This watts to amps calculator takes watts or kilowatts, works both ways, and also gives the kVA figure, because generators, UPS units and transformers are rated in kVA rather than kW and confusing the two is how equipment gets undersized.
Why is a generator rated in kVA and not kW?
Because kVA is what limits the machine. A generator or transformer is constrained by how much current its windings can carry and how much voltage its insulation can stand, and the product of those two is apparent power in volt-amps. How much real work that current does depends on the load's power factor, which the generator has no control over.
The relationship is simple: kVA = kW ÷ power factor. So a generator rated 100 kVA delivers 80 kW into a load at 0.8, and only 65 kW into a load at 0.65.
This is how equipment gets undersized
Someone adds up their loads in kilowatts, buys a generator with a matching kVA number, and finds it cannot carry them. A 1500 VA UPS supplies about 975 watts to typical mixed IT equipment at a power factor of 0.65, not 1,500. If you are sizing anything, work out the kVA your loads need, not the kW.
What are the formulas?
| Supply | kW to amps | Amps to kW |
|---|
| DC | I = kW × 1000 ÷ V | kW = V × I ÷ 1000 |
| Single-phase AC | I = kW × 1000 ÷ (V × PF) | kW = V × I × PF ÷ 1000 |
| Three-phase, line to line | I = kW × 1000 ÷ (√3 × V × PF) | kW = √3 × V × I × PF ÷ 1000 |
The 1000 is just the kilo prefix. Everything else is the same relationship as watts to amps, so if a figure looks wrong by a factor of a thousand, that is where to look first.
Which voltage should I use for three-phase?
The line-to-line voltage, which is the number a three-phase supply is normally quoted at: 400 volts in most of Europe and Australia, 208 or 480 in North America. This calculator assumes line to line.
If you have the line-to-neutral figure instead, 230 volts against 400 for example, the two differ by exactly √3. Using the wrong one puts the answer out by 73%, which is a large enough error to notice but not always large enough to look obviously wrong, which is what makes it dangerous.
Common questions
- How many amps is 10 kW?
- At 400 volts three-phase and a power factor of 0.9, about 16 amps. At 230 volts single-phase and the same power factor, about 48 amps. At 120 volts single-phase, about 93 amps. The voltage matters more than anything else here, which is why heavy loads are put on the highest voltage available.
- What power factor should I assume if I do not know?
- For a mixed commercial load, 0.85 to 0.9 is a reasonable planning figure. For a motor-heavy industrial load, 0.8. For modern IT equipment with corrected supplies, 0.95 or better. If you are sizing rather than estimating, get the actual figure: a wrong power factor moves the current by tens of percent.
- What is the difference between kW and kWh?
- kW is a rate and kWh is a quantity. A 10 kW load running for three hours uses 30 kWh. Your bill is in kWh because you pay for energy consumed; a breaker is rated in amps because it limits the rate. Confusing the two is common and always produces an answer out by a factor of however many hours.
- Does this work for solar inverters?
- For the AC output, yes: an inverter's rated kW divided by the grid voltage and power factor gives its maximum output current, which is what the circuit feeding it has to carry. The DC side is different, and specifying DC circuits for a solar array has its own rules that this calculator does not cover.