Our 3.6kWh array has been in 5 years & saved £3.5k. It cost £5.3 k so the payback is 7.6 yrs as a crude non DCF calculation. It includes an Immersun switch to heat water.
It faces due south & has generated 20.5MWh.
The FIT & export payments make up £1.48k the rest I measure as the non exported (i.e. used in house power) x the relevant cost to buy off Shell.
The 4.2kWh batteries & inverter went in later in Nov 2022. They cost £2.95k as they were ordered pre covid so by the time they arrived they were cheap. They have discharged 1529 kWh. Based upon the cost of a unit from Shell Energy at the time of discharge they have saved me £494.
So the payback is 6 years. Have more battery capacity wouldn't help as the savigs aren't there & I feel this is optimised for us given the UK weather.
I agree both don't take into account the sudden jump in interest I could be earning on the capital because of higher rates or the need to replace inverters at some stage.
It is also for now though untaxed income unlike a bank acount. As evn retired I pay tax this is good news plus it makes me feel good to save the planet
I hope that helps.
