Solar charge controller size calculator

Enter your panels' label figures, how they are wired and your battery voltage. You get the controller's amp rating with a safety margin, the voltage it has to be built to take on the coldest morning, the PV fuse size, and what a PWM controller would cost you in power.

Inputs

W

The watt rating (Pmax) on the panel label.

V

On the panel label or data sheet, at 25 °C. A "12 V" panel is usually 21–24 V.

V

Also on the label. A "12 V" panel is usually 17–20 V.

A

On the label. The controller must carry this, with a margin.

Series adds voltage: two 12 V panels in series make 24 V.

Parallel adds current. Each string is the series panels above.

MPPT turns extra panel voltage into charging current. PWM needs panels whose voltage matches the battery and is cheaper.

Panel voltage rises in the cold, so size for the coldest weather you will see. Victron’s manual adds 10% in cold climates.

V

Victron’s MPPT 100/30 takes 100 V; Morningstar’s TriStar 125 V. Exceeding it destroys the controller.

The NEC rates the controller at 125% of the array’s short-circuit current. Set 0 to see the bare charging current.

MPPT controller to look for

40A or more

27.8 A (400 W ÷ 14.4 V) plus 25% margin is 34.7 A. Rated for at least 54 V of panel voltage.

A 40 A MPPT controller carries 34.7 A and takes 53.7 V on the coldest morning, inside its 100 V limit.

Array
400 W
Voltage on a 14 °F morning
53.7 V
Array short-circuit current
10.5 A
PV fuse or breaker
16.4 A or the next size up
Power into the battery, MPPT
392 W
What your array needs and what the controller offers

2 panels of 200 W (1 string of 2) make 400 W into a 12 V battery. With an MPPT controller that is 27.8 A, so with a 25% margin look for a 40 A controller that accepts at least 54 V from the panels (48.0 V at 25 °C, more on a 14 °F morning). The array’s short-circuit current is 10.5 A and its fuse should be at least 16.4 A. The bars compare the voltage and current your array needs with what the controller offers: A 40 A MPPT controller carries 34.7 A and takes 53.7 V on the coldest morning, inside its 100 V limit.

Safety first

Solar panels make power whenever there is light, and a series string can reach a high voltage. A charge controller that is given more voltage than its rating is destroyed, and a fault between the panels and the battery can start a fire.

  • Fuse every circuit, including the panel wiring to the controller, using the fuse size above or the maker’s. The fuse protects the wire, so it must not be bigger than the wire’s rating.
  • Follow the manufacturer’s instructions for the controller, the panels and the battery, including their limits on voltage, current and series and parallel connections.
  • Follow your local electrical code and any marine or RV standard that applies to your vehicle or boat. In the US, the NEC’s article 690 covers the disconnects and fuses for a solar array.
  • Have a qualified person check the installation before you switch it on, and disconnect the panels (or cover them) while you work.

How it works

MPPT current   = array watts ÷ battery charging voltage
PWM current    = panel short-circuit current × strings in parallel
rating to buy  = current × (1 + margin), rounded up to a size on sale
cold voltage   = panel Voc × panels in series × (1 + 0.34% × degrees below 25 °C)
PV fuse        = array short-circuit current × 1.25 × 1.25
  • Charging voltage is 14.4 V on a 12 V battery, 28.8 V on 24 V and 57.6 V on 48 V, Victron’s default absorption settings, so the current comes out slightly lower than at the nominal voltage.
  • MPPT controllers need the array at maximum power to be at least 5 V above the battery voltage to start (Victron). PWM controllers need panels whose voltage is close to the battery’s.
  • The margin is 25% by default, the NEC’s 125%. Renogy’s 2026 guide adds none to the MPPT figure. Set it to zero to see the bare current.
  • The cold-weather rule uses a coefficient of 0.34% per degree, from Victron’s example module. Your panel’s data sheet gives its own, usually between 0.25% and 0.35%. Victron’s manual adds 10% to the Voc in cold climates as a rule of thumb.

Worked example

Two 200 W panels in series (Voc 24 V, Vmp 19.5 V, Isc 10.5 A each) charge a 12 V battery with an MPPT controller. The coldest morning is -10 °C.

  • Array: 400 W, 48 V open-circuit at 25 °C, 39 V at maximum power, 10.5 A of short-circuit current.
  • Current: 400 ÷ 14.4 = 27.8 A, plus 25% is 34.7 A, so look for a 40 A controller.
  • Voltage: 48 V × (1 + 0.34% × 35) = 53.7 V on the coldest morning, well inside a 100 V controller.
  • Fuse: 10.5 × 1.25 × 1.25 = 16.4 A, so a 20 A fuse or breaker.
  • A PWM controller on the same array would pass about 151 W (14.4 V × 10.5 A) of the 400 W, because the panels would be pulled down from 39 V to the battery’s voltage.

