Solar Charge Controller Calculator
Enter the battery voltage, panel specs, array layout and lowest temperature to check cold Voc and size the charge controller current.
Step-by-step
- Enter valid values to begin.
Cold voltage follows NEC 690.7 with the module temperature coefficient: Voc,cold = Voc × (1 + β × (Tmin − 25)). Output current uses 125% of P / Vbatt for MPPT at the nominal battery voltage and 125% of the summed Isc for PWM. The PWM mismatch estimate assumes the array is pulled down to a charge voltage of about 1.2 × nominal (14.4/28.8/57.6 V) and uses STC Vmp, so it is an approximation. Standard ratings 10/20/30/40/60/80/100 A are typical catalogue sizes. Always read the controller datasheet and verify the NEC edition and local AHJ.
Solar Charge Controller Sizing: Cold Voc and Charge Current
Core Engineering Principles
A charge controller can fail in two ways, and both are decided before it is switched on. The first is voltage. A panel’s open-circuit voltage rises as the cells get colder, by roughly 0.28% per degree below 25 °C on modern silicon modules. On a clear winter morning a string rated 111 V at test conditions can read over 120 V with no load, and the controller input must survive that. Exceed the limit and the input stage can fail permanently on the first cold day. That is why NEC 690.7 requires temperature-corrected maximum voltage, and why we use the record low for the site, not the average winter night.
The second is current. An MPPT controller converts the array’s higher voltage to battery voltage, so output current is array watts over battery volts, plus 25% for continuous duty. A PWM controller connects the array to the battery directly, so the array is dragged to battery voltage and the current is about the panel short-circuit current. That is where PWM wastes power: a 93 V array forced to 57.6 V gives up more than a third of its output. PWM is fine only when array and battery voltages match.
IMPPT = Parray / Vbatt × 1.25 • IPWM = Npar × Isc × 1.25
PWM loss ≈ (Vmp,array − Vcharge) / Vmp,array
NEC & Standard References
NEC Article 690 applies: 690.7 sets maximum PV system voltage using the module temperature coefficient, and 690.8 sizes circuits at 125% of short-circuit current. UL 1741 covers the listing of PV charge controllers in North America. IEC 62109-1 and 62109-2 define the safety of power converters for PV systems, and IEC 62509 specifies the performance and functioning of battery charge controllers. Verify the adopted NEC edition and local AHJ requirements.1. Voc,cold = 37.0 × (1 + 0.0028 × 35) = 37.0 × 1.098 = 40.63 V per panel.
2. Array = 3 × 40.63 = 121.9 V, which is 81% of 150 V, so it passes.
3. Current = 2,400 / 48 × 1.25 = 62.5 A, so choose the next standard size, 80 A.
4. Max series string = floor(150 / 40.63) = 3 panels. Array limit = 80 × 48 / 1.25 = 3,072 W.
5. On PWM the 93 V array would be pulled to about 57.6 V, a loss of roughly 38%.
- Connect the battery first. The controller must see battery voltage before the array, and the array comes off first.
- Don’t add a fourth panel because it fits. It puts cold Voc at 162 V, above the 150 V limit.
- Fuse both sides. Protect the PV input and the battery output, rated to the wire.
- Keep a margin. Aim for 90% or less of the Voc limit.
- Check the datasheet for over-sizing. Many controllers clip, but some are damaged.