Grid-Tie Inverter Sizing Calculator
Enter array and inverter ratings plus module temperature data to get the DC/AC ratio, clipping estimate and valid string lengths.
Step-by-step
- Enter valid values to begin.
The clipping figure is a simple planning model, not a simulation: each day is a half-sine of output whose peak is the DC/AC ratio × 0.92 (typical peak array output after temperature, soiling and conversion loss) scaled by three sky types (1.0 clear, 0.6 partly cloudy, 0.25 overcast) and three seasonal peak factors (1.0, 0.93, 0.82); the site class sets how often each sky occurs. Energy above the inverter rating on each day is summed over the year. It ignores cloud-edge over-irradiance and tracker or east-west layouts; use PVsyst, SAM or PVWatts for a bankable number. String limits use the temperature-corrected Voc and Vmp and the 125% Isc factor of NEC 690.8. The 1.1–1.4 DC/AC range is common practice, not a code limit. Verify the NEC edition, utility interconnection rules and local AHJ.
Grid-Tie Inverter Sizing: DC/AC Ratio and the String Window
Core Engineering Principles
An inverter is rarely sized equal to the array. A panel reaches nameplate only under a bright, cold, well-aimed 1,000 W/m², a few hours a year, so an inverter rated to the full kWp spends its life lightly loaded. Designers therefore choose a DC/AC ratio of about 1.1 to 1.4. The inverter clips the rare peaks, the cost per delivered kWh drops, and the energy lost is often 1–3%. Beyond 1.4 clipping grows quickly. Our estimate is a simple daily curve model, fine for a first pass; use an hourly simulation for a bankable number.
The second job is voltage. Every inverter has an MPPT window, and the string must stay inside it all year. The cold morning sets the upper limit: open-circuit voltage must stay below the maximum DC input, and the cold maximum-power voltage below the top of the MPPT window. The hot afternoon sets the lower limit: cell temperatures near 70 °C drop Vmp by about 15%, and below the MPPT minimum the inverter stops tracking just when you want output. Finally, check current: with the 125% factor one 13.7 A string needs 17.1 A, so a 22 A input takes only one.
Nmin = ⌈ MPPTmin / Vmp,hot ⌉ • Strings/MPPT = ⌊ Imax / (1.25 × Isc) ⌋
V(T) = V25 × (1 + β (T − 25))
NEC & Standard References
IEEE 1547 sets the interconnection and interoperability requirements for distributed resources, including anti-islanding and voltage and frequency ride-through. UL 1741, with its supplement for grid support utility interactive inverters, is the listing standard used in North America, and IEC 62109 covers inverter safety. NEC Article 690 requires temperature-corrected voltage (690.7) and 125% circuit sizing (690.8), and NEC Article 705 covers interconnection. Verify the adopted code edition and the requirements of the local authority having jurisdiction.1. DC/AC = 9.6 / 8 = 1.20.
2. Clipping model gives about 0.7%, roughly 96 kWh of 13,440 kWh per year.
3. Cold Voc = 37 × 1.098 = 40.63 V; cold Vmp = 34.69 V; hot Vmp = 26.26 V.
4. Max = min(14, 14) = 14; min = ceil(120 / 26.26) = 5. A string of 12 is valid (488 V cold).
5. 24 modules = 2 strings, one per MPPT; each draws 1.25 × 13.7 = 17.1 A, within 22 A.
- Cold voltage is the hard limit. One module too many can exceed the maximum DC input on the first cold morning and destroy the input stage.
- Check the hot minimum. Short strings on a hot roof can drop out of the MPPT window in summer afternoons.
- Respect the input current rating. Extra parallel strings overheat connectors.
- Treat the clipping figure as an estimate. Confirm with a simulation.
- Use a listed inverter. Anti-islanding must work, and the AC side needs a lockable disconnect.