Charge Controller Sizing Calculator

Pick MPPT or PWM, enter your array and your bank voltage, and get the controller amp rating to buy. The two types are sized off completely different quantities, which is why a single rule of thumb gets one of them wrong. MPPT is sized on array power; PWM is sized on array short-circuit current.

Nominal bank voltage: 12, 24 or 48. Using nominal rather than absorption voltage gives the higher, safer current.

Panel count times panel wattage, at STC.

NEC 690.8(A) treats PV current as continuous and applies 1.25. Raise it to 1.56 if you want the full 690.8 stacking.

Checks an existing unit against this array. A controller's array-watt ceiling is its amp rating times the bank voltage, so a 30 A unit on a 12 V bank tops out at 360 W no matter how many panels are wired to it.

Free download

Get the printable solar cable and fuse charts

DC cable ampacity with the ambient and bundling corrections, how far each size runs at 12, 24 and 48 V, PV string fuse sizes from Isc, and battery-to-inverter current by inverter size. Three pages, made to pin by the battery bank.

We store your email with EmailOctopus to send the charts and occasional solar sizing tips, nothing else. Unsubscribe any time. Privacy policy.

How It Works

MPPT: sized on power

An MPPT controller is a DC-to-DC converter. It runs the array at its maximum power point and converts that power down to whatever the battery needs, so the current that comes out of it is set by power, not by the array current:

Controller A = Array W ÷ Battery V × 1.25

Because power is conserved and voltage falls, current rises. A 3 kW array feeding a 12 V bank would need 250 A of controller, which does not exist. That is the real reason large arrays run at 48 V.

PWM: sized on current

A PWM controller is a switch, not a converter. When it closes, the array is connected straight to the bank and dragged down to battery voltage, so the current through it is simply the array short-circuit current:

Controller A = Panel Isc × Strings in parallel × 1.25

Array wattage does not enter into it. Two 200 W panels in parallel and two 100 W panels in parallel can need the same PWM controller if their Isc figures match. Isc is used rather than Imp because a shorted controller output is a normal fault condition, and because Isc is the number the standards size against.

Worked example: 3 kW at 48 V, MPPT

  • Output current = 3000 ÷ 48 = 62.5 A
  • With the 1.25 factor = 62.5 × 1.25 = 78.1 A
  • Next standard size = 80 A controller
  • That 80 A unit tops out at 80 × 48 = 3,840 W of array on this bank

Worked example: two 165 W panels at 12 V, PWM

Two panels in parallel, each Isc 9.5 A, Imp 8.9 A, Vmp 18.5 V:

  • Array Isc = 9.5 × 2 = 19 A
  • With the 1.25 factor = 23.8 A, so a 30 A controller
  • Power actually collected = 8.9 × 2 × 12 = 214 W, not the 330 W on the labels

That last line is the PWM decision in one number. The panels are rated at 18.5 V but PWM holds them at 12 V, so roughly 35% of the nameplate never arrives. On a 12 V bank with 12 V-nominal panels that gap is tolerable and PWM is cheap. With 60-cell or 72-cell grid panels, whose Vmp sits near 30 V or 40 V, PWM throws away two thirds of the array and MPPT pays for itself immediately.

MPPT Array Limits by Controller Size

The maximum array a given MPPT controller can carry, in watts, at each nominal bank voltage. This is the same figure manufacturers print on the box, and it is just the amp rating multiplied by the bank voltage. It is why the same controller is advertised as a 780 W unit and a 3,120 W unit at the same time.

Controller rating Max array at 12V Max array at 24V Max array at 48V
10 A120 W240 W480 W
20 A240 W480 W960 W
30 A360 W720 W1,440 W
40 A480 W960 W1,920 W
50 A600 W1,200 W2,400 W
60 A720 W1,440 W2,880 W
80 A960 W1,920 W3,840 W
100 A1,200 W2,400 W4,800 W
150 A1,800 W3,600 W7,200 W

These are the controller's own limits. The other limit is the maximum PV input voltage, which is set by the array's open-circuit voltage on the coldest morning of the year rather than at room temperature. Work that out with the series vs parallel calculator before committing to a string length, because exceeding it destroys the controller instantly and is not covered by any warranty. The long-form walkthrough of that failure, with a 150 V MPPT example, is charge controller sizing and cold Voc.

Frequently Asked Questions

What size charge controller do I need?

For an MPPT controller, divide your total array wattage by your battery bank voltage and multiply by 1.25, then round up to the next size sold. A 2,000 W array on a 24 V bank gives 2000 divided by 24, which is 83.3 A, times 1.25 is 104 A, so a 150 A controller or two smaller ones. For a PWM controller, ignore wattage entirely: multiply the panel short-circuit current by the number of parallel strings, then by 1.25.

Why is MPPT sized on watts and PWM sized on amps?

An MPPT controller converts power, so it trades voltage for current: array power divided by battery voltage gives the output current, and a higher array voltage produces more output current, not less. A PWM controller only switches, so whatever current the array produces passes straight through it at battery voltage. Sizing a PWM unit on watts will undersize it whenever the panels have a Vmp well above the bank voltage.

Where does the 1.25 factor come from?

NEC 690.8(A) treats photovoltaic output as a continuous current and requires it to be multiplied by 1.25 for equipment and conductor sizing. It also covers the real-world case of irradiance above 1000 W per square metre, which happens on bright cold days with cloud-edge reflection and can push a panel above its rated Isc. Some designers stack the 1.25 twice, giving 1.56, when sizing overcurrent protection; the same 1.56 factor appears on the PV fuse and breaker sizing calculator.

Can I connect more solar than the controller is rated for?

Some MPPT controllers permit it and simply clip their output at the rated current, which is a deliberate design choice on systems that want more winter production and accept clipping in summer. Check the manufacturer's data sheet, because others treat it as an overload and shut down or void the warranty. Exceeding the maximum PV input voltage is never acceptable on any controller; that is a hard limit set by the switching components.

Is MPPT worth it on a small 12 V system?

It depends on the panels, not the system size. With true 12 V nominal panels whose Vmp is around 18 V, PWM loses roughly a third of the array and the controller costs a fraction as much, so on a 200 W setup the maths often favours PWM. With 60-cell or 72-cell grid-tie panels, whose Vmp is 30 V to 40 V, PWM would collect less than half the nameplate power and MPPT recovers its cost almost immediately. Run both cases through the calculator above and compare the usable watts.

Related Calculators

Continue with these related solar tools

Solar Panel Series vs Parallel Calculator

Array voltage and current for any series and parallel arrangement, including cold-weather Voc

Solar System Size Calculator

What size solar system you need in kW, from daily kWh use, peak sun hours and system losses

PV Fuse and Breaker Sizing Calculator

Next standard fuse or breaker size for a PV string, controller or inverter circuit

Embed This Calculator

Add this free calculator to your website. Copy the code below and paste it into your page.

Solar Sizing Calculator

Free solar and off-grid sizing calculators

More Calculators

© 2026 Solar Sizing Calculator. All rights reserved. A Nomogram Works site.

Privacy Policy Terms of Service Cookie Policy