DC Power Calculator
DC power, equivalent load resistance, energy use and running cost.
What it does
Power in a DC circuit is P = V × I. The calculator also gives the equivalent load resistance, the energy used over time, the power drawn from the wall through your supply’s efficiency, and the running cost.
Where it's used
Sizing power supplies, battery budgets and estimating the running cost of DC equipment.
How to use it
- Enter the voltage and the average current.
- Enter hours per day and number of days.
- Enter your tariff (price per kWh in your currency).
- Enter the power-supply efficiency.
The formula explained
- P
- V × I, in watts
- E
- P × t / 1000, in kWh
- η
- supply efficiency — input power = P / η
- Cost
- energy × tariff
Worked example
A 12 V, 3 A device running all month.
Inputs:
- Voltage: 12 V
- Current: 3 A
- Hours of use per day: 24 h
- Days: 30 days
- Electricity tariff: 8 /kWh
- Supply efficiency: 85 %
The calculator gives:
- Load power: 36 W
- Equivalent resistance: 4 Ω
- Input power (incl. losses): 42.35 W
Tips and common mistakes
- Use the average current, not the peak, for energy and cost.
- The bill is for energy from the wall, so supply efficiency matters.
- Leave 20–30 % headroom when choosing a supply’s rating.
Questions people ask
What is the difference between watts and watt-hours?
Watts measure how fast energy is used; watt-hours measure how much was used in total. A 36 W device running for 10 hours uses 360 Wh, or 0.36 kWh — the unit on an electricity bill.
How long will a battery run my device?
Divide the battery’s energy in watt-hours by the load in watts. A 12 V 7 Ah battery holds 84 Wh, so a 36 W load runs for about 2.3 hours in theory. In practice expect less: lead-acid batteries last longer if you use only about half their capacity.
How is the running cost worked out?
Energy from the wall (load power ÷ supply efficiency × hours) times your tariff. The example device draws 42.35 W from the wall around the clock, which is about 30.5 kWh in 30 days — about 244 at a tariff of 8 per kWh.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.