Amps to kW Calculator
Amps × volts gives watts; divide by 1,000 for kilowatts. So 40 amps at 240 volts is about 9.6 kW.
Voltage
3 kW
12.5 amps at 240 volts is about 3 kW.
Runs in your browser — nothing is sent anywhere. Estimates assume nominal voltage and ideal conditions; confirm with a licensed electrician and local code before sizing wire or breakers.
What each breaker can safely carry
The raw rating vs. the safe continuous load (the 80% rule — what actually keeps the breaker from tripping on a load running 3+ hours).
| Breaker | Watts @120V | Watts @240V | Safe continuous @120V |
|---|---|---|---|
| 15 A | 1,800 W | 3,600 W | 1,440 W |
| 20 A | 2,400 W | 4,800 W | 1,920 W |
| 30 A | 3,600 W | 7,200 W | 2,880 W |
| 40 A | 4,800 W | 9,600 W | 3,840 W |
| 50 A | 6,000 W | 12,000 W | 4,800 W |
The formula
Multiply amps by volts, then divide by 1,000. kW = (Amps × Volts) ÷ 1000.
Example: A 100-amp service at 240 volts: 100 × 240 ÷ 1000 = 24 kW of capacity (before the 80% continuous rule).
One caveat that keeps people safe: This is a guide for estimation only. Conversions assume nominal voltage and ideal conditions; real installations vary with power factor, voltage drop, temperature, wire type, and continuous vs. brief use. It doesn't replace the National Electrical Code or a licensed electrician — always confirm with a pro and local code before sizing wire, breakers, or circuits.
Hiring out the wiring?
Before you pay anyone to touch your panel or wiring, make sure they're actually licensed. You can check a contractor's license on StateCreds — our sister site.
Verify a contractor's license by state →Common questions
- How many kW is 100 amps?
- At 240V, 100 amps is about 24 kW; at 120V it's about 12 kW. Whole-home service is typically 240V.
- How many kW is 50 amps?
- About 12 kW at 240V, or 6 kW at 120V.
- Should I use the 80% rule?
- For continuous loads, yes — usable continuous capacity is about 80% of the rating. A 100-amp service shouldn't be loaded to a full 24 kW continuously.