System Expectations
Output
The maximum instantaneous output from a solar power system should be approximately 85% of its rated power (kWp). This is because at elevated temperatures the panels produce less power, and there are further losses in the wiring and inverter. This is taken account for in most calculations of expected output energy yield (kWh/year).
The available solar energy falling on a PV system varies with location. The solar radiation resource can also vary by 10% from year to year.
Annual yield also varies with panel inclination (angle from horizontal), orientation (degrees from solar north), local weather, shading, and other site-specific factors including length of wiring run and temperature of inverter.
For greatest ("optimal") annual power generation, the PV system should generally face due north and be inclined at same angle as the local latitude – commonly a 30° pitch is used.
Small variations from optimal orientation and inclination don't significantly reduce the amount of power output
For this reason and for sake of neat appearance, panels are typically installed flush-mounted to the best available roof.
Shade has a much greater impact on system output. Due to the nature of PV panels, a small amount of shade can cause a large reduction in system output. The entire solar array should be un-shaded from 10am – 3pm year round in order to have acceptable performance.
Local conditions such as shade from nearby trees or hills may therefore mean the system installer chooses an orientation other than north-facing.
Payback
The payback for a system depends upon its upfront cost and its revenue
Upfront cost varies significantly from company to company, and also depends on the REC Zone of the installation location. Companies using Chinese panels and, or inverters have lower upfront costs however, Future Sustainability represents neither of these products and suggests buyer beware when dealing with non brand name product.
Revenue depends on the import electricity price and any net or gross feed-in tariff, plus system performance
Payback for customers in states with net feed-in tariffs depends upon the house's pattern ("profile") of electricity consumption.
Customers that don't use much power during the day will have quicker payback than customers with high daytime power consumption.
Paybacks from 5 - 10 years in south-east Queensland are possible on 1.5 kW systems for $38 REC/STC prices: 5 year payback if the entire amount of power is export (unlikely), 10 year payback if no power is export at all.
The difference between residential and commercial electricity consumption patterns means that the above results do not usually apply to PV systems on commercial buildings.
Because of the gross FiT, paybacks in as little as 3-6 years are possible in NSW (6-7 years in the ACT), which are not dependent upon household power consumption. Minimum paybacks in these states occur for 1.5 kW systems, though NSW customers with a short FiT should definitely consider a larger system as insurance against future price rises.
It is always worthwhile upgrading an inverter if there is available un-shaded roof for future panel expansion. An inverter with slightly higher capacity has a small incremental cost increase, whereas a second inverter has a comparatively larger cost. Some limits on panel-inverter compatibility apply.
