Solar
Solar Charge Controller Calculator
Charge-controller sizing must satisfy both array current with a 125% planning margin and the cold-weather open-circuit voltage of the complete panel string. Production inputs follow the dated NLR PVWatts, EIA, and Census-derived sources documented on the Solar Data Sources page.
Calculate minimum controller charge current and cold-weather PV input voltage from array watts, panel Voc, series count, temperature, and bank voltage.
Calculated result
Controller requirements
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
Data provenance: NLR PVWatts v8 grid awaiting its first private-key refresh; the visible peak-sun-hours fallback is active. EIA residential state averages updated 2026-08-13. ZIP centroids use the 2025 Census Gazetteer. Calculator lookups use checked-in data rather than sending inputs to those source agencies; information submitted through quote or email forms is handled under the Privacy policy. Read the solar data provenance and limitations.
What this calculator returns
The controller must meet both the charge-current and cold-input-voltage requirements. Compare those values with every applicable equipment limit.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Required output current = array watts / bank voltage × 1.25. Cold string Voc = panel Voc × series count × [1 + coefficient × (minimum temperature °C − 25°C)].
- Parray
- Solar array nameplate power in watts DC.
- Vbank
- Nominal battery-bank voltage.
- Voc
- Panel open-circuit voltage from the product datasheet.
- βVoc
- Panel open-circuit-voltage temperature coefficient per degree Celsius.
- Nseries
- Number of panels connected in each series string.
The 1.25 multiplier is a visible planning margin. Use the panel datasheet coefficient and the lowest design temperature for the site.
Worked example
| Input | Value |
|---|---|
| Solar array power | 1200 W |
| Panel open-circuit voltage | 44 V |
| Panels in series | 2 |
| Temperature units | Fahrenheit (°F) |
A 1,200 W array charging 24 V requires 62.5 A after the 125% margin. Two 44 V Voc panels at −4°F and −0.29%/°C reach about 99.5 V open circuit.
Reference table
| Array power | 12 V bank at 125% | 24 V bank at 125% | 48 V bank at 125% |
|---|---|---|---|
| 400 W | 41.7 A | 20.8 A | 10.4 A |
| 800 W | 83.3 A | 41.7 A | 20.8 A |
| 1,200 W | 125.0 A | 62.5 A | 31.3 A |
| 2,400 W | 250.0 A | 125.0 A | 62.5 A |
Frequently asked questions
Why is panel open-circuit voltage temperature-adjusted?
Panel Voc generally rises in colder conditions, so the entered coefficient and minimum temperature estimate the cold string voltage.
Is the current result the controller's input or output requirement?
It is a planning output-current requirement based on array watts, bank voltage, and the visible 1.25 margin.
Why use cold temperature for Voc?
PV open-circuit voltage generally rises as cells get colder, so the cold condition can control the input-voltage requirement.
Assumptions and limitations
- Planning math only; not a controller, conductor, fuse, breaker, or code selection.
- Controller topology, conversion limits, battery charge profile, and manufacturer array rules require separate review.
- Use site-specific minimum temperature and product datasheets.
Method and sources
Review the solar data sources, calculation methodology, and electrical formulas for the references most relevant to this calculation. The broader technical sources index records source scope and provenance. Content release .
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