What size charge controller for a 400 watt solar panel?
400 W of panels on a 12 V battery sends up to about 33.3 A through an MPPT controller — 41.7 A with 25% headroom, so a 50 A controller (a 40 A one would run at its limit on the best days).
How to work it out
- MPPT: 400 W ÷ 12 V = 33.3 A
- × (1 + 25% headroom) = 41.7 A → 50 A controller
Four hundred watts is a popular size for van and small-RV arrays: one big panel, two 200 W or four 100 W panels. The controller depends on the battery voltage and how the panels are wired.
Adjust for your project
More options: Headroom
- Current with headroom
- 41.7 A
- Current with no headroom
- 33.3 A
- At other battery voltages (MPPT, with headroom)
- 24 V: 21 A · 48 V: 10 A
- Also check the controller’s maximum PV input voltage against your array’s cold open-circuit voltage — that is the limit that damages controllers. Use the series and parallel calculator.
Controller size for other array sizes
| Solar array watts | Current with no headroom | Current with headroom | Controller size to look for |
|---|---|---|---|
| 100 | 8.3 A | 10.4 A | 15 A |
| 200 | 16.7 A | 20.8 A | 30 A |
| 300 | 25 A | 31.3 A | 40 A |
| 400 | 33.3 A | 41.7 A | 50 A |
| 600 | 50 A | 62.5 A | 70 A |
| 800 | 66.7 A | 83.3 A | 100 A |
| 1000 | 83.3 A | 104.2 A | 104 A |
| 1200 | 100 A | 125 A | 125 A |
What to know about a controller for 400 W
- On 24 V it halves. 400 W ÷ 24 V is about 16.7 A, or about 21 A with headroom — a 30 A controller (20 A would be tight).
- Check the volts too. Four 100 W panels all in series reach about 97 V on a 14 °F morning (Victron’s example panel) — too close to a 100 V controller; two strings of two stay near 48.5 V. See four solar panels in series vs parallel.
- PWM is a poor fit here. With PWM, 400 W of 12 V panels in parallel has a short-circuit current around 24 A (four of Victron’s 6.12 A example panels), and gives up more output in cool weather than MPPT.
- Real current is usually lower. 33 A is the full-sun, cool-panel figure; on a hot afternoon the same array delivers noticeably less.
The raw conversion is on 400 watts to amps at 12V, and the daily output on how much power a 400W solar panel produces per day. Smaller arrays: what size charge controller for a 200 watt solar panel.
Different size? Use the full Charge Controller Size 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) - 400 W residential-size solar panelPremium price
Fewer, larger panels for a cabin, shed or ground mount. Their higher voltage usually means an MPPT controller.
See options on Amazon (opens in a new tab)
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Frequently asked questions
What size charge controller for 400 watts on a 24 V battery?
About 20.8 A with headroom — a 30 A controller.
What size PWM controller for 400 watts of 12 V panels?
With an array Isc of about 24.5 A, 30.6 A with headroom — a 40 A PWM controller.
What size charge controller for 400 watts with no headroom?
33.3 A — a 40 A controller at its limit in full sun.
Other common sizes
- What size charge controller for a 200 watt solar panel — 30 A MPPT on 12 V (a 20 A one would run flat out)
- What size charge controller for 1000 watts of solar — 60 A MPPT on 24 V (too much for one on 12 V)
Sources
- 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.
- 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%.