Unlike other power plants, it’s not uncommon to see solar PV projects have their capacity quoted in DC (direct current) terms; adding up all the ‘standard test condition’ (STC) capacities of the solar modules that make up the plant. However grid operators (and business planners) are most interested in AC capacity, which caps the saleable output of the plant.
Conversion from DC to AC happens in the plant’s inverter and the ratio of these two capacities, DC/AC, known as the ‘inverter load ratio’ (ILR), is rarely 1. More often, it will be something in the range 1.1 – 1.3 (i.e. DC capacity is 10-30% greater than the AC output).
Arriving at the right ratio trades off the clipping of output on the sunniest days (and hence some loss of revenue) against savings in capex (a smaller inverter). As well as those capex savings, choosing a smaller inverter can also optimise its efficiency more of the time. The charts below illustrate, in very simplistic terms, these tradeoffs.
Using modelled PV generation data for a hypothetical single-tracked 20MW PV system here in southern England, the top chart shows how increasing the ILR means more clipped hours (those during which the DC output of the panels exceeds the AC export capacity); and more revenue loss.
The lower chart adds up these revenue losses over an assumed 15-year inverter lifetime, selling at £50/MWh and including discounting (at 5%), then compares against the capex saved (assumed £70/kW) by choosing a smaller inverter. The optimum ILR is just below 1.2. Go above an ILR of 1.3 and revenue losses start to outweigh capex savings.
(NB. In practice, arriving at the optimum ILR for a solar power plant involves a much more comprehensive set of calculations and considerations than this simple illustration!)