Volts to Watts: How to Convert Volts to Watts

How do you convert volts to watts?

Multiply the volts by the amps. Watts = volts times amps. That is the whole conversion, and it is why you can never do it from voltage alone: without the current, there is no answer. A device on a 12 volt battery pulling 5 amps draws 60 watts (12 times 5). A 120 volt appliance pulling 10 amps draws 1,200 watts. A 240 volt water heater element pulling 20 amps draws 4,800 watts.

If you know the watts and one of the other two numbers, the same equation runs backward. To find the current a device pulls, amps = watts / volts, so a 1,200 watt hair dryer on a 120 volt outlet draws 10 amps. To find the voltage, volts = watts / amps. This three-way relationship (sometimes called the power formula or the PIE formula, for Power = current I times voltage E) is the same one behind sizing a fuse, a wire, or an inverter. If you are trying to move between power units instead, converting watts to kilowatts is a separate, simpler step of dividing by 1,000.

Why can't you convert volts to watts directly?

Because volts and watts measure two different things, and the missing piece is current. Voltage is the electrical pressure pushing the current; wattage is the power actually delivered. The classic analogy is water in a pipe: volts are the water pressure, amps are the flow rate (how much water moves), and watts are the total work the water does. A high pressure with no flow does no work, so knowing the pressure alone (the volts) cannot tell you the power (the watts).

This is why a spec that lists only a voltage, like a 120V outlet or a 48V battery, is not telling you the power. The outlet can deliver anywhere from a fraction of a watt (a phone charger) up to about 1,800 watts on a standard 15 amp circuit, all at the same 120 volts. What sets the watts is how many amps the plugged-in device pulls. So any honest volts-to-watts answer has to ask: at how many amps? For a quick way to check whether a battery bank can even run a given load, the what can a 100Ah battery run breakdown works the watt-hour side of the same idea.

How many watts is 120 volts?

A 120 volt circuit delivers watts equal to 120 times whatever current is flowing, so there is no single number. At the two standard US household circuit sizes, the ceilings are 1,800 watts on a 15 amp circuit (120 times 15) and 2,400 watts on a 20 amp circuit (120 times 20). Those are the maximums the breaker allows; most plugged-in devices pull far less. A 60 watt lamp on a 120 volt outlet is drawing only half an amp.

Here is the 120V table for the amp draws people look up: 120V at 1A = 120W; at 5A = 600W; at 10A = 1,200W; at 12A = 1,440W; at 15A = 1,800W; at 20A = 2,400W. In real wiring you never load a circuit to its full rating continuously. The 80 percent rule means a 15 amp circuit is meant for about 1,440 watts of continuous load and a 20 amp circuit for about 1,920 watts, which is why a 1,500 watt space heater is the practical limit on many outlets. Actual circuit and breaker work is a job for a licensed electrician; the math here is only for sizing and sanity checks.

How many watts is 240 volts?

A 240 volt circuit delivers 240 times the amps, so the watt figures get large fast: 240V at 20A = 4,800 watts; at 30A = 7,200 watts; at 40A = 9,600 watts; at 50A = 12,000 watts. This is why big loads like an electric range, a clothes dryer, a water heater, a central air conditioner, and an EV charger run on 240V. Doubling the voltage doubles the watts a given current can carry, so 240V moves the same power at half the amps, which lets it use thinner, cheaper wire than 120V would for the same job.

The breaker size caps the watts. A 240V, 30 amp breaker allows up to 7,200 watts, and under the 80 percent continuous rule about 5,760 watts for something that runs for hours, like an EV charger. A 240V, 50 amp circuit allows 12,000 watts, or about 9,600 watts continuous. These numbers are also why whole-home backup and off-grid systems are built around 240V: it is the only way to move 10 kW or more without enormous current. Choosing a system voltage for a battery bank is a related tradeoff covered in 12V vs 24V vs 48V solar.

Does 1 volt equal 1 watt? Is 5 volts 5 watts?

No. 1 volt does not equal 1 watt, and 5 volts is not 5 watts, unless exactly 1 amp happens to be flowing. Because watts = volts times amps, a voltage only equals the same number of watts in the single case where the current is 1 amp. At 1 volt and 1 amp you get 1 watt; at 5 volts and 1 amp you get 5 watts. Change the amps and the watts change with them: 5 volts at 2 amps is 10 watts, and 5 volts at 0.5 amps is 2.5 watts.

This trips people up with USB and phone chargers, which are labeled in volts. A 5V USB port is not 5 watts; a basic one at 1 amp is 5 watts, a 2.4 amp port is 12 watts, and a fast charger running 5V at 3 amps is 15 watts. The voltage stayed the same while the power tripled, because the amps changed. Same rule, every time: without the current, a voltage tells you nothing about the watts.

Does the AC power factor change volts to watts?

Yes, for alternating current with motors or electronics, the real formula is watts = volts times amps times power factor. Power factor is a number from 0 to 1 that accounts for current that sloshes back and forth without doing useful work. For a purely resistive load like a heater, an incandescent bulb, or a stovetop element, the power factor is effectively 1.0, so watts = volts times amps holds exactly. That is why the heater and water-heater examples above need no correction.

For motors, compressors, and some electronics, the power factor is lower, often 0.7 to 0.9, so the true watts are less than volts times amps would suggest. The volts-times-amps figure by itself is called apparent power, measured in VA (volt-amps), and it is what inverter and generator makers often rate their equipment in. That is why a 1,800 VA inverter may only deliver about 1,500 real watts to a motor load. For DC, like a solar panel or a battery, there is no power factor, so watts = volts times amps is exact. To turn a panel's voltage and current into daily energy for your location, run it through the solar panel calculator rather than doing it by hand.

How does volts to watts apply to solar and batteries?

In DC solar and battery systems the conversion is exact and you use it constantly: watts = volts times amps, with no power factor to worry about. A solar panel's power is its maximum power voltage (Vmp) times its maximum power current (Imp), so a panel at 18 volts and 5.5 amps is about 100 watts. A 12 volt battery delivering 10 amps is putting out 120 watts, and the same battery pushing 50 amps is 600 watts. This is also how you check whether a wire or a charge controller can handle a load: amps = watts / volts, so a 1,200 watt inverter on a 12V battery pulls 100 amps on the battery side, which is a lot of current and needs thick cable.

Higher system voltages move the same watts at fewer amps, which is the whole reason larger setups use 24V or 48V instead of 12V: less current means smaller wire and less loss. A 2,400 watt load is 200 amps at 12V but only 50 amps at 48V. Once you are dealing with energy over time rather than instant power, you leave this formula behind and move to watt-hours and kilowatt-hours; if that distinction is fuzzy, kW vs kWh and watts to kWh lay it out. Any hardwiring of an inverter or battery into a home panel is a licensed electrician's job, not a DIY conversion.

Frequently asked questions