Ohm's Law and Kilowatt Calculator
A free Ohm calculator and kilowatt calculator built around V = I × R and P = V × I. Enter any two of voltage, current, resistance, or power and the calculator returns the rest, plus active and reactive power for AC loads. Use it as an Ohm's Law solver, a kW calculator, or a starting point for a kWh calculator estimate (kW × hours = kWh).
Formulas Used
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About this Ohm's Law calculator
Enter any two of voltage, current, or resistance and the calculator solves for the rest using Ohm's Law (V = I × R). It also returns power: apparent power S in volt-amps, active power P in watts, and reactive power Q in VARs when you supply a power factor below 1.0. For DC circuits, leave the power factor at 1.0.
From kW to kWh: a quick electricity bill calculator
Watts (W) and kilowatts (kW) measure how fast a load draws energy. Kilowatt-hours (kWh) measure the energy used over time. The full kWh calculator formula is just: kWh = kW × hours of use. A 1500 W space heater running for 8 hours uses 1.5 kW × 8 h = 12 kWh. At a US average rate near $0.16/kWh, that's about $1.92 per day.
For battery and DC sizing, the equivalent unit is the amp-hour (Ah). The amp hour calculator formula is Ah = A × hours. A 10 A load running for 5 hours pulls 50 Ah. To convert Ah to Wh, multiply by the system voltage: 50 Ah × 12 V = 600 Wh = 0.6 kWh.
Amperage chart: common load draws at 120 V
| Load | Watts | Amps @ 120 V | kWh per 8 h |
|---|---|---|---|
| LED bulb | 10 W | 0.08 A | 0.08 |
| Laptop charger | 65 W | 0.54 A | 0.52 |
| Refrigerator (running) | 150 W | 1.25 A | 1.20 |
| Microwave | 1000 W | 8.3 A | 8.0 |
| Space heater | 1500 W | 12.5 A | 12.0 |
| Window AC | 900 W | 7.5 A | 7.2 |
| Hair dryer | 1800 W | 15 A | 14.4 |
Need a resistors-in-parallel calculator?
Ohm's Law works on a single equivalent resistance. If you have two or more resistors in series or parallel, calculate the equivalent first, then plug it in here. The dedicated circuit solver handles series, parallel, and mixed networks (including AC impedances) and shows the parallel resistor formula step-by-step.
Working in the trade?
Ohm's Law shows up on every electrical exam — and on every troubleshooting call. ClarkTE training programs cover the math and the field skills together, taught by working PEs and master electricians.
See ClarkTE trainingOhm's Law and power FAQ
How do I convert amps to kilowatts?
For DC or unity power factor: kW = V × A / 1000. For single-phase AC: kW = V × A × pf / 1000. For balanced three-phase: kW = √3 × V_LL × A × pf / 1000. Power factor is typically 0.8-0.9 for industrial loads.
What is the Ohm's Law wheel?
A Pie chart showing every combination of V = IR, P = VI, P = I²R, and P = V²/R, so given any two of voltage, current, resistance, or power, you can solve for the other two. The calculator above implements the same wheel — enter any two values and it returns the rest.
How do I convert kW to kWh?
Kilowatt-hours measure energy used over time: kWh = kW × hours of use. A 1.5 kW load running for 8 hours uses 12 kWh. At a US average rate near $0.16/kWh, that's roughly $1.92 a day.