400 watts to amps at 12V
400 watts at 12 volts is 33.3 A. For 400 W of solar panels charging a 12 V battery, an MPPT controller delivers at most about 31 A at a 13 V charging voltage.
How to work it out
- 400 W ÷ 12 V = 33.3 A
People ask this for two different reasons: a 400 W load on a 12 V battery (a TV, a blender, a small cooler on an inverter), or 400 W of solar panels charging one. The number is the same; what it sizes is not.
Adjust for your project
- Energy per hour of running
- 0.4 kWh
| Voltage | Amps | Amps × volts |
|---|---|---|
| 12 V | 33.3 A | 400 W |
| 24 V | 16.7 A | 400 W |
| 48 V | 8.3 A | 400 W |
| 120 V | 3.3 A | 400 W |
| 240 V | 1.7 A | 400 W |
Amps for common loads at the voltage above
| Power | Current |
|---|---|
| 100 | 8.3 A |
| 300 | 25 A |
| 500 | 41.7 A |
| 1000 | 83.3 A |
| 1500 | 125 A |
| 2000 | 166.7 A |
| 3000 | 250 A |
Load or solar: what 33 amps sizes
- As a load, 400 W through a 90% inverter is about 37 A from the battery. A 100 Ah LiFePO4 battery used to 80% runs it a little over two hours.
- As solar, 400 W of panels is a peak figure. An MPPT controller converts it to charging current at battery voltage — Victron’s example is watts ÷ 13 V — so about 31 A in full midday sun, much less early and late.
- Controller size. That is why 400 W of panels on 12 V pairs with a controller rated around 40 A; what size charge controller for a 400 watt solar panel works it through with headroom.
- At 24 V it halves. The same 400 W is 16.7 A at 24 V, which is why many systems above a few hundred watts move to 24 V.
For the daily energy those panels make, see how much power a 400W solar panel produces per day. A larger 12 V load is on how many amps is 1000 watts at 12 volts.
Different size? Use the full Watts to Amps Calculator.
What you’ll need
- MPPT solar charge controllerMid-range price
Converts a higher panel voltage into charging current, so you can wire panels in series and lose less in the cables. Size it on output amps and maximum PV voltage.
See options on Amazon (opens in a new tab) - DC clamp meterMid-range price
Clamps around one cable to read DC amps without breaking the circuit — checks what an inverter or fridge actually draws from the battery. Make sure it says DC; many cheap clamps read AC only.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
100 watts to amps at 12V?
8.3 A.
200 watts to amps at 12V?
16.7 A.
400 watts to amps at 12V through a 90% inverter?
About 37 A from the battery.
Other common sizes
- How many amps is 1000 watts at 12 volts — 83.3 A (92.6 A through an inverter)
- How many amps is 1500 watts at 120 volts — 12.5 A
Sources
- Virginia Cooperative Extension, BSE-301: Estimating Appliance and Home Electronic Energy Use (2020) (opens in a new tab) — Wattage × hours ÷ 1,000 = kWh per day; watts = amps × volts when only amps are listed; a refrigerator runs at full wattage about a third of the time; typical nameplate wattages.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Victron Energy — Which solar charge controller: PWM or MPPT? (2020) (opens in a new tab) — MPPT output current = panel watts ÷ battery voltage (100 W ÷ 13 V = 7.7 A); a PWM controller pulls the panel down to battery voltage (81 W from a 100 W panel at 25 °C, about the same as MPPT at 75 °C); temperature-coefficient method; example panel Voc 21.6 V, −0.35 %/°C.