Oversizing a PV array, also referred to as undersizing a PV inverter, involves installing a PV array with a rated DC power (measured @ Standard Test Conditions) which is larger than an inverter’s rated AC output power (i.e. DC @ STC > AC). It can be a valuable tool for system designers seeking to deliver a maximum amount of energy at a lowest possible specific cost. Reasons for oversizing PV arrays and important factors to consider are summarised below.
1. Make better use of the inverter’s AC output
PV modules have ratings which define how they will operate. Their power, current and voltage ratings are all defined at Standard Test Conditions (STC). STC are defined as operating at:
Air Mass 1.5
However it is obvious that a PV module would very rarely be subjected to these conditions under real world operating scenarios. Operating conditions can vary throughout the day and temperature can greatly impact the output power of a PV array. As the temperature of a PV array increases, its voltage and power will decrease. Typically at solar noon (maximum solar irradiation), a PV array will have its STC output power de-rated by between 20-25%, due to the array operating above 25ºC. That would mean that at solar noon on a clear sunny day a 100kWp PV array would probably be generating approximately 77kW. That’s 23% of the array’s rated power not being delivered!
If a PV array will never deliver its rated power, sizing an inverter to match that array’s typical peak power can make better use of the inverter’s AC output capacity.
2. Lower the specific cost of energy delivered
By oversizing a PV array, a lower cost of delivered energy can be realised (lower $ or S$/kWh). Oversizing a PV array will increase the cost of PV modules and array racking for a system. However, since this can be achieved without necessarily increasing either the quantity of rating of other balance of system components, the increased energy production is achieved with a lower $ or S$/kW installed cost. This in turn yields in a lower specific cost of energy delivered by the system. By oversizing a PV array with a 5kW inverter, the annual energy yield of a system can be increased by over 28% for only a ~10% increase in the total cost of installation.
3. Reduce inverter costs
By oversizing a PV array, the DC energy output of that array can better match the rated AC power of an inverter. This means that an inverter with a lower AC rating (hence lower cost) can be used. Consequently, this can decrease the relative cost of inverters compared to the total system cost.
4. Achieve favourable energy output when installing inverters in limited space
Inverters sometimes need to be installed in specific locations, either due to constraints from the owner or local electrical regulations. This may mean it would not be possible to install as many inverters at a site as would be desired for a perfectly sized system. However by oversizing PV arrays, it may be possible to achieve almost the same annual energy output with fewer installed inverters. For example, a 100kWp PV array with three STP25000TL-30 inverters (i.e. 75kW of inverters) would only produce ~2% less annual energy compared to the same PV array with four STP25000TL-30 inverters (i.e. 100kW of inverters). This means that there is only a ~2% lower energy output for 25% fewer inverters.
5. Maximise the value of daytime energy to the system owner
For a business which operates during normal business hours, the value of daytime energy from their PV system might be different depending on individual circumstances. The PV output may be used to avoid peak-capacity grid charges or to offset constant loads which may be operating on the site. In such cases, oversizing a PV array could provide a business with greater certainty in their energy costs, especially given the low price of PV modules in today’s market. By oversizing a PV array, the inverter can reach its rated AC capacity earlier in the day, and continue operating at that point until late in the afternoon as shown in the following graph.