Calculate Electrical Power and Running Cost
This electrical power calculator does two jobs. First, it solves the power equation P = V × I: enter any two of power (watts), voltage (volts) and current (amps), and it finds the third. Second, enter how many hours per day the device runs and your electricity rate, and it estimates the energy used per day in kilowatt-hours (kWh) and the monthly cost.
Formulas Used
Example: a space heater on a 120 V circuit drawing 12.5 A uses 1,500 W. Running it 4 hours a day uses 6 kWh per day. At $0.15 per kWh, that is about $27 per month.
- Power: P = V × I. Voltage: V = P ÷ I. Current: I = P ÷ V.
- Daily energy (kWh) = watts × hours per day ÷ 1,000.
- Monthly cost = daily kWh × 30 × rate per kWh.
Finding Your Numbers
Most appliances list their wattage or amperage on a label or nameplate. Standard U.S. household outlets supply about 120 V, while large appliances like dryers and ovens use 240 V. Your electricity rate in $/kWh is printed on your utility bill.
Frequently Asked Questions
What is the difference between watts and kilowatt-hours?
Watts measure how fast energy is used at a given moment. Kilowatt-hours measure the total energy used over time. A 1,000 W device running for one hour uses 1 kWh, which is what your utility bills you for.
How many amps can a household circuit handle?
Common U.S. circuits are 15 A or 20 A. For continuous loads, the usual guideline is to use no more than 80% of the rating, about 1,440 W on a 15 A, 120 V circuit.
How can I lower my electricity bill?
Use this calculator to find your highest-cost devices. Then reduce their run time, switch to energy-efficient models (look for ENERGY STAR) and unplug electronics that draw standby power.