Site survey, system design and component selection
Survey first, design second
A survey records roof covering, pitch, azimuth, rafter size and spacing (fixings must land on structure, not just sarking), roof age and condition, wind zone, shading from chimneys, trees, dormers and future growth, cable routes, inverter position, meter position and earthing arrangement (TN-S, TN-C-S or TT). Structural loading matters: a typical framed module is roughly 20–25 kg, so a 4 kWp array adds only around 15 kg/m² spread over the roof - modest, but the point loads at the fixings are not.
String design and component selection
Match module string voltage to the inverter MPPT window at both temperature extremes, and never exceed the inverter maximum DC voltage or the module maximum system voltage. Use the cold condition for maximum Voc and the hot condition for maximum current. Design current for cables and fuses is conventionally 1.25 × Isc, and any string fuse must not exceed the module's maximum series fuse rating. Check DC:AC ratio (commonly 1.1–1.3), inverter start-up voltage, export limitation requirements, and whether the DNO connection is likely to be G98 or G99. Planning may be needed for listed buildings, conservation areas or prominent front elevations.
Worked example - cold-temperature Voc
Module Voc = 40.5 V, temperature coefficient of Voc = −0.29 %/°C, expected minimum cell temperature −10 °C, 12 modules in series. Voc(max) = 40.5 × [1 + (−0.0029 × (−10 − 25))] = 40.5 × 1.1015 = 44.6 V per module, so the string is 44.6 × 12 ≈ 535 V. That string must sit below the inverter's maximum DC input (often 550–600 V) and below the module's 1000 V system rating.
Traps
Using STC Voc and ignoring temperature; sizing on Isc without the 1.25 factor; forgetting that Voc rises as temperature falls; assuming a north-facing or heavily shaded roof can be rescued by a bigger inverter.