Volts to Watts Calculator

Volts to watts needs one more number than most people expect: the current. Watts equals volts multiplied by amps, so a 12V battery supplying 10 amps delivers 120 watts, while a 120V outlet at that same 10 amps delivers 1,200 watts. Voltage alone has no wattage attached to it.

The volts to watts formula, explained

Three quantities describe any circuit.

• Volts are pressure, and they belong to the supply: a US socket near 120V, a vehicle battery near 12V, a lithium cell near 3.7V.
• Amps are flow — the current the load pulls at that pressure. This one belongs to the appliance, and drops to zero when you switch it off.
• Watts are the two multiplied: the unit every battery, inverter and solar specification is written in.

So a voltage on its own carries no wattage. A live 120V outlet with nothing plugged in delivers zero watts; put a kettle on it and it delivers 1,500.

When you know resistance instead of current

For a purely resistive load — a heating element, an incandescent bulb — resistance substitutes for current: watts equals volts squared divided by ohms. A 120V element measuring 12 ohms dissipates 1,200W; a 12V element measuring 6 ohms dissipates 24W. The shortcut collapses on motors and switch-mode supplies, whose resistance shifts with load. Run it backwards for the watts to volts calculator and the watts to amps calculator.

Quick-reference conversion table

Supply voltages down the left, current across the top, watts in the body. Every figure assumes DC, or AC at a power factor of 1.0; for motors, use the power factor table below. Battery rows are nominal: a 12V lead-acid bank swings from about 11V flat to 14.6V on charge, so treat those as good to roughly 10%.

Every cell is the same multiplication, so it scales cleanly: 120V at 7A is simply 840W. The bottom row catches people out. 240V at 30A is 7,200W, which is why a dryer is not a load a portable unit takes on.

Why this conversion decides what you buy

Products are sold in watts and watt-hours, but nearly every component that limits what you can do is rated in volts and amps. Converting connects the two.

Battery banks

Amp-hours mean nothing until a voltage is attached. A 100Ah bank at 12V nominal stores roughly 1,200Wh; the same 100Ah in a LiFePO4 pack at 12.8V stores about 1,280Wh; at 48V it stores four times as much again. Identical headline number, three different purchases. The amp-hours to watt-hours calculator does that step.

Solar panels and charge controllers

A panel’s wattage is its maximum-power voltage multiplied by its current there: a nominal 12V panel works near 18V at 5.5A in full sun, which is where the 100W label comes from. The controller is rated in amps, which is a hidden wattage ceiling. A 30A controller charging a 12V battery at 14.4V tops out near 430W of array; on a 24V bank at 28.8V it handles about 860W. Buy 600W of panel for that controller on 12V and a third is wasted daily. Our solar sizing guide covers the array.

Inverters and the DC current problem

The conversion also tells you what happens on the battery side of an inverter. Run a 1,000W AC load through one that is 90% efficient and the battery must supply about 1,111W. Divide by the system voltage:

• At 12V, roughly 93 amps — heavy cable, hot terminals, measurable voltage drop.
• At 24V, roughly 46 amps.
• At 48V, roughly 23 amps.

Same watts, same job, a quarter of the current. That is why serious off-grid systems moved to 48V, and why 12V is a poor foundation for a 3,000W inverter. A finished unit settles that internally, leaving continuous output as the figure you choose. The power station finder filters on output and capacity together, and the catalogue is on the power stations page.

AC, DC and power factor

On DC — batteries, solar panels, 12V appliances — volts multiplied by amps gives watts exactly. The same holds on AC with a resistive load: kettles, toasters, heating elements, incandescent bulbs.

Motors, compressors, pumps and cheap switching supplies do not. Their voltage and current drift out of step, so the circuit carries more current than the delivered power accounts for. Volts multiplied by amps then gives volt-amps, apparent power; real watts are that figure multiplied by the power factor, a number between 0 and 1. Below, one circuit under six loads.

Two habits fall out of that table. Sizing a battery, assume a power factor of 1.0: it overstates consumption rather than understating it. Sizing an inverter, use the real figure, because an inverter is limited by the current it passes. The VA to watts calculator does that directly.

Safe continuous load and the 80% rule

Multiplying a circuit’s voltage by its breaker rating needs one correction. Under US electrical code a continuous load — anything drawing for three hours or more — may occupy only 80% of the breaker rating. Convert, then take four fifths.

The 20A row is highlighted because most households quietly rely on it: a 1,920W continuous budget, which a 1,500W space heater plus a 700W microwave has already overrun. The 30A RV row explains the other familiar complaint, since 2,880W is why air conditioning plus a microwave trips a pedestal.

The DC rows work differently. No code rule derates a battery management system, but the same discipline pays, because a BMS current limit is a wattage ceiling in disguise: a 12V pack with a 100A BMS cannot deliver much beyond 1,200W whatever inverter is bolted to it. Treat a power station the same way. A unit rated 2,000W holds 2,000W, but running near 1,600W leaves surge headroom for a compressor start.

Frequently asked questions