Appliance wattage is volts × amps. Enter the voltage (120 V for standard US outlets, 240 V for large appliances) and the amp draw from the nameplate, pick a surge factor for motor startup, and the tool returns running watts and starting (surge) watts. The preset — 120 V, 6 A, ×2 — gives 720 running watts and 1,440 starting watts.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
Running watts
720 W
Starting (surge) watts
1,440 W
Formula
Running watts = volts × amps (W = V × A). Starting (surge) watts = running watts × surge factor. Use a factor of 1 for purely resistive loads (heaters, incandescent bulbs, toasters), about 2 for ordinary induction motors, and 3 for hard-starting motors such as compressors, air conditioners and deep-well pumps. The nameplate usually lists amps; read it at the operating voltage.
Worked example
A small fan motor on a 120-volt circuit draws 6 amps. Running watts = 120 × 6 = 720 W. Because it's a motor, apply a surge factor of 2: starting watts = 720 × 2 = 1,440 W. So a generator or inverter feeding this fan must supply 720 watts continuously and tolerate a 1,440-watt spike for the fraction of a second it takes the motor to spin up.
This tool is a lookup for the two figures that drive backup-power sizing: the running watts an appliance draws steadily and the starting watts it spikes to at switch-on. Resistive loads such as heaters, kettles and incandescent bulbs have no meaningful surge, so their two figures match; anything with a motor or compressor — a fridge, a pump, an air conditioner, a power tool — can spike to two or three times its running draw for a fraction of a second. Use these numbers to build a load list for the generator or inverter calculators, but treat them as typical ranges: the nameplate on your own appliance, or a reading from a plug-in meter, always beats a chart.
Multiply amps by volts: watts = amps × volts. A 6-amp device on a 120-volt circuit uses 720 watts. For 240-volt appliances, use 240 in the formula. This works directly for DC and resistive AC loads; for motors and electronics with a power factor below 1 the true watts are lower than volts × amps, but using the nameplate amps at line voltage gives a safe, conservative figure for sizing.
What surge factor should I use for my appliance?
Use 1 for resistive loads with no moving parts — space heaters, toasters, kettles and incandescent bulbs draw the same watts at startup as when running. Use about 2 for common motor loads like fans, drills and refrigerators. Use 3 for hard-starting motors: air-conditioner compressors, well pumps and large power tools. When in doubt, read the locked-rotor amps (LRA) on the nameplate, or choose the higher factor to stay safe.
Where do I find the amps and volts for my appliance?
Look at the nameplate or rating label, usually on the back or base of the appliance, or in the manual. It lists voltage (120 V or 240 V) and either amps or watts. If only watts is given, you already have running watts. If only amps is shown, multiply by the voltage. Motor appliances may also list LRA (locked-rotor amps) for the startup surge.
Why are starting watts higher than running watts?
Electric motors draw a large inrush current at the instant they start, because they must overcome inertia and the rotor is not yet generating a back-EMF to limit current. This spike typically lasts a fraction of a second but can be two to three times the running draw. Sizing a generator, inverter or circuit only to the running watts risks tripping, stalling or brownout during startup.
Does this work for 240-volt appliances?
Yes. Enter 240 as the voltage and the appliance's amp draw at 240 V. For example, a 240-volt water heater pulling 18.75 amps uses 240 × 18.75 = 4,500 watts. Large US appliances — dryers, ranges, central AC, water heaters — run on 240-volt circuits, so confirm the voltage on the nameplate before calculating, since using 120 V would halve the result.
Source: W = V × A; motor starting surge typically 2–3× running watts (locked-rotor inrush). · All sources