🔌 PV String Sizing Calculator
Design a PV string against your inverter's voltage limits: temperature-corrected Voc and Vmp, the MPPT window, and the DC/AC ratio. Prevents over-voltage in cold weather and under-voltage in hot weather.
🔌 Module & Inverter
📊 String Design
For solar designers and installers sizing strings to inverter specifications.
How it is calculated
· Cold Voc: Voc_max = Voc × (1 + α × (Tmin − 25) ÷ 100), α ≈ −0.34 %/°C.
· Hot Vmp: Vmp_min = Vmp × (1 + α × (Tmax − 25) ÷ 100).
· Max modules = floor(Vmax ÷ Voc_max); min modules = ceil(MPPT_min ÷ Vmp_min); target ≈ Vmax × 0.85 ÷ Vmp.
Referenced standards
| Standard | Scope |
|---|---|
| IEC 62548 | Photovoltaic arrays — design requirements |
| NEC 690.7 | Maximum voltage of PV systems |
Worked example — 550 W module, 10 kW inverter
1. 550 W module (Voc 49.5 V, Vmp 41.5 V), 10 kW inverter (Vmax 1000 V, MPPT 90–520 V), Tmin −10 °C, Tmax 60 °C.
2. Cold Voc = 49.5 × (1 − 0.0034 × (−35)) = 55.4 V; hot Vmp = 41.5 × (1 − 0.0034 × 35) = 36.6 V.
3. Max modules = floor(1000 ÷ 55.4) = 18; min = ceil(90 ÷ 36.6) = 3.
4. Recommended ≈ round(1000 × 0.85 ÷ 41.5) = 18 modules per string.
FAQ
How do I size a PV string against inverter voltage limits?
The string's temperature-corrected open-circuit voltage Voc must stay below the inverter maximum (commonly 1000 V or 1500 V), and the operating voltage Vmp must stay inside the MPPT window. Use the module temperature coefficients: Voc rises roughly 0.25-0.35%/deg C below 25 deg C. A 550 W module with Voc 49.5 V allows about 18-20 in series at -10 deg C on a 1000 V inverter.
What DC/AC ratio should a PV system use?
DC/AC ratio is array DC capacity divided by inverter AC rating. Typical values are 1.1-1.3 for most sites, up to 1.5 in high-latitude or high-albedo locations, to capture more energy when modules rarely reach STC output. Over 1.3-1.4 the inverter clips more peak power, and the marginal energy gain falls below the extra module cost.
Why does temperature affect PV string voltage?
Solar cell voltage drops as temperature rises. Module datasheets give temperature coefficients, typically -0.29%/deg C for Voc and -0.34%/deg C for power. At -10 deg C the voltage is about 10% above the 25 deg C value, so strings must be short enough not to exceed inverter limits in winter; at 70 deg C the voltage is ~13% lower, so strings must stay above the MPPT minimum.
How many modules per string is typical for a 550 W module?
With a 550 W module (Voc ~49.5 V, Vmp ~41.5 V) on a 1500 V inverter, strings of 28-32 modules are common; on a 1000 V inverter, 18-22 modules. The exact count depends on the minimum site temperature and the inverter MPPT range, so the calculator checks both the -10 deg C Voc and the 70 deg C Vmp bounds.
What is the difference between 1000 V and 1500 V PV systems?
1500 V systems allow longer strings (fewer combiner boxes, less cable and DC loss) and cut balance-of-system cost about 3-8% for utility plants, but need 1500 V-rated modules, inverters and protection. 1000 V remains standard for rooftop and small C&I. The calculator's string designer supports both voltage classes.
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