Charge controller size calculator
An MPPT controller is sized on the current it sends to the battery: array watts ÷ battery volts. A PWM controller is sized on the array’s short-circuit current. Add some headroom and pick the next size up.
Enter your measurements
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
MPPT: 400 W ÷ 12 V = 33.3 A
× (1 + 25% headroom) = 41.7 A → 50 A controller
- 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.
Quick answers for common sizes
| Example | Answer |
|---|---|
| What size charge controller for a 400 watt solar panel | 50 A MPPT on 12 V (33.3 A minimum) |
| 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) |
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 |
Worked examples: what size charge controller for a 400 watt solar panel, what size charge controller for a 200 watt solar panel and what size charge controller for 1000 watts of solar.
MPPT: size on output current
An MPPT controller is a DC-to-DC converter. It runs the panels at their maximum power point and turns that power into battery current: amps = watts ÷ battery volts. Victron’s worked example is a 100 W panel delivering 7.7 A into a 13 V battery. This calculator divides by the nominal 12, 24 or 48 V, which gives a slightly higher, safer figure.
MPPT amps = array watts ÷ battery volts × (1 + headroom) PWM amps = array short-circuit current (Isc) × (1 + headroom)
MPPT controllers carry two ratings, often in the name — “100/30” means 100 V maximum PV input and 30 A maximum charge current. The amps come from this calculator; the volts must be above your array’s cold open-circuit voltage.
PWM: size on short-circuit current
A PWM controller is a switch that connects the panels straight to the battery, pulling them down to battery voltage. The current it must carry is close to the panels’ short-circuit current, so it is sized on the array’s Isc. PWM suits small 12 V systems with 12 V-class panels wired in parallel; it can’t use the extra voltage of series strings.
Common sizes
| Array | 12 V battery | 24 V battery | 48 V battery |
|---|---|---|---|
| 200 W | 21 A → 30 A | 10 A → 15 A | 5 A → 10 A |
| 400 W | 42 A → 50 A | 21 A → 30 A | 10 A → 15 A |
| 600 W | 63 A → 70 A | 31 A → 40 A | 16 A → 20 A |
| 800 W | 83 A → 100 A | 42 A → 50 A | 21 A → 30 A |
| 1200 W | 125 A → split | 63 A → 70 A | 31 A → 40 A |
| 2000 W | 208 A → split | 104 A → split | 52 A → 60 A |
Mistakes to avoid
- Ignoring the voltage limit. Too many panels in series can exceed the controller’s maximum PV voltage on a cold morning, even if the amps are fine.
- Using 12 V panels in series with PWM. The extra voltage is wasted.
- Forgetting the battery’s own limits. The controller’s charge current shouldn’t exceed what the battery accepts; Victron lists maximum charge currents on its LiFePO4 data sheets.
- Wiring the panels before the battery. Many controllers need to see the battery first to detect the system voltage — follow the manual’s connection order.
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) - PWM solar charge controllerBudget price
Simple and cheap for one or two 12 V panels on a 12 V battery. It ties the panel to battery voltage, so it harvests less than MPPT in cool weather.
See options on Amazon (opens in a new tab) - Digital multimeterBudget price
Reads battery voltage, panel open-circuit voltage and continuity. The first tool to reach for when a solar system is not charging.
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
What size charge controller do I need for 400 watts of solar?
On a 12 V battery, about 33 A with no margin and 42 A with 25% headroom — a 40 A controller at its limit, or 50 A with room to spare. On 24 V, about 21 A with headroom. See what size charge controller for a 400 watt solar panel.
MPPT or PWM?
PWM is fine for one or two 12 V panels in parallel on a 12 V battery in a warm climate. MPPT harvests more in cool weather (Victron measured 81 W vs 100 W from a 100 W panel at 25 °C) and lets you wire panels in series. Above a few hundred watts, MPPT is the usual choice.
What does 100/30 mean on a charge controller?
On many MPPT controllers, 100 is the maximum PV input voltage and 30 the maximum charge current in amps. Your array’s cold open-circuit voltage must stay below the first number; your array watts ÷ battery volts should sit at or below the second.
Can I put more panels on a controller than its rating?
Some MPPT controllers allow more panel watts than their current rating and simply limit the output — but never exceed the maximum PV voltage. Check the controller manual before over-paneling.
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%.