People often wonder how solar professionals are able to determine the optimum size for a solar panel installation. The answer is they base it on the consumption of the premises during the peak, sunlight hours.
On the Energis website at www.energis.com.au, there is solar power calculator for the convenience of customers. On pressing ‘Calculate’, the calculator will quickly return your annual savings and the pay-back period of the proposed solar system. The first question asked is “Enter your quarterly bill in dollars” Given the bill value in dollars, together with the percentage of daytime use (the third question), they can deduct the fixed fees and then calculate the number of kilowatt hours (kWhs) consumed during the day, using the average peak rate tariff for your area. If you already know your quarterly consumption you can short-cut this process by answering question 2.
Clean Energy Council accredited solar designers have access to an online tool known as a solar estimator. This accesses data from the Australian Solar Handbook to estimate the generated output 
for a solar system located in any part of Australia. From this tool a table can be prepared showing the average annual output per day per kilowatt of installed solar PV. For instance, in Melbourne, a north-facing, unshaded, pitched roof holding 1kW of solar panels should generate an average of 4-4.2kWh per day. This figure will vary from about 2kWh/day in winter to 6kWh/day in summer. Remember these are average figures so there may be days when you generate 8kWh and others when you hardly generate anything. Remember that these figures are for a 1kW system. For a 10kW system the figures are ten times as much.
The calculator also uses another table of postcodes and zone ratings to calculate the dollar value of the Federal Government incentives known as STCs. These are subtracted from the retail price to provide a point-of-sale discount to the customer.
Finally the calculator divides the net retail price by the annual savings in dollars to determine the 
pay-back period.
Given the number of daylight kilowatt hours consumed per kilowatt of solar, the calculator will determine the size of the proposed system in kilowatts. Remember that these calculations assume that the premises are facing north, unshaded and at an inclination of about 30o. If your premises vary from these ideal conditions then a professional designer will need to modify your quote by using the CEC estimator for your location and conditions.
As mentioned above, the aspect (azimuth) of the system affects the generation output. In Melbourne, a solar panel array facing due west at 30o will lose about 14% of its output, east is 15%.
Also affecting the output is the inclination of the array – if they lie flat the loss is 14%. Depending on your location relative to the tropics, the optimum inclination could be anywhere from 15o to 40o.
However, if you have a roof with both east and west aspects, this could be an advantage, particulary if you are home for breakfast and the evening meal. Modern inverters can behave like two inverters in one by optimising the power from the west independently from the power from the east. These inverters have multiple power-point trackers.
Once you have a ball-park figure from the calculator, contact Energis Pty Ltd on 1300 782 217 to arrange an obligation-free appraisal.
