This page explains how we handle data and calculations. The formula for each calculator is also shown on its own page, under "How it is calculated".
Principles
- Every market or official figure is stored with its source link, source name, the period it covers and the date we last checked it.
- Calculators read those figures from one file per dataset, so a correction is made once and appears everywhere.
- If we cannot confirm a figure, we do not fill it with an approximate value. We ask you to enter it or leave the field without a default.
- We round only when showing a result, never during the calculation.
- The calculation code is separate from the screen and is tested against known cases and edge cases (zero, negative, empty and extreme values) before a calculator is published.
- Example values used in worked examples are labeled as assumptions. They are not statements about typical equipment.
Data sets
These are the data sets currently used by the calculators.
Data sets, sources and verification dates| Data | Source | Used for | Last checked |
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| Average residential electricity price by state (August 2026) | U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A (Form EIA-861M) | Default price in the electricity calculators | November 2, 2026 |
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| Average residential natural gas price by state (2024) | U.S. Energy Information Administration, Natural Gas Prices, Average Residential Price (annual, dollars per thousand cubic feet), 2024; heat content from https://www.eia.gov/dnav/ng/ng_cons_heat_dcu_nus_a.htm (U.S. natural gas delivered to consumers, 2024) | Gas price in the heating and water heater calculators | November 3, 2026 |
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| Average residential use, price and monthly bill by state (2024) | U.S. Energy Information Administration, 2024 Average Monthly Bill - Residential (Forms EIA-861, 861S and 861U) | Default price and usage in the cost, cooling, water heater, car, propane and battery calculators | November 2, 2026 |
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| Btu conversion factors (kWh, therm) | U.S. Energy Information Administration, Units and calculators explained: British thermal units (Btu), "Sample Btu conversion factors" | Converting between kWh, therms and Btu | November 3, 2026 |
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| CPSC portable generator safety distance | U.S. Consumer Product Safety Commission, news release of May 27, 2026: operate generators outdoors only, at least 20 feet from homes and buildings, with the exhaust directed away from windows, doors and vents | Safety note in the generator calculator | November 4, 2026 |
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| ENERGY STAR connected thermostat savings criteria | ENERGY STAR, Smart Thermostats key product criteria, Table 2: Connected Thermostat Energy Savings Criteria (ENERGY STAR Program Requirements for Connected Thermostats, Version 1.0) | Smart thermostat savings calculator | November 3, 2026 |
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| ENERGY STAR dehumidifier sizing and efficiency | ENERGY STAR, Dehumidifiers (recommended capacity range for a portable dehumidifier by room size and dampness) and Dehumidifiers key efficiency criteria (minimum Integrated Energy Factor, Version 6.0) | Dehumidifier size calculator | November 3, 2026 |
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| ENERGY STAR minimum AFUE and HSPF2 | ENERGY STAR, Air Source Heat Pumps key product criteria (HSPF2, cold climate tier) and Furnaces key product criteria (https://www.energystar.gov/products/heating_cooling/furnaces/key_product_criteria, AFUE) | Starting efficiencies in the heating calculators | November 3, 2026 |
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| ENERGY STAR minimum UEF for water heaters | ENERGY STAR, Residential Water Heaters Key Product Criteria (minimum UEF to be certified) | Water heater calculator examples | November 3, 2026 |
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| ENERGY STAR recommended insulation R-values | ENERGY STAR, Recommended Home Insulation R-Values (recommended insulation levels for retrofitting existing wood-framed buildings) | Insulation R-value calculator | November 3, 2026 |
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| ENERGY STAR room air conditioner sizing chart | ENERGY STAR, Room Air Conditioners: "Find the correct cooling capacity for your room size" and "Adjust as necessary" | AC BTU calculator | November 3, 2026 |
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| EPA label assumption: 15,000 miles a year | U.S. Department of Energy and EPA, fueleconomy.gov, Electric Vehicle Label: how annual fuel cost is estimated | Starting miles in the electric car calculator | November 3, 2026 |
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| Physical constants for heating water | U.S. Energy Information Administration, Units and calculators explained: British thermal units (definition of the Btu as the heat to warm one pound of water by one degree Fahrenheit); weight of a U.S. gallon of water: common reference value | Water heater calculator | November 4, 2026 |
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| Residential propane price and heat content | U.S. Energy Information Administration, Residential Propane Weekly Heating Oil and Propane Prices (U.S. average, dollars per gallon excluding taxes); heat content from EIA, Units and calculators explained: British thermal units (1 gallon of propane = 91,452 Btu) | Propane vs electric heating calculator | November 3, 2026 |
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The state prices come from Table 5.6.A of the EIA Electric Power Monthly. EIA describes values for 2026 as preliminary estimates, and the table reports a single month, so it is a snapshot and not an annual average. EIA states that its information products may be used and distributed, and asks for an acknowledgment that includes the publication date. Source: U.S. Energy Information Administration (release of October 23, 2026).
The average use, price and bill by state come from the EIA table "Average Monthly Bill - Residential" for 2024, built from EIA survey forms 861, 861S and 861U. These are annual averages across all residential customers in a state, not forecasts for a single home or month.
What the results are and are not
- Results are estimates for planning and comparison. They are not an engineering calculation, an energy audit, a quote or advice.
- Cost results price the energy used at one flat price per kWh. They leave out fixed monthly charges, demand charges, taxes and fees, which differ by utility.
- Your own utility rate, the real power an appliance draws and how you use it can all differ from the defaults.
See also the disclaimer.
Calculators and their formulas
- Appliance Electricity Cost Calculator, last reviewed November 2, 2026.
- kWh to Dollars Converter, last reviewed November 2, 2026.
- Monthly Electric Bill Calculator by State, last reviewed November 2, 2026.
- Electric Space Heater Cost Calculator, last reviewed November 3, 2026.
- Electric Car Charging Cost Calculator by State, last reviewed November 3, 2026.
- Pool Pump Running Cost Calculator, last reviewed November 3, 2026.
- AC BTU Calculator, last reviewed November 3, 2026.
- Window AC vs Central AC Cost Calculator, last reviewed November 3, 2026.
- Heat Pump vs Gas Furnace Cost Calculator, last reviewed November 3, 2026.
- Dehumidifier Size Calculator, last reviewed November 3, 2026.
- Smart Thermostat Savings Calculator, last reviewed November 3, 2026.
- Propane vs Electric Heating Cost Calculator, last reviewed November 3, 2026.
- Tank vs Tankless Water Heater Cost Calculator, last reviewed November 3, 2026.
- Home Battery Size Calculator, last reviewed November 3, 2026.
- Generator Size Calculator for Your Home, last reviewed November 3, 2026.
- Insulation R-Value Calculator by Climate Zone, last reviewed November 3, 2026.
Corrections
If you find a mistake in a formula or a figure, tell us through the contact page. We correct it and update the date on the page.