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Solar Panels Power System Payback Calculator

This payback calculator will help you understand the factors involved in purchasing a Solar Panels PV Power System. Before you start you will need:

  • accurate solar power system quotes
  • the pitch and orientation of your roof (assuming panels are mounted flat on roof, else the array tilt and facing direction)
  • your existing tariff rates
  • your anticipated solar tariff rates (obtained from your electricity provider)
  • your average daily power usage§ during Peak ( 2pm-11pm), Shoulder (7am-2pm) and Off-peak (11pm-7am) tariff times.

You can compare different scenarios to find the best payback solution for your situation, for example you can:

  • Compare two different quotes and systems of different quality (degradation rate)
  • Compare two different system sizes at different installation angles (installation angles can have a large effect on payback times)
  • Determine if a tilt frame is financially viable - simply compare two different install angles and upfront costs
  • Determine the financial viability of the newer grid connected battery storage systems coming on the market - simply set the "power export balance" values to 0.001

For more details about this calculator, scroll below.

* Use a deviation measured from Solar North (not Magnetic North) - for this use Google maps. Note: East or West facing arrays significantly change the percentage of power collected during peak and shoulder times, which effect Net feed-in systems, see the System Results section to change this percentage.

^ Note, these rates are expected to change over to smart meter Time of Use (TOU) tariff rates in the coming years, use the input box directly below these rates to choose when you expect this to happen for your area.

~ After the specified number of years this will change over the old tariff rates into the new smart meter TOU rates for the non-solar home projections.

§ When averaging your daily power usage keep in mind that most electricity providers allocate Shoulder tariff rates for weekends and public holidays, hence this power use needs to be included in the Shoulder tariff average and not the Peak average.

¤ Some providers charge extra for homes with solar systems.

† After the initial solar feed-in tariff period the calculator will use the subsequent feed-in tariff you specified (power inflation adjusted). Note: for this calculator the initial solar feed-in rate is contractually fixed and is not subject to the power inflation rate, while the subsequent feed-in tariff is power inflation adjusted (reflecting pricing movements to the wholesale power rates).

†† Some solar companies require that you periodically service your PV system or you will void the Warranty.

‡ Yearly average per day for a new system.

** The derate factor sets the overall efficiency of a system in converting from the specified DC rating of the panels to AC power, typically 75% to 80%.

§§ Typically 0.5% for good systems and 1.0% for average systems.

¥ This specifies the average breakdown of solar power collected during sun hours between the shoulder and peak tariff times, i.e. 70% during shoulder 30% during peak. It's relevant for Net feed-in systems as it has an effect on the balance of power you'll feed-in at specific times. The 70% values assumes a North facing array, a 2pm changeover from shoulder to peak tariff and shoulder tariff times during weekends. Note: this percentage will be significantly lower for West facing systems, you will need to approximately estimate this percentage by taking into consideration the change over time during weekdays and the weekend tariffs rates from your supplier.

¤¤ For net solar feed-in systems the "Power export balance" value under the "Advanced Details" section alters the percentage of solar power exported to the grid during the initial and subsequent solar feed-in periods (see below for more info).



Balancing Your Net Feed-in Percentage

If you're on a Net feed-in tariff, you need to pay attention to the percentage of power feed into the grid as this can have a large effect on payback times. The calculator approximates the percentage exports using a complex function integrating over a typical domestic load curve, however this can be altered as follows.

Power export balance Amount Exported Export Situation
<0.01 0% export - nearly all solar power is used Equivalent to a battery storage system
0.5 Biased toward using available solar power Domestic solar usage priority
1.0 Balanced 50% export Typical domestic scenario
5 Biased toward exporting solar power Domestic export priority
>1000 100% export - nearly all solar power is exported Equivalent to Gross feed-in

Hence for example if you received a good initial feed-in tariff and you think you'll achieve a better feed-in percentage, for example by being careful when you use power, you can increase exported power by setting the "Power export balance" to a value between 2 - 5. On the other hand you may have a low subsequent solar feed-in tariff (after the initial feed-in tariff expires) and a high peak and/or shoulder tariff rate and hence at that time you may choose to shift some electricity usage to coincide with good solar days, to minimize your export feed-in percentage and maximally off-set high peak/shoulder tariff rates. In this case you can set the subsequent "Power export balance" to 0.5 - 1.

Details About this Solar Calculator

  • This calculator sums all initial outlays, earnings and costs and subjects the balance to interest rates. Hence it assumes both debts and credits, incur/earn the same relative amount of interest. This is equivalent to a mortgage scenario where initial outlays incur interest, while in later years cost savings are used to pay down the mortgage faster. Tax on solar feed-in earnings is not included. The graph shows the net difference in debt position between solar and non-solar homes.
  • This calculator assumes the same tariff rates will be charged for non-solar homes as are charged for solar homes when the anticipated smart meter time tariff rates are mandated in Australia.
  • This calculator uses average daily solar insolation data from the Bureau of Meteorology (BOM) and angle corrections fitted to tables supplied by the Clean Energy Council (CEC) in the document "Grid-Connected PV Systems - System Design Guidelines", Issue 3 July 2007 November 2009. Please note yearly variations in insolation can be substantial.
  • Return On Investment (ROI) can be worked out by adding the Initial Outlay (including cumulative interest paid on it to negate the interest charged) to the Net Benefit chart line.

Useful links

  • Solar feed-in rates
  • Google maps (use to estimate your roof deviation from Solar North, use positive values up to 90° to specify East deviations and negative values up to -90° to specify West deviations)
  • ATA Forums

Disclaimer: This free to use calculator is provided as is. While we have endeavoured to make this calculator as accurate as possible we take no responsibility for accuracy of the assumptions, data or advice it provides. This calculator should be cross checked with other information and should not be used as the sole means in making your purchasing decisions.