Inverter and grid connection
Inverter sizing ratio (DC/AC)
- where
- DC/AC ratio
- array peak power (kWp)
- inverter rated AC output (kW)
- array oversizing (%)
Explain this equation
Compares the array size with the inverter rating. Use it to check an inverter choice or explain clipping to a customer.
P_dc is the array's STC rating and P_ac what the inverter can put out. Arrays rarely reach their STC rating in UK conditions, so a modest oversize keeps the inverter working nearer its best for more of the day. X shows the oversize as a percentage.
A plain ratio of the two ratings.
Sense check: 6 kWp on a 5 kW inverter is 1.2, 20% oversized. Around 1.1 to 1.3 is common; a little energy is clipped on the brightest days.
Watch out: check the inverter's maximum DC input power and current, which set the real limit on oversizing.
How much array peak power sits behind each kW of inverter output. A moderate oversize is normal because modules rarely reach STC output in the UK.
Enter your values to see the result.
Good to know
UK arrays rarely produce their full STC rating, so ratios a little above 1 are common and lose little energy to clipping. Always check the inverter datasheet's maximum PV input power and maximum input current per MPPT, which are hard limits. The inverter AC rating, not the array kWp, is what counts for G98/G99.
Common mistake
Treating the array kWp as the export capacity for the DNO application; G98/G99 use the inverter's rated AC output.
Limits
Ratio only; does not estimate clipping losses. Use design software or PVGIS hourly data for that.
Sources
- IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd edition (2022)
- ENA Engineering Recommendation G98: Requirements for the connection of fully type tested micro-generators (up to and including 16 A per phase) in parallel with public LV distribution networks
- Module manufacturer datasheet: STC values (Voc, Vmp, Isc, Imp, Pmax), temperature coefficients of Voc, Pmax and Isc, NOCT/NMOT and dimensions
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.