Solar Charge Controller Size Calculator (PWM / MPPT)
Solar Charge Controller Size Calculator (PWM / MPPT)
The controller your array needs: minimum current from the panels’ short-circuit rating with the 125% factor, the cold-weather open-circuit voltage your controller must survive, and what MPPT gains over PWM.
Charge controller size
Two 590 W panels in series (Isc 14.13 A, Voc 52.63 V) on a 24 V bank, MPPT, coldest 5 °C
Current, cold voltage, and what MPPT adds
- I(sc)
- panel short-circuit current at STC, from the datasheet
- 1.25
- the factor NEC 690.8(A)(1) applies to rated I(sc)
- β
- temperature coefficient of V(oc) in %/°C, a negative number
- T(min)
- lowest expected ambient temperature, per NEC 690.7
Worked example
Two 590 W panels in series (Isc 14.13 A, Voc 52.63 V) on a 24 V bank, MPPT, coldest 5 °C
Array = 2 × 590 = 1,180 W; MPPT output current = 1,180 ÷ 24 = 49.17 A, so a 50 A or 60 A controller
PV-side current = 14.13 × 1 string × 1.25 = 17.66 A — that is what the array wiring and fusing must carry
Cold V(oc) = 52.63 × (1 + (5 − 25) × (−0.25) ÷ 100) = 55.26 V per panel, 110.52 V for two in series, inside a 150 V controller
PWM would pull this array to 28.8 V and harvest about 389 W instead of 1,145 W
What MPPT gains over PWM, full sun
| Array and battery | PWM harvest | MPPT harvest | Difference |
|---|---|---|---|
| 12 V battery, one 36-cell 100 W panel (Vmp 18 V) | 80 W | 97 W | +21% |
| 24 V battery, two 590 W panels in series (Vmp 87.4 V) | 389 W | 1145 W | +194% |
| 48 V battery, two 590 W panels in series | 778 W | 1145 W | +47% |
Sizing a solar charge controller
Two numbers decide whether a controller fits: the current it must pass and the voltage it must survive. Get the second one wrong and the controller dies on the first cold, sunny morning.
Current. For a PWM controller the array’s current flows straight through, so the rating must cover the short-circuit current of all the parallel strings with a margin: NEC 690.8(A)(1) sets the maximum current of a PV source circuit at 125% of rated I(sc), which for the example is 17.66 A. An MPPT controller is a converter: it takes the array’s power in at a high voltage and puts it out at the battery’s, so its output current is the array’s power divided by the battery voltage — 1,180 W ÷ 24 V = 49.17 A, which needs a 50 or 60 A unit. The PV-side figure still matters for the cable and the string fuse.
Cold voltage. A panel’s open-circuit voltage rises as it gets colder, by the temperature coefficient on its datasheet — −0.25%/°C on the example panel. NEC 690.7 asks for the maximum system voltage to be worked out at the lowest expected ambient temperature, which is exactly the calculation here: at 5 °C the panel’s 52.63 V becomes 55.26 V and two in series reach 110.52 V. Put four in series at −10 °C and the string reaches 228.94 V, well past a 150 V controller. Always use your site’s record low, in the morning before the panels warm up.
PWM or MPPT. A PWM controller is a switch: it connects the array to the battery, so the array is dragged to the battery’s voltage and the power collected is that voltage times the panel’s current. That is fine when the panel is built for the job: on a 36-cell panel with a maximum-power voltage around 18 V charging a 12 V bank, MPPT gains only about 21%. It is terrible with today’s large panels, whose maximum-power voltage is 30–45 V: most of the panel is thrown away. An MPPT controller converts instead of connecting, holds the array at its maximum-power point, and — as Victron puts it — harvests 10% to 40% more in the conditions that matter, cold cells, very hot cells and low light. Above a few hundred watts, MPPT is the only sensible choice.
Once the controller is chosen, size the battery it charges with the solar battery bank calculator, the array with the solar panel size calculator, and the cable from the array with the AWG wire size calculator.
Frequently asked questions
What size solar charge controller do I need?
For MPPT, divide the array’s watts by the battery voltage: 1,180 W on a 24 V bank needs 49 A, so a 50 or 60 A controller. For PWM, take the panels’ short-circuit current × the number of strings × 1.25.
Why multiply Isc by 1.25?
Because irradiance can exceed the 1,000 W/m² of standard test conditions and push a panel above its rated short-circuit current. NEC 690.8(A)(1) takes 125% of rated Isc as the maximum current of a PV source circuit, and controller manufacturers ask for the same margin.
How do I calculate maximum PV voltage in cold weather?
Voc × (1 + (Tmin − 25) × β ÷ 100) × panels in series, with β the temperature coefficient of Voc in %/°C. At 5 °C a 52.63 V panel with −0.25%/°C reaches 55.26 V; two in series, 110.52 V.
Is MPPT worth it over PWM?
Almost always, above a few hundred watts. Victron measures 10–40% more harvest in cold, very hot or low-light conditions. PWM only makes sense with a small array whose panel voltage is already close to the battery’s.
Can I connect more panels than the controller’s rating?
Never above its maximum PV voltage. Exceeding the current or wattage rating is different: a good MPPT controller simply limits its output, and some manufacturers allow deliberate oversizing. Check that manufacturer’s own limit before relying on it.
What happens if the array voltage is too high?
The controller’s input stage fails, usually permanently and usually not under warranty. Work the voltage out at your coldest expected temperature, not at today’s.
Related calculators
References
- NFPA 70 National Electrical Code, Article 690. 690.8(A)(1): the maximum current of a PV source circuit is 125% of the rated short-circuit current. 690.7: the maximum voltage is the open-circuit voltage corrected to the lowest expected ambient temperature, using the module’s temperature coefficient.
- JinkoSolar. Tiger Neo N-type 72HL4-(V) 580–605 W datasheet, model JKM590N: 590 W, Voc 52.63 V, Isc 14.13 A, Vmp 43.71 V, Imp 13.50 A, temperature coefficient of Voc −0.25%/°C, 2,278 × 1,134 mm, 22.84% efficient.
- Victron Energy. Which solar charge controller: PWM or MPPT? (2014): an MPPT controller harvests 10% to 40% more than PWM when cells are cold, very hot, or in low light; PWM suits small systems with cell temperatures between 45 °C and 75 °C.
- World Bank / ESMAP / Solargis. Global Photovoltaic Power Potential by Country, 2020, and the Global Solar Atlas (globalsolaratlas.info) for the long-term average at your own site.