How to use this calculator
- Enter or choose array size (DC).
- Enter or choose dC/AC ratio.
- Enter or choose panel open-circuit voltage (Voc).
- Enter or choose voc temperature coefficient.
- Enter or choose lowest expected cell temperature.
- The inverter AC rating updates as you type. Use Copy result or Share to keep it.
Method
AC kW = DC kWp ÷ DC/AC ratio; Voc(cold) = Voc × (1 + β × (Tmin − 25)); Max panels per string = ⌊Vmax ÷ Voc(cold)⌋
- β
- Voc temperature coefficient as a fraction per °C (negative)
- Tmin
- lowest cell temperature, °C
- Vmax
- inverter maximum DC input voltage
Worked example
- AC rating = 6.6 ÷ 1.2 = 5.5 kW.
- Voc at −10 °C = 49.5 × (1 + (−0.0027) × (−35)) = 49.5 × 1.0945 = 54.18 V.
- Panels per string = 600 ÷ 54.18 = 11.07, so at most 11.
A 6.6 kWp array at a 1.2 ratio suits a 5.5 kW inverter, with no more than 11 of these panels per string on a 600 V input.
What the result means
A ratio above 1 slightly clips output on the sunniest hours but makes better use of the inverter the rest of the time. The string limit is a hard safety limit: exceeding the inverter’s maximum voltage can damage it.
Assumptions and limits
- The temperature coefficient is linear over the range.
- All strings use identical panels.
- Does not check the MPPT voltage window, maximum input current or minimum string length; check them against the datasheets.
- Local electrical codes may require extra margins; a qualified installer must design the final system.
Questions
Why does panel voltage rise in the cold?
Silicon cell voltage falls as temperature rises and rises as it falls. On a clear, freezing morning the open-circuit voltage is at its highest.
Is it safe to oversize the array?
Within the inverter maker’s stated DC limits, yes; the inverter simply caps its AC output. Never exceed the maximum DC voltage or current.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.