF2 7.8.
• Shower: minutes × people × 365 × (your flow − 1.8 gal/min). Hot water is taken as 70% of shower water with a 70°F rise; water heaters at 60% (gas) or 92% (electric).
• Toilets: 5 flushes per person a day × (your gal/flush − 1.28).
• LEDs: each old bulb saves 51 W while on.
• Solar: kW × daily sunshine (kWh/m²) × 365 × 0.80. Value at your rate, capped at your yearly use when you enter a bill.

What it doesn't do

It isn't an energy audit, a blower-door test or a quote. It doesn't know your utility's tariff details, time-of-use rates or net-metering rules, and it doesn't include incentives.