Home › Calculators › Amps to Watts Calculator
Amps to Watts Calculator
Enter a current and the voltage it was measured at, and you get the real power figure every battery, inverter and solar spec is written in.
Enter the current and the supply voltage to get the power draw.
Watts = Amps × Volts × Power factor
To convert amps to watts, multiply the current in amps by the voltage the device actually runs on. Ten amps on a 120V US household circuit is 1,200 watts. The same ten amps drawn from a 12V battery is only 120 watts. Voltage is what makes those answers differ tenfold.
The amps to watts formula, explained
- Amps are current: how much electricity is flowing. It is what a clamp meter reads and what a fuse, a breaker or a shore power pedestal is rated for.
- Volts are the pressure behind that flow, and they belong to the supply, not the appliance. A US wall socket sits near 120V, a vehicle battery near 12V, a single lithium cell near 3.7V.
- Watts are the power actually delivered. Current alone tells you nothing until you know the pressure behind it.
So the same current reading means very different things depending on where you took it. Twenty amps from a 12V battery is 240W, a load any power station shrugs off. Twenty amps from a household socket is 2,400W, more than most portable units can deliver at all.
The mistake that wrecks this calculation. Applying wall voltage to a current measured on the battery side, or the reverse. An inverter turns 12V DC into 120V AC, so one 1,200W load reads roughly 105A at the battery and about 10A at the socket. Both are correct. Multiply 105A by 120V and you get 12,600W, which is nonsense.
Quick-reference conversion table
Watts for round current values at the four voltages that come up in portable power. These are nominal figures: a 12V battery sits between about 11V flat and 14.6V on charge, so treat that column as good to roughly 10%. The 120V and 240V columns assume a resistive load.
| Current | 12V battery | 24V battery | 120V US mains | 240V circuit |
|---|---|---|---|---|
| 1 A | 12W | 24W | 120W | 240W |
| 2 A | 24W | 48W | 240W | 480W |
| 3 A | 36W | 72W | 360W | 720W |
| 5 A | 60W | 120W | 600W | 1,200W |
| 7.5 A | 90W | 180W | 900W | 1,800W |
| 10 A | 120W | 240W | 1,200W | 2,400W |
| 12 A | 144W | 288W | 1,440W | 2,880W |
| 15 A | 180W | 360W | 1,800W | 3,600W |
| 20 A | 240W | 480W | 2,400W | 4,800W |
| 25 A | 300W | 600W | 3,000W | 6,000W |
| 30 A | 360W | 720W | 3,600W | 7,200W |
| 40 A | 480W | 960W | 4,800W | 9,600W |
| 50 A | 600W | 1,200W | 6,000W | 12,000W |
For a 48V system, double the 24V column. For individual lithium cells at 3.7V nominal, multiply the current by 3.7: a cell delivering 2A is producing 7.4W. The reverse conversion is on the watts to amps calculator.
Why this conversion matters for power stations and solar
Portable power has an awkward split running through it. The products are sold in watts and watt-hours, but almost every component that limits what you can do is rated in amps. Converting is how you find out whether the parts fit together.
Solar panels and charge controllers
A nominal 12V panel works near 18V, so one producing 5.5A is making about 99W — that is where the 100W label comes from. The real limit is the charge controller, rated in amps of output. A 30A controller charging a 12V battery at 14.4V tops out near 430W of panel; the same controller on a 24V battery handles about 860W. Buy 600W of panel for that 30A controller on 12V and you waste a third of the array every sunny day. Our solar sizing guide covers the rest.
Battery limits and inverters
A battery management system has a continuous discharge current limit, and that limit is a wattage ceiling in disguise. A 12V pack with a 100A BMS cannot deliver more than about 1,200W whatever inverter is bolted to it; on a 24V pack the same BMS supports about 2,400W. It stays invisible until you multiply. For stored capacity, use amp-hours to watt-hours.
Shore power and hookups
Campground pedestals are labelled in amps only. A 30A RV service is a single 120V leg, so 3,600W in total — less than people assume, and the reason air conditioning plus a microwave trips it. A 50A service is two 120V legs, giving 12,000W. The comparison tool filters on continuous output rather than capacity.
Why so many units stop at 1,800W. A US 15A outlet at 120V is 1,800W, so an inverter built to replace one has little reason to exceed it. Need more from a single socket and you want a bigger output rating, not a bigger battery. Browse power stations on output.
DC, AC and power factor
On direct current — batteries, solar, the 12V side of a vehicle — amps multiplied by volts is watts exactly, with nothing left over. The same holds on AC when the load is resistive: heaters, kettles, toasters and incandescent bulbs.
