The kW calculator that gets the math right.

A kilowatt (kW) is a unit of power equal to 1,000 watts. Power measures the rate at which energy is produced, transferred, or consumed. One kilowatt sustained for one hour equals one kilowatt-hour (kWh), the unit your utility bills you in.

kW is the most useful unit when you size electrical equipment because it lets you bridge between voltage, current, mechanical output (horsepower), thermal output (BTU/hr) and apparent power (kVA) using a handful of well-known formulas. Underneath all of them sits Ohm's Law (V = I × R) and its power form, Watt's Law (P = V × I): voltage, current, resistance, and power are never independent of each other.

Core kW formulas

The calculator above updates as you type, so there's no submit button to press. Follow four steps to get a result.

1. Pick a direction. Use the toggle to switch between kW → Amps and Amps → kW.
2. Select your current type. Choose DC, AC single-phase, or AC three-phase (line-to-line or line-to-neutral).
3. Enter the voltage. US homes run 120V or 240V; US commercial and industrial sites typically run 480V three-phase.
4. Enter the power factor for AC circuits. Use 1.0 if the load is purely resistive (heaters, incandescent bulbs), or 0.8 as a working estimate if you don't know the exact value. DC has no power factor field because there's no phase shift to account for.

If you don't know your power factor, check the equipment nameplate first. Motors, transformers, and fluorescent ballasts list it directly. When no nameplate value is available, 0.8 is a safe planning estimate for a mixed commercial load, and resistive loads like water heaters and electric ranges are close enough to 1.0 that the difference rarely matters.

Every AC circuit carries two kinds of power. Active power (also called real power), measured in watts (W) or kilowatts (kW), is the power actually consumed and converted into light, heat, or motion. Apparent power, measured in volt-amps (VA) or kilovolt-amps (kVA), is the total power the source must supply to deliver that active power.

The gap between the two is reactive power, measured in kVAR, the power that inductive and capacitive loads store and release each cycle without doing useful work. The three are related by kVA² = kW² + kVAR², and the ratio between active and apparent power is the power factor: kW = kVA × PF.

This distinction matters in three practical ways. Wiring and breakers are sized for current, which tracks apparent power, so a low-PF load needs heavier conductors than its kW rating alone suggests. Generators and UPS units are rated in kVA, and their real kW output drops as PF drops. And many commercial utility contracts charge a penalty when a facility's average power factor falls below about 0.9.

Power factor (PF) is the ratio of real power (kW) to apparent power (kVA), always between 0 and 1. A PF of 1.0 means the load is purely resistive and every watt supplied does useful work. A PF of 0 would mean all supplied power is reactive, stored and released without producing output, a theoretical extreme you won't see in practice.

Low power factor is a cost problem, not just a technical curiosity. A motor running at PF 0.7 draws noticeably more current than one at PF 0.95 for the same real-power output, which means thicker conductors, larger breakers, and, on many commercial rate plans, a monthly penalty charge.

How to calculate power factor

Real power in watts: W = |VA| × PF = |VA| × |cos φ|

Single-phase PF: PF = 1000 × kW / (V × A)

Three-phase PF (line-to-line): PF = 1000 × kW / (√3 × V × A)

Three-phase PF (line-to-neutral): PF = 1000 × kW / (3 × V × A)

Full formulas and correction methods are on the dedicated power factor calculator.

If your power reading is in watts rather than kilowatts, divide by 1,000 first. If your voltage is in kilovolts, multiply by 1,000 first so both sides of the formula use consistent units.

These tables assume a purely resistive load (power factor 1.0) so you can read off an estimate at a glance. For motors and other inductive loads, use the calculator above with the correct power factor.

Need the full breakdown with reactive power and worked examples? Visit kVA to kW or kW to kVA.

Mechanical, electrical, and metric horsepower each use a slightly different constant. See HP to kW or kW to HP for the full comparison and motor nameplate guidance.

Power (kW) and energy (kWh) are different quantities: power is a rate, energy is power multiplied by time. Enter an appliance's power draw, how many hours a day it runs, how many days, and your electricity rate to see the energy and cost below.

