Solar string sizing calculator: panels in series and parallel
This solar string sizing calculator works out how many solar panels you can wire in series on one string and how many strings in parallel each inverter or charge-controller MPPT input can take. It corrects the panel voltages for your coldest morning and hottest afternoon, compares them with the inverter’s maximum DC voltage and MPPT window, checks the currents, and shows a pass or fail for each limit.
Enter the figures from the panel and inverter datasheets — nothing is assumed about a particular brand — and either type your own design or press Use maximum to fill the largest one that fits.
How to use the Solar String Sizing Calculator
- Copy the panel’s STC values from its datasheet: Pmax, Voc, Vmp, Isc, Imp and the temperature coefficients.
- Enter the inverter’s maximum DC voltage, MPPT voltage range, maximum input and short-circuit currents per MPPT, and how many MPPT inputs it has.
- Set the record low temperature for the site and the hottest cell temperature; switch to °F if you prefer.
- Type panels per string, strings per MPPT and MPPT inputs used — or press Use maximum.
- Check the green, amber and red badges, read the working, and copy the design check.
Solar String Sizing Calculator features
Cold Voc correction
Voc × (1 + βVoc × (Tmin − 25 °C)) sets the most panels in series below the maximum DC voltage.
MPPT window
Vmp is corrected for cold and hot cells so the string stays inside the tracking range all year.
Current checks
Strings per MPPT from the short-circuit limit, an amber warning when operating current will be clipped, and the NEC 690.8 125 % current.
Pass / fail badges
Each limit is shown clearly for the design you enter.
Array summary
Total panels, kWp and DC/AC ratio.
The string sizing formulas
Panel voltage rises as it gets colder. NEC 690.7(A) requires the maximum voltage to be calculated at the lowest expected ambient temperature, using the manufacturer’s temperature coefficient where one is given (IEC 62548 uses the same idea): Voc(cold) = Voc × (1 + βVoc × (Tmin − 25)) with β as a negative fraction per °C. The maximum number in series is the inverter’s maximum DC voltage ÷ Voc(cold), rounded down.
Default example: a 410 W panel with Voc 37.6 V and −0.27 %/°C at −10 °C gives 37.6 × (1 + 0.0027 × 35) = 41.15 V, so ⌊600 ÷ 41.15⌋ = 14 panels. In summer heat Vmp falls: at a 70 °C cell temperature and −0.35 %/°C, 31.4 V becomes 26.45 V, so at least ⌈100 ÷ 26.45⌉ = 4 panels keep the string above a 100 V MPPT minimum.
A published check (IAEI Magazine, John Wiles, 2017): a 68.2 V module at −0.245 %/°C and −40 °C reaches 79.04 V, allowing 7 modules on a 600 V inverter and 12 on 1,000 V. The calculator reproduces that result.
Series vs parallel
- Series adds voltages and keeps current the same: more panels per string means higher voltage, thinner cables and lower losses, up to the voltage limit.
- Parallel adds currents: each MPPT input has a maximum short-circuit current, so strings in parallel are limited to that ÷ the panel’s Isc. Parallel strings on one MPPT should have the same number and type of panels and face the same way.
- Operating current above the inverter’s maximum input current is not dangerous but the extra power is clipped; short-circuit current above the inverter’s Isc limit is not allowed.
- For conductors and fuses, NEC 690.8(A)(1) takes the sum of parallel Isc × 125 %; conductor ampacity then needs another 125 % (690.8(B)) — check cable sizes with the wire gauge calculator.
Choosing temperatures
For the cold limit, use the lowest expected ambient temperature — commonly the ASHRAE extreme annual mean minimum design dry-bulb for the nearest weather station. For the hot limit, take the hottest ambient and add about 25–35 °C for roof-mounted panels in full sun. Size the whole system first with the solar panel calculator, and for an off-grid battery bank use the solar battery calculator.
Solar String Sizing Calculator FAQ
How many solar panels can I put in series?
Divide the inverter’s maximum DC input voltage by the panel’s Voc corrected to your coldest temperature, and round down. For a 37.6 V panel at −10 °C on a 600 V inverter that is 14. The solar string sizing calculator also checks the MPPT window.
How do I calculate the temperature-corrected Voc?
Voc(cold) = Voc × (1 + coefficient × (lowest temperature − 25 °C)), with the coefficient in %/°C divided by 100. With −0.27 %/°C at −10 °C, the factor is 1.0945.
Should solar panels be wired in series or parallel?
Usually in series up to the voltage limit, because higher voltage means lower current and smaller cable losses. Add parallel strings only when one string reaches the limit and the MPPT current allows.
What happens if the string voltage is above the inverter maximum?
The inverter can be damaged and its warranty voided, and codes do not allow it. Remove a panel from each string.
What if Vmp falls below the MPPT minimum?
In hot weather the inverter cannot track the maximum power point, so output drops or the inverter stops. Add a panel to the string.
Does this replace the inverter maker’s string tool?
No — it is a check with your datasheet values. Use the maker’s tool and follow the electrical code that applies where you live.