How We Calculate

Every Home Energy Lab calculator follows the same idea: start from a physical quantity you can find on a label, a meter or a bill, convert it into the unit your utility charges for, and multiply by the price you pay. This page lists the formulas, the default values and where they come from, so you can check any result by hand.

All calculations run in your browser. The numbers you enter are not sent to us or stored by us.

The core chain: watts → kilowatt-hours → dollars

Most electrical devices are estimated in three steps.

1. Power. How many watts the device draws while running, from its label, its specifications or a plug-in energy meter.
2. Energy. Power multiplied by running time. kWh = watts × hours ÷ 1,000
3. Cost. Energy multiplied by your electricity rate. cost = kWh × rate

For example, a 1,500 W space heater running for one hour uses 1.5 kWh. At 17.5¢ per kWh, that hour costs about 26¢.

To turn a daily figure into longer periods, we use year = day × days per week × 52.14 and month = year ÷ 12. Using 365 ÷ 7 weeks per year, rather than 52, keeps monthly and yearly totals consistent with each other.

Default electricity rate

When you don't enter your own rate, calculators start with 17.5¢ per kWh, the average U.S. residential electricity price for 2025 from the U.S. Energy Information Administration (EIA) Short-Term Energy Outlook.

Rates vary widely by state, utility, season and plan. For the most accurate result, use the price on your own bill. If your bill lists several per-kWh charges (supply, delivery, riders), add them together.

Appliances that cycle

Some devices don't run at full power the whole time they are switched on.

• Air conditioners. Power is estimated from cooling capacity and efficiency: watts = BTU/h ÷ EER. The compressor runs only part of the time, so average draw is watts × duty cycle, multiplied by hours per day and the months of the cooling season.
• Space heaters. The same model: rated wattage × thermostat cycling × hours on, over a heating season.
• Refrigerators. These run around the clock and cycle constantly, so we start from yearly energy use, either the kWh figure on the EnergyGuide label or a reading from an energy meter scaled to a full year (8,760 hours).
• Gaming PCs. The day is split into gaming, idle and sleep, each with its own wall draw. Monitors and peripherals are added while the computer is awake.

Measured readings

If you measured a device with a smart plug or energy meter, the Smart Plug Energy Calculator works backwards from the reading: average watts = kWh measured ÷ hours measured. That average already includes cycling and standby, which makes it more reliable than a nameplate rating for most appliances.

Lighting

Bulbs are compared at equal brightness (lumens), because that is how you would actually swap them. Typical wattage at 800 lumens is 60 W for incandescent, 14 W for CFL and 9 W for LED. Rated lifetimes used for replacement comparisons are 1,000 hours (incandescent), 8,000 hours (CFL) and 15,000 hours (LED). Payback time is the upfront cost divided by yearly savings.

Natural gas

Gas utilities bill in therms, but meters usually measure CCF (hundreds of cubic feet).

• therms = CCF × therm factor. We use 1.037 as the default factor, typical for U.S. pipeline gas. Your bill shows the exact factor for your area.
• bill = therms × price per therm + fixed monthly charge

The default price of $1.50 per therm is an example, not a national average. Gas prices change significantly by region and season, so use the all-in price from your bill.

Home heating

To compare heating systems, we first estimate how much heat a home needs over a season, from its floor area and a climate-based heat demand (from about 8,000 BTU per square foot per year in warm climates to 50,000 in very cold ones). The same heat is then delivered by each system:

One kWh equals 3,412 BTU and one therm equals 100,000 BTU. Climate-based heat demand is only a starting point: insulation, air sealing and thermostat settings can change real use considerably.

Water, sewer and hot water

• Units. One CCF equals 748 gallons.
• Water and sewer bill. Usage × price per 1,000 gallons for water and for sewer, plus fixed monthly charges. Tiered rates are calculated block by block, the way utilities apply them.
• Hot water. Heating one gallon of water by 1°F takes 8.34 BTU. heat = gallons × 8.34 × temperature rise, divided by the water heater's efficiency (Uniform Energy Factor, or COP for heat pump models), then converted to kWh or therms.

The default water and sewer rates are examples. Water pricing is set locally and differs more between cities than almost any other utility.

Laundry

One load is split into the four parts that land on different bills: the washer motor, water and sewer, heating the hot water, and drying. Washer water use ranges from about 14 gallons (front-load ENERGY STAR) to 35 gallons (older top-load with agitator). A typical electric dryer load uses about 3 kWh; a gas dryer uses about 0.2 therms plus a little electricity for the drum and fan.

Total home utilities

The Home Utility Cost Calculator adds up the bills you enter for each month. Months you leave empty are filled with your average month and marked as estimates, so the yearly total is a projection rather than a measured figure.

What our estimates leave out

Calculators estimate usage-based costs. A real bill may also include:

• fixed customer or connection charges;
• time-of-use, tiered or seasonal pricing;
• taxes, riders and fuel-cost adjustments;
• minimum bills, credits, and differences in billing period length.

Real appliance use also depends on age, maintenance, settings, weather and habits. Treat every result as an estimate for planning and comparison, not a quote. See our Disclaimer for details.

Corrections

We review formulas and default values when better data becomes available, such as a new EIA annual figure. If you think a number or method is wrong, please tell us through the Contact page.