A watts to amps calculator comes down to one line, amps = watts ÷ volts, the same relationship the U.S. Department of Energy uses for appliance energy estimates. This guide gives the formula, a ready-made chart for common voltages, and the two corrections people forget: inverter losses and power factor. Use it to size wire, fuses and breakers, and to tell whether a battery can run a load.
The Formula
Amps = Watts ÷ Volts. For a 1,500W heater on 120V: 1,500 ÷ 120 = 12.5A.
Watts to Amps Calculator Chart
| Watts | 12V | 24V | 48V | 120V | 240V |
|---|---|---|---|---|---|
| 100W | 8.3A | 4.2A | 2.1A | 0.83A | 0.42A |
| 500W | 41.7A | 20.8A | 10.4A | 4.2A | 2.1A |
| 1,000W | 83.3A | 41.7A | 20.8A | 8.3A | 4.2A |
| 1,500W | 125A | 62.5A | 31.3A | 12.5A | 6.3A |
| 2,000W | 166.7A | 83.3A | 41.7A | 16.7A | 8.3A |
| 3,000W | 250A | 125A | 62.5A | 25A | 12.5A |
Calculated values (W ÷ V). Real current is higher once losses and power factor are included, as explained below.
Correction 1: Inverter Losses (DC Side)
When a battery runs an AC load through an inverter, the battery supplies the load plus the inverter’s losses. Example math with an assumed 90% efficiency: 1,000W AC ÷ 0.90 = 1,111W DC, and 1,111 ÷ 12V ≈ 93A. Check your inverter’s datasheet for its actual efficiency.
Correction 2: Power Factor (AC Side)
For motors and some electronics, current is higher than watts ÷ volts because of power factor (PF). Amps = Watts ÷ (Volts × PF). A 1,000W load at PF 0.8 on 120V draws 1,000 ÷ 96 ≈ 10.4A, not 8.3A. That’s why inverters list both VA and W.
Using the Number
- Wire size: pick wire whose ampacity covers the current. NEC 210.19 and 215.2 size branch circuits and feeders at 125% of continuous loads (3+ hours).
- Fuse or breaker: protects the wire, so size it to the wire’s ampacity.
- Battery runtime: use watt-hours, not amps alone (see the amp-hours vs watt-hours guide).
Split-Phase 240V Loads
A 240V appliance like a dryer or a mini-split draws its current on both hot legs at once. A 4,800W dryer pulls 20A on each leg, not 40A total. That’s why its breaker is a two-pole 30A unit, sized for the 20A load with headroom.
Example math: 4,800 ÷ 240 = 20A.
Amps to Watts (Reverse)
Watts = Amps × Volts. A 15A, 120V circuit can deliver 1,800W at most, and about 1,440W for continuous loads at 80%.
Worked Example: Can My 12V Battery Run a Coffee Maker?
Illustrative inputs: a 1,000W coffee maker, a 12V LiFePO4 battery (12.8V nominal, 100Ah), and an inverter assumed at 90% efficiency.
| Step | Math | Result |
|---|---|---|
| DC power needed | 1,000 ÷ 0.90 | about 1,111W |
| DC current | 1,111 ÷ 12.8 | about 87A |
| Brew time | 6 minutes (0.1 hour) | |
| Energy used | 1,111W × 0.1h | about 111Wh |
| Share of a 1,280Wh battery | 111 ÷ 1,280 | about 9% |
The energy is small, but the current is high, so the battery’s continuous discharge rating, the cable and the fuse all need to handle about 90A. Check the battery datasheet’s max continuous current; battery C-rating explained shows how. Size the inverter with what size inverter do I need.
Extended Chart: Common Household Loads
| Appliance (typical label) | Watts | Amps at 120V | Amps from 12V battery (90% inverter) |
|---|---|---|---|
| LED TV, 55 in. | 100W | 0.8A | 9.3A |
| Refrigerator (running) | 150W | 1.3A | 13.9A |
| Microwave (input) | 1,200W | 10A | 111A |
| Hair dryer (high) | 1,800W | 15A | 167A |
| Space heater | 1,500W | 12.5A | 139A |
Illustrative wattages; check your own label. DC column = W ÷ 0.9 ÷ 12. Appliance guides: refrigerator, microwave and TV.
Circuit Capacity Table (120V)
| Breaker | Max watts | 80% continuous load |
|---|---|---|
| 15A | 1,800W | 1,440W |
| 20A | 2,400W | 1,920W |
| 30A | 3,600W | 2,880W |
The 80% column follows the NEC rule that continuous loads (three hours or more) be sized at 125%, which is the same as loading a breaker to 80%. For 240V circuits, double the watts. See 120V vs 240V split phase.
Three-Step Method for Any Load
- Find the watts on the label. If only amps are listed, watts ≈ amps × volts.
- Divide by system voltage. On DC from an inverter, divide by efficiency first.
- Add headroom for continuous loads (×1.25), then size wire and the fuse. See what size fuse do I need.
Motor Loads and Surge
Compressors and pumps draw several times their running current for a fraction of a second at startup. The running-amps number sizes the wire; the surge number sizes the inverter. Details are in surge watts vs running watts.
FAQ
How many amps is 1,500 watts at 120V?
12.5A.
How many amps does a 2,000W inverter draw from a 12V battery?
At full load, about 167A before losses, and roughly 185A at 90% efficiency.
Do I use 110V or 120V?
US nominal voltage is 120V. Older references say 110V, but use 120V for the math.
How many amps is 3,000 watts at 240V?
3,000 ÷ 240 = 12.5A on each leg.
Is 12.8V or 12V right for LiFePO4 math?
12.8V is the nominal voltage of a 4-cell LiFePO4 pack, so it gives slightly more accurate Wh numbers; 12V is fine for quick, conservative estimates.
Sources: U.S. Department of Energy, “Estimating Appliance and Home Electronic Energy Use” (energy.gov); NFPA 70 (NEC) 2023, Articles 210, 215 and 240; U.S. DOE Energy Saver appliance energy estimates; NFPA 70 (NEC) 210.19 and 215.2. Accessed Oct 2026.
Related: Allwei 1500W Pure Sine Wave Inverter Review, Pro-Chaser 400W Car Power Inverter Review, EcoFlow Delta 3 Max Efficiency and Idle Draw
3 replies on “Watts to Amps: How to Convert at 12V, 120V and 240V (Calculator + Chart)”
[…] Example sizing only. Use the inverter manual’s recommended fuse and the cable maker’s ampacity. The cable choice comes from the wire gauge chart for 12V, 24V and 48V, and the current from watts to amps. […]
[…] = watts ÷ volts. See watts to amps at 12V, 120V and 240V for the […]
[…] A 12V system needs heavy cable, a properly sized fuse and a battery rated for that discharge. See watts to amps at 12V, 120V and 240V and battery C-rating […]