Panel watts and inverter watts are different ratings
PV modules are rated in DC watts while an inverter is rated for AC output. The ratio between array DC capacity and inverter AC capacity is often called the DC/AC ratio or inverter loading ratio (ILR). NREL uses this ratio explicitly in its PV system modelling and notes that the appropriate value depends on location and design.
Why designers may oversize the DC array
A PV array rarely operates at nameplate output for every daylight hour. A higher DC/AC ratio can keep the inverter better loaded during lower-irradiance periods, but when available DC power exceeds what the inverter can convert, output can be limited or “clipped.” Some clipping can be a deliberate economic design trade-off; excessive clipping or operation outside electrical limits is not.
The limits that matter before the ratio
- Maximum DC input voltage: string open-circuit voltage must remain inside the inverter’s absolute limits, including cold-weather effects.
- MPPT operating window: string voltage must sit in the range where the inverter can track power properly.
- Input current per MPPT: module/string current and parallel strings must not exceed supported input values.
- Maximum permitted PV array size: use the manufacturer’s documented oversizing limit for the exact model.
- Local interconnection/export rules: the allowed AC export or inverter capacity may be constrained separately from array size.
A “1.2” or “1.3” rule of thumb cannot replace the exact inverter datasheet, module electrical data, temperature calculations, roof orientation and local interconnection requirements.
Use production modelling for the trade-off
NREL’s PVWatts and professional design tools can show how orientation, weather and system losses affect expected annual energy and clipping. The correct design is the one that stays inside equipment limits and produces sensible lifetime economics for the site.