Well let’s do the maths for me. We’re a household of 3 for now.
So I buy electricity for the immersion at 7.5p kwh and export at 4.1 kwh. So the lost “free” energy costs me 3.4p kwh. (FWIW gas costs about the same until October, albeit with higher losses in the process).
£400 / £0.034 = 11,767 kwh of energy needing diverted to break even.
My HW takes about 4kwh to heat each morning.
So 2,941 cycles / 365 = 8 years assuming I could fully solar charge it every day which I won’t be able to.
Let’s assume excess solar generation for 2/3rds of the year and we’re now up to 12 years.
If my maths is right it won’t pay for itself before the end of its likely life.
Your maths is right, but as ever with solar it requires certain criteria and we never know if they will remain. Plus like everything right now, supply issues currently exist. If they suddenly become a total no brainer they will become hard to get.
Heating water with the 7.5p tariff requires that tariff and its a one off pretty much as far as pricing is going.
Also its got a barrier to entry (as discussed over and over assuming that barrier is tested correctly).
For me its £335. I would also need a time controller for the immersion so its £35ish for that if I wanted to use 7.5p energy.
Also your usage would seem low for a tank of hot water 24/7. I have a modern decent tank (megaflow) its listed as losing (I forget exact number) 1-2kwh per day.
It takes just under 4kwh to heat 75 litres from 15-60 and thats ignoring the tanks losses.
We use about 8kw (via gas) for a couple and this time of year its only HW its doing. We aren't heavy users either (its only for bathing basically, no washing up etc), although other half will probably average 1 bath a week plus 7 showers where as I only shower never bath really.
My plan right now, although it has to evolve is to do similar, heat my tank via the 7.5p units (assuming I get that deal, don't switch until tomorrow) then iboost once its installed any excess units over the top of my battery storage and usage. Should top up the tank a bit more before it fully heats in the evening on higher generation days, and hopefully mean very low export to grid. If you dont have battery storage the units they can consume will likely be a lot higher. I was talking to someone the other week or was exporting a lot this time of year, they check out and bought a diverter within a few days now they just need to wait for delivery and get it installed.
Availability of tariff, usage, incoming water temp, actual generation from panels, battery storage, plenty of variables that affect individual payback