String Sizing at −18 °C with Bifacial Gain
A customer signs for 20 × 550 W bifacial modules on a ground mount, all on one south plane, feeding a 7.6 kW single-phase string inverter with two MPPTs. Module data: Voc 49.6 V, Vmp 41.7 V, Isc 14.0 A, Imp 13.2 A, βVoc −0.25%/°C. Inverter data: max DC input 11.4 kW, max input current 20 A per MPPT, max Isc 30 A per MPPT, MPPT range 100–500 V, startup 125 V, max DC voltage 600 V. Site record low is −18 °C; design cell temperature at record high is 70 °C. The design assumes 5% bifacial gain. The draft design is two strings of 10, one string per MPPT.
1. Find the maximum and minimum modules per string and confirm the string voltage window.
Read the worked answer
Cold-weather open-circuit voltage governs the maximum string length.
Voc at −18 °C with a 25 °C reference and −0.25%/°C: factor = 1 + (−0.0025)(−18 − 25) = 1 + 0.1075 = 1.1075 Voc(cold) = 49.6 × 1.1075 = 54.9 V per module
- 600 V ÷ 54.9 V = 10.9 → 10 modules maximum (11 modules = 604 V, over the 600 V limit).
Minimum length is set by the hot-cell Vmp and the MPPT window. Approximating Vmp with the Voc coefficient: Vmp(70 °C) = 41.7 × [1 + (−0.0025)(70 − 25)] = 41.7 × 0.8875 = 37.0 V per module
- 10 modules = 370 V → inside 100–500 V and well above the 125 V startup. Even 3 modules (111 V) clears the MPPT floor and 4 (148 V) clears startup, so the top end is the binding constraint, not the bottom.
Conclusion: 10 per string is legal but leaves only 51 V (8.5%) of headroom to 600 V. Anything that raises Voc - a colder-than-record event, or a different module - breaks it. Verify the AHJ's required design low temperature (record low vs. ASHRAE 2% low) before locking the design.
2. Evaluate the proposed 2 × 10 configuration on current, DC/AC ratio, and clipping, and state whether you would sign it.
Read the worked answer
Voltage: 10 × 54.9 V = 549 V cold < 600 V ✓
Current per MPPT: one string per input gives Imp 13.2 A (limit 20 A) and Isc 14.0 A (limit 30 A) ✓. Note that paralleling two strings on one MPPT would be 26.4 A Imp against a 20 A limit - not allowed on this inverter.
DC input power: 20 × 550 W = 11.0 kW vs. 11.4 kW max DC input ✓ (thin margin).
DC/AC ratio: 11.0 ÷ 7.6 = 1.45. With 5% bifacial gain the array's effective output exceeds the inverter's rating for several hours on cool clear days, so the inverter will clip. At 1.45 in a good-sun resource, annual clipping losses typically run on the order of 1%, which is usually a good economic trade - but it is a trade the customer should have been shown, not discovered.
Verdict: sign it, with two conditions - (1) confirm the AHJ accepts the design low temperature used; (2) disclose the clipping in the proposal. The configuration is valid only because each MPPT carries exactly one string; the layout has no slack for a third string.
3. After signing, the customer asks to add 4 more modules for 24 total. What changes and what do you tell them?
Read the worked answer
You cannot get to 24 on this inverter. Show them why with numbers.
- 24 modules in two strings = 12 per string → 12 × 54.9 V = 659 V cold, well over the 600 V maximum. Not an option at any temperature on this inverter.
- 10 per string is the ceiling, and only one string can land on each MPPT (26.4 A exceeds the 20 A input limit). So the hardware maximum is 2 × 10 = 20 modules, 11.0 kW.
- Adding 4 modules therefore requires a different inverter: one with a third MPPT, a higher DC voltage rating, or a higher per-MPPT input current paired with a re-check of the 600 V/1000 V limit and the 690.7 voltage calculation.
What to present:
- Keep 20 modules on the existing inverter (no change order).
- Quote an inverter swap and re-run the string voltage, current, and 120% busbar/OCPD checks - the service panel and AC disconnect may also change.
- Re-run the permit and interconnection application; the approved AC size on the interconnection agreement is what the utility sized the feeder for.
Never add modules to a string to please a customer without redoing the cold-temperature Voc - that is the single most common way PV systems get designed over 600 V.