Motors, compressors, pumps and cheap switching supplies do not behave that way. Their current and voltage drift out of step, so amps multiplied by volts gives volt-amps, apparent power, rather than watts. Real power is that figure multiplied by the power factor, a number between 0 and 1 describing how much of the current does useful work.
| Current at 120V | PF 1.0 heater | PF 0.9 electronics | PF 0.8 fridge, pump | PF 0.6 small motor |
|---|---|---|---|---|
| 1 A | 120W | 108W | 96W | 72W |
| 2 A | 240W | 216W | 192W | 144W |
| 5 A | 600W | 540W | 480W | 360W |
| 8 A | 960W | 864W | 768W | 576W |
| 10 A | 1,200W | 1,080W | 960W | 720W |
| 12 A | 1,440W | 1,296W | 1,152W | 864W |
| 15 A | 1,800W | 1,620W | 1,440W | 1,080W |
| 20 A | 2,400W | 2,160W | 1,920W | 1,440W |
If a nameplate quotes both watts and amps, divide the watts by the amps and then by the voltage to recover the manufacturer’s own power factor rather than guessing at it. The VA to watts calculator works the same problem in reverse.
Size an inverter on volt-amps, not watts. The power factor makes the wattage look smaller, but the inverter still supplies the full current. A 15A compressor load at power factor 0.7 is 1,260W of real power and 1,800VA of apparent power, and 1,800 is what the inverter has to survive. Sizing on the smaller number is how people end up with a unit that trips under a load it was rated for.
Safe continuous load and the 80% rule
Converting amps gives you an instantaneous ceiling, not a number to design around. Under US electrical code a circuit carrying a continuous load, meaning anything drawing for three hours or more, may use only 80% of the breaker’s rating. Convert first, then take four fifths.
| Breaker | Continuous amps | Continuous watts at 120V | Continuous watts at 240V |
|---|---|---|---|
| 15 A | 12 A | 1,440W | 2,880W |
| 20 A | 16 A | 1,920W | 3,840W |
| 30 A | 24 A | 2,880W | 5,760W |
| 40 A | 32 A | 3,840W | 7,680W |
| 50 A | 40 A | 4,800W | 9,600W |
In a US home, 15A and 20A circuits are the 120V ones and 30A upward are usually 240V. The 20A row is highlighted because it is the one most people are quietly relying on: a 20A kitchen circuit is a 1,920W continuous budget, and a 1,500W space heater plus a 700W microwave has already blown through it.
Apply the same discipline to a portable unit. Manufacturers publish a continuous rating and a higher surge rating, and the continuous figure is the one to design against. Running an inverter at 95% of rated output for hours is how you discover its thermal derating. Our sizing guide turns an appliance list into a continuous watt figure.
A breaker protects the wire, not your appliance. The rating describes what the cable behind the wall can carry safely, not what is sensible to plug in. Loading a circuit to its full converted wattage is fine in bursts and a poor habit.
Frequently asked questions
How do I convert amps to watts?
Multiply the current in amps by the voltage of the supply. On DC, and on AC with a resistive load such as a heater, that is the whole answer. For motors and compressors, multiply again by the power factor, typically 0.6 to 0.85, to get real watts.
How many watts is 1 amp?
It depends entirely on the voltage. One amp is 120W on a US 120V outlet, 240W on a 240V circuit, 24W on a 24V bank, 12W on a 12V battery and 3.7W on a lithium cell. A fixed watts-per-amp figure always hides an assumed voltage.
How many watts is 30 amps?
3,600W on a 120V supply, which is what a 30A RV shore power hookup gives you. The same 30A is 7,200W on a 240V circuit and only 360W from a 12V battery. For a continuous load the 80% rule cuts the 120V figure to 2,880W.
How many watts can a 20 amp breaker handle?
2,400W at 120V as an instantaneous maximum, but 1,920W is the number to plan around. Code limits a continuous load, anything running three hours or more, to 80% of the breaker rating. On 240V the figures are 4,800W and 3,840W.
Should I use 110V, 115V or 120V in the calculation?
Use 120V. It is the nominal standard for US residential supply and the older figures are historical. The gap matters less than it looks: at 15A, 110V gives 1,650W against 1,800W at 120V, and your supply voltage wanders across a comparable range anyway.
Why does my inverter show different amps at the battery and at the outlet?
Because power is conserved but current is not. The same wattage at a tenth of the voltage needs ten times the current, so a 1,200W load reads about 10A at a 120V socket and roughly 105A at a 12V battery. Both are correct for their own side of the inverter.
Next step. Once you have a watt figure, the question becomes how long you can sustain it. The power station finder filters on continuous output and capacity together, so you can check both constraints at once.