For example, a 2 kW appliance running 8 hours a day for 30 days uses 480 kWh. At the 2025 US residential average of roughly $0.16/kWh, that's about $76.80 for the month. Your actual rate can range from about $0.11/kWh in low-cost states like Idaho to $0.40/kWh or more in Hawaii, so swap in your own utility rate for an accurate figure. See the electricity bill calculator for tiered and time-of-use rate plans.

Enter your electric power current needs in amperes

• A two-bedroom home typically draws around 30 A.
• A three-bedroom home typically draws around 45 A.
• A small business typically draws 50-100 A depending on equipment.

Enter the voltage coming into your home or business

• Most US homes receive 240V split-phase service.
• Most US commercial buildings receive 480V three-phase service.

Choose single-phase or three-phase

• Houses are almost always single-phase.
• Businesses with large motors or HVAC equipment are almost always three-phase.

These are service-capacity figures, the size of the wire and breaker feeding the circuit, not what actually gets consumed minute to minute. Use the home electrical load calculator to add up your specific appliances, or the generator size calculator if you're sizing backup power.

Sizing a generator from a kW load

Generators are rated in kW (real power, what does work) and kVA (apparent power, what sets conductor and alternator size). For continuous loads, size the generator so the operating load sits between 50% and 80% of prime rating, diesels suffer wet-stacking below 30% load, and you want headroom above. To account for motor inrush, the generator must momentarily absorb 6-8 times the rated current of the largest motor at start. That's why a 30 kW continuous load in a building with a 10 HP motor may need a 50 kW generator, and it's why a circuit breaker sized only for steady-state current will nuisance-trip on motor startup.

Load demand, the actual peak draw a panel or generator has to cover at once, is almost always lower than the sum of every connected device's nameplate rating, because not everything runs simultaneously. Electricians apply demand factors from NEC Article 220 to estimate realistic peak load instead of over-sizing equipment for a peak that never happens.

kW vs HP: shaft power, electrical input, and metric quirks

Nameplate horsepower is the mechanical output at the shaft. Electrical input is higher because of losses. A NEMA Premium 10 HP motor at 92% efficiency draws about 8.11 kW input. Two HP conventions exist: mechanical (US) HP = 745.7 W, and metric (PS) HP = 735.5 W. Always check the standard when reading European nameplates.

BTU/hr, tons, and how HVAC people talk

HVAC engineers use BTU/hr for capacity and "tons" for refrigeration. One refrigeration ton equals 12,000 BTU/hr, the rate of heat needed to melt one short ton of ice in 24 hours. In kW: 1 ton = 3.517 kW. So a 5-ton residential AC is roughly 17.6 kW of cooling output, but its electrical input is much lower because the system moves heat rather than generating it; a 5-ton heat pump at COP 3.5 draws only about 5 kW of electricity.

Breakers, wire gauge, and circuit protection

Once you know the amps a load draws, a circuit breaker protects the wire feeding it by tripping before the conductor overheats. NEC 210.19 requires sizing continuous loads (running 3+ hours) at 125% of calculated current before picking a breaker: a 16 A continuous load needs a 20 A breaker at minimum. Skipping that margin is one of the most common sizing mistakes on residential and light-commercial jobs.

How much is a kilowatt-hour, really?

The US residential average in 2025 hovers near 17 cents/kWh; Hawaii pays roughly 40 cents and Idaho roughly 11 cents. In Europe, expect 25-40 cents/kWh. Multiply your daily kWh by your tariff to get the daily cost. Use the electricity bill calculator to model time-of-use plans and the EV charging cost calculator for vehicle-specific math.

Quick reference: typical loads in kW

When the calculator says "N/A"

A few inputs produce no meaningful answer. Power factor must be between 0.01 and 1.00. Voltage and current must be positive. If hours = 0 in a kWh/kW conversion, the result is undefined, so the calculator shows N/A rather than guess. Negative power factors only appear in regenerative systems (a motor acting as a generator) and are out of scope for normal sizing.