Optimised PV size

Optimised PV size

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I’m often asked to help determine whether a proposed PV system really will produce the savings that has been claimed, and whether this is an optimum solution. Too often I see back-of-the-envelope estimates based on monthly or annual electricity bills that don’t give the full picture. To really understand, you need to dig a bit deeper.

Obtaining the 15 or 30 minute electricity meter interval data and matching that up with an output profile for solar PV, adjusted for location, roof orientation etc is the only robust method of predicting electricity bills with and without solar PV. A good model, and a bit of expertise, will allow an optimisation calculation to show the actual returns after considering all the variables.

Modelling of PV in an energy system normally shows that a small system, which displaces 100% purchased energy has good financial returns. This is because 100% of the electricity produced offsets electricity purchases and due to the daytime production 5/7ths is during peak tariff periods. As the PV size is increased, the output will displace more imported electricity, and the financial return improves due to installation cost scale economies. At larger sizes, the PV output begins to exceed site consumption and export to the grid. While the import peak electricity may be 15-20 c/kWh, exported electricity is 6-8 c/kWh. This might lead you to conclude that the best system size is one where the peak solar output just equals the site load. However, I generally find that the optimum size is slightly larger.

PV size vs paybackIn the example for the graph, the actual site peak demand was 150kW. The modelling showed that a larger PV system at 250kW would better meet their objectives.

Critical factors include:

  • whether the network company will allow energy to be exported to the grid
  • site consumption
  • weekend consumption
  • energy rates
  • tariff structure (particularly demand tariffs)
  • exported energy buy-back rates from the retailer
  • roof orientation
  • whether the size of the PV array is eligible for Small-scale Technology Certificates (STC’s) or Large-scale Generation Certificates (LGC’s)


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