Tips from the road

  • Put more panels in series, not parallel, when the controller allows it. Higher voltage means lower current, so thinner and cheaper wire, and a smaller controller.
  • Do not guess the temperature. The lowest overnight temperature you will really see matters, because a cold clear morning is when panels reach their highest voltage and a bright one is when the controller is busiest.
  • A 12 V “panel” is not 12 V. Its open-circuit voltage is around 22 V and its maximum-power voltage around 18 V. Check the label rather than the name.

FAQ

What size charge controller do I need for my solar panels?

For an MPPT controller, divide the array watts by the battery's charging voltage (14.4 V for a 12 V battery), then add a margin and round up to the next size. Renogy's example is 400 W ÷ 14.4 V = 27.8 A, so a 30 A controller. The calculator adds the NEC's 25% margin by default, which turns that into 35 A, so a 40 A controller. Then check the panel voltage, which matters just as much.

What is the difference between MPPT and PWM?

A PWM controller connects the panels to the battery and switches the current on and off, so the panels are pulled down to the battery's voltage and the extra volts are wasted. An MPPT controller converts them into extra charging current. Victron's data sheet says its ultrafast MPPT improves harvest by up to 30% over PWM. PWM is cheaper and fine for small arrays whose voltage matches the battery.

Why does the cold weather matter?

A panel's open-circuit voltage rises as it gets colder. Victron's example module gains 0.34% for every degree below 25 °C, so three 22.2 V panels in series make 66.6 V at 25 °C and 74.5 V at -10 °C. A controller's maximum PV voltage is a hard limit, and Victron says exceeding it damages the controller permanently.

How big a fuse does the solar array need?

Morningstar, quoting the NEC, says the PV fuse or breaker is at least 125% of 125% of the array's short-circuit current, which is 156% of Isc, rounded up to the next standard size. Two panels in parallel at 8 A each need 25 A or the next size up.

Can I connect more panel watts than the controller is rated for?

With an MPPT controller you can, within limits. Victron's data sheet says an MPPT limits its input power if more PV is connected, and its own sizing calculator allows up to 130% of the controller's maximum PV power. The two limits that cannot be exceeded are the maximum PV voltage and the maximum PV short-circuit current, and each is checked on its own.

Sources

  1. Solar Charge Controller Sizing and How to Choose One, Renogy. MPPT charging current is array watts ÷ battery charging voltage (400 W ÷ 14.4 V = 27.8 A, so a 30 A controller; 400 W ÷ 28.8 V = 13.9 A on 24 V); the array's open-circuit voltage must stay under the controller's maximum PV voltage including cold-weather increase; PWM needs closely matched panel and battery voltages.
  2. SmartSolar Charge Controllers MPPT 100/30 and 100/50 data sheet, Victron Energy. 100/30 is 30 A with 440 W (12 V) and 880 W (24 V) of PV power, 100 V open-circuit and 35 A short-circuit maximum; the array must exceed the battery voltage by 5 V to start; maximum efficiency 98%; ultrafast MPPT improves harvest by up to 30% over PWM.
  3. Matching Victron Energy solar modules to the new MPPT charge regulators, Victron Energy. Multiply the panel Voc by the panels in series and check it at the coldest temperature (three 22.2 V modules at -10 °C reach 74.5 using -0.34%/°C); multiply the panel Isc by the strings in parallel; a PWM controller's output area is Vbatt × Isc, up to 30% smaller than the maximum power point.
  4. ProStar MPPT operation manual, Morningstar. Array Voc at the coldest module temperature must not exceed the voltage ratings; NEC 690.9 sets the PV disconnect at 1.25 × array Isc and the PV fuse at 156% of array Isc.

Formula and sources last checked September 30, 2026. How we test formulas.

  • Series and parallel wiring calculatorWire batteries or solar panels in series, parallel or both and see the voltage, capacity and current you get, with a wiring diagram that updates as you type.
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  • Solar panel size calculatorHow many watts of solar panels you need for an RV, van, boat or cabin from daily watt-hours and peak sun hours, with a chart of panel output against load.
  • Battery bank size calculatorHow many amp-hours of battery you need for an RV, van, boat or cabin, from your daily watt-hours and days of autonomy, with a 24-hour state-of-charge chart.
  • DC wire size and voltage drop calculatorFind the AWG wire size for a 12, 24 or 48 V DC circuit from current and distance. Checks voltage drop and ampacity with NEC and Coast Guard tables.
  • Amp-hour and battery runtime calculatorHow long a battery runs a load. Enter amp-hours, voltage, battery type and watts. Covers LiFePO4 and AGM, inverter losses and lead-acid discharge-rate loss.

Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.