Electric Bill Calculator

Electricity is billed per kilowatt-hour (kWh).

The formula:

• kWh = Watts × Hours / 1,000
• Cost = kWh × Rate per kWh

Example (metric and imperial contexts): A 100W bulb running 10 hours/day for 30 days:

• Energy = 100 × 10 × 30 / 1,000 = 30 kWh
• At $0.12/kWh (US avg): $3.60/month
• At EUR 0.25/kWh (EU avg): EUR 7.50/month
• At GBP 0.28/kWh (UK avg): GBP 8.40/month

Typical appliance consumption:

Unit conversions:

• 1 kWh = 3,600,000 joules = 3,412 BTU

The whole bill comes down to one formula: energy in kilowatt-hours times your rate. A kilowatt-hour is 1,000 watts running for an hour, so a 100-watt device left on for ten hours uses one kWh. Multiply by the rate printed on your utility bill and you have the cost; the rate is the figure that varies most between regions and seasons, so plug in your own rather than an assumed one.

Two things surprise people. First, it’s runtime, not wattage, that usually dominates: a low-power device left on around the clock can cost more than a high-power one used briefly, which is why an always-on fridge or router quietly adds up. Second, phantom loads are real, the few watts that TVs, chargers, and consoles draw in standby, running up the bill all year. The biggest savings nearly always come from whatever runs longest, like heating, cooling, and water heating, not from the gadgets that feel power-hungry but run for only minutes.