How WattCost Actually Calculates Every Number

Every number on WattCost comes from one formula: certified annual kWh × state electricity rate (¢/kWh) ÷ 100 = annual cost. No weighting, no adjustments, no proprietary scoring — across 9,441 certified models, if you multiply the two public numbers yourself, you get WattCost's figure exactly.

The formula, worked through once

Take the median certified refrigerator: 345 kWh a year, certified by the DOE test procedure behind every ENERGY STAR label. Multiply by the U.S. average electricity rate, 18.83¢/kWh, and divide by 100: 345 × 18.83 ÷ 100 = $64.96 a year. That's the entire calculation — the same one behind every model, category, and state page on the site.

Two public data sources, nothing proprietary

The kWh figure comes from the EPA's ENERGY STAR certified-products database (data.energystar.gov), the same standardized laboratory numbers printed on the yellow EnergyGuide label — WattCost's current snapshot was certified through July 2026. The rate figure comes from the U.S. Energy Information Administration's Electric Power Monthly, Table 5.6.A, which publishes the average residential price of electricity by state; WattCost uses the April 2026 figures, putting the U.S. average at 18.83¢/kWh. Neither number is WattCost's own estimate — both are public, both are checkable at the source.

The same model, priced in every state

Because the formula only changes on the rate side, the same certified model produces very different bills depending on where it's plugged in. That median 345 kWh refrigerator costs $42.61 a year in North Dakota, the cheapest state, and $160.84 in Hawaii, the priciest — a 3.8x gap for an identical appliance, entirely from the local electricity rate.

What gets left out, on purpose

• Models missing the energy field for their category are dropped entirely — WattCost never estimates a certified figure to fill a gap.
• Duplicate SKUs are clustered. Retailer variants sharing a brand, normalized model number, and identical energy figures are merged into one canonical model.
• Gas clothes dryers and gas or propane water heaters are excluded — their running cost is mostly fuel, not electricity, so a kWh figure would mislead.
• Signage displays are excluded from computer monitors, and models not marketed in the United States are excluded everywhere.
• Ceiling fans and pool pumps get transparent wattage calculators instead — ENERGY STAR certifies them for something other than annual kWh, so there's no certified figure to build a cost page from.

What this doesn't tell you

The formula is exact, but it answers a narrower question than "what will my bill be." Test conditions aren't your house — real usage (thermostat settings, load sizes, climate, hours of use) moves actual consumption above or below the certified figure. State averages hide utility-specific variation — tiered pricing and time-of-use plans mean a specific bill may differ from the state average. And ENERGY STAR only covers certified models, so "power-hungry" on WattCost means power-hungry among certified models, not the worst performers on the market at large. These are comparison estimates for choosing between appliances, not bill forecasts — see the full methodology page for the complete list.

How often the numbers update

ENERGY STAR data is re-pulled and electricity rates are refreshed monthly, with EIA's rate data typically landing about two months after the fact. WattCost's current snapshot: ENERGY STAR data through July 2026, EIA rates for April 2026, site rebuilt July 2026.

FAQ

What's the exact formula WattCost uses? Annual cost ($) = certified annual kWh × state electricity rate (¢/kWh) ÷ 100. Monthly cost divides that by 12, daily cost by 365 — no other adjustment is applied.

Where does the energy-use (kWh) data come from? The EPA's ENERGY STAR certified-products database, published at data.energystar.gov — standardized laboratory figures measured under each category's DOE test procedure, the same numbers behind the EnergyGuide label.

Where does the electricity-rate data come from? The U.S. Energy Information Administration's Electric Power Monthly (Table 5.6.A), which reports the average residential price of electricity by state. WattCost currently uses the April 2026 figures.

Does WattCost ever estimate a number when certified data is missing? No. A model missing its category's certified energy field is dropped from the dataset entirely rather than assigned an estimated figure — the one hard rule the whole pipeline is built around.

Why don't ceiling fans or pool pumps have certified cost pages? ENERGY STAR certifies them for a different metric — airflow or pumping efficiency — not annual kWh, so there's no certified annual-cost figure to build a model page from. They get transparent wattage calculators instead.

How often is WattCost's data updated? Monthly. ENERGY STAR's certified-products list is re-pulled and EIA's state electricity rates are refreshed on the same cadence, though EIA's own data typically arrives with about a two-month lag.