Top 7 Reasons Why You Should Consider Oversizing Your PV Array
Author: Darvin Tocmo Date Posted:30 January 2017
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) is known as oversizing a PV array or also referred as undersizing a PV inverter. It can be a valuable tool for system designers seeking to deliver a maximum amount of energy at a lowest possible specific cost. Below are the top 7 reasons on why considering to oversize your PV array really matters.
1. Specific cost of energy delivered will be lowered
By oversizing a PV array, a lower cost of delivered energy can be realised (lower $ or €/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 €/kW installed cost. This in turn yields in a lower specific cost of energy delivered by the system. An example comparison shows that 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.
2. Favourable energy output when installing inverters in limited space will be achieved
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, using 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.
3. 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: * 25º Celsius * Air Mass 1.5 * Insolation 1000W/m2 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 above25ºC. 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.
4. Better match the inverter to the PV array, in the event an inverter needs to be replaced
Sometimes if an inverter which is no longer within its warranty fails, it is not always possible to replace it with the same model inverter. In such cases an inverter of a different AC output power may need to be purchased and installed. By installing an inverter with a lower AC output power, the existing PV array could be better matched to the inverter’s capacity and the replacement cost to the system owner minimised.
5. Value of daytime energy to the system owner will be maximized
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.
6. Inverter costs will be reduced
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.
7. Make the most if the East-West PV arrays
PV arrays are often installed to maximise energy output and so are tilted towards the equator (south facing in the northern hemisphere, north facing in the southern hemisphere). Sometimes however the array plane available for installing PV modules with these ideal orientation conditions may not be as great as other less ideal array planes. In cases where the area available for east and west facing orientations is greater, a PV array might be split into some east facing strings and some west facing strings. Since an east and west PV array will peak in output power at different times of the day, it is possible to greatly oversize a PV array (e.g. install a DC input power equal to the inverter AC output power for EACH of the east and west PV arrays). Using an inverter’s sizing capability in such a way can deliver greater overall energy output, and a more levelled AC output each day. Conclusion There can be many different reasons to install an oversized PV array. Given PV array’s rarely operate at their rated peak power, oversizing a PV array can make better use of an inverter’s rated AC output and deliver a lower cost/watt system resulting in a lower specific cost of energy delivered ($ or €/kWh). When oversizing PV arrays it is important that an inverter’s critical input limits are never exceeded and to always comply with local electrical regulations and to use appropriately qualified system designers and installers.