Pay Back - TonyM1958/FoxESS-Cloud GitHub Wiki

Worked example of how I calculate system payback and return on investment (ROI).

Inverter Data

If you look in the Fox ESS app or web site and look at your Analysis Statistics, you will see Total. These are the values your inverter energy counters have registered since it was installed:

fox values

I also track the data via Home Assistant, so I can use the values I get there to validate these numbers:

ha values

The numbers I am interested in are:

  • Imported: 7.90MWh used to date.
  • Exported: 4.95MWh exported to date.
  • Consumed: consumption (measured by Home Assistant) is 16.75MWh and the number reported by Fox is 18.10MWh. This is because the Fox data is calculated and includes system losses of 1.35MWh, giving an overall system efficiency of 92.5%. If you only have the number from the Fox web site, multiple by 0.925 to allow for these losses.

Note: if your solar is AC coupled, the load energy provided by Fox ESS may not show what you consumed correctly.

Payback

If I didn't have solar and battery, I would have consumed 16.75MWh. Using a standard variable tariff, this would have cost around 26p / kWh so my import cost to date would have been £4,355.

Using a smart tariff and solar:

  • my import was 7.90MWh and most of this was off-peak rate at 8.5p/kWh so my actual import cost was £671.50.
  • my export of 4.95MWh was at around 14p/kWh so I have been credited £693.

Overall, my pay back to date is £4355 - £671.50 + £693 = £4,376.5

You can make this a lot more complicated by trying to allow for changes in tariff but then you should allow for inflation, cost of money etc. For simplicity, just work out the payback using today's typical prices.

Return on investment

My system has been installed for 1,162 days and the cost including installation and VAT was £11,995.

The payback to date is £4,376.5 or an average of £3.77 per day. Recouping the original cost will take 3,184 days (8 years, 9 months).

Self Sufficiency

While you are looking at the data, you can also work out your self sufficiency to date. There are 2 versions:

  • Absolute Self Sufficiency: is based on the ratio of what you imported and consumed.
    1 - 7.90 / 16.75 = 52.8%
  • Net Self Sufficiency: takes account of what you also exported, as this offsets what you imported.
    1 - (7.90 - 4.95) / 16.75 = 82.4%
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