Watts & Amps
Give it any two of volts, amps and watts. It solves the third, sizes the circuit, and prices the running cost.
Watts
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watts · 15 A circuit- Volts
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- Amps
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- Watts
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- Continuous limit
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- Cost a month
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Single-phase or DC. Three-phase adds a √3.
The power triangle
Watts = volts × amps. That one line answers most household electrical questions, and running it backwards answers the rest. Volts is the pressure the utility supplies, amps is the flow the appliance draws, watts is the work getting done. Know two, and the third is arithmetic — which is what the form above does while you type.
The useful direction is usually watts to amps: a nameplate gives you watts, but breakers and wire are sold in amps. A 1,500 watt heater on 120 volts is 12.5 amps, comfortably inside a 15 amp circuit until something else joins it. The same heater wired for 240 volts draws 6.25 amps and can run on much smaller wire.
The 80 percent rule
A breaker's rating is not a budget you may spend fully for hours. Anything running three hours or more is a continuous load, and NEC 210.19(A) sizes the circuit at 125 % of it — which read backwards means a continuous load may use no more than 80 % of the breaker. On a 15 amp circuit that is 1,440 watts; on a 20 amp circuit, 1,920. The tag above prints both the instantaneous draw and that continuous ceiling.
Power factor, briefly
With a heater or a filament bulb, current and voltage rise and fall together and volts × amps is exactly watts. Motors and switching supplies pull them slightly out of step: the wire still carries the full current (the VA), but the useful power (the watts) is less. Power factor is the ratio between the two. Leave it at 1 for resistive loads; use around 0.8 when sizing for a motor.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- The formula
- Watts = volts × amps × power factor — DC and single-phase AC
- Power factor
- 1 by default: correct for resistive loads, low for motors
- Continuous loads
- The 80 % line is NEC 210.19(A) read backwards from the breaker
- Running cost
- Your own kWh rate — the US residential average is about 17 cents
Questions from the aisle
How many watts is 15 amps at 120 volts?
1,800 watts. A 15 amp circuit is only rated for continuous use to 80% of that, though — 1,440 watts — which is why two space heaters on one circuit trip the breaker.
How do I convert watts to amps?
Divide the watts by the volts. A 1,500 watt heater on a 120 volt circuit draws 12.5 amps. On 240 volts the same heater draws 6.25 amps, because the same power moves at twice the pressure.
What is power factor and do I need it?
For anything resistive — heaters, incandescent bulbs, toasters — leave it at 1. Motors and some electronics draw current slightly out of step with the voltage, so their real power (watts) is less than volts × amps (VA). A power factor of 0.8 is a typical motor figure.
What can I actually put on a 20 amp circuit?
2,400 watts on paper, 1,920 watts for anything running three hours or more. That is the 80% continuous-load rule, and it is why a 1,500 watt heater plus a 900 watt coffee maker is one appliance too many.