PrefInvestor wrote:Well funduffer your calculations seem to completely exclude consideration of the solar panels other than to say it’s not worth using them !.
We have 4KW solar system and it can fully charge our 8.2Kw battery (at zero cost),
Not zero cost, you get paid for your exported energy that went into the battery, and wouldn't have earned interest on the cost of the panels?
I read FD as saying the best thing was to export the solar and charge the batteries overnight, IF you could get overnight supply cheaper than the daytime export rate.
PrefInvestor wrote:Using a cheap off peak tariff might be good in the winter months, though the new Octopus Flux tariff you can get paid 19.3p for your export and 31.4p during the 16:00-19:00 peak time slot. There are issues though with smart meters, loss of inflation proof FIT payments and the fact that it’s a variable tariff and can be varied by them any time they see fit – so we are still on their standard variable tariff (SVT) tariff ATM.
Hmm, now that's where battery could be economical if you were allowed to charge overnight on a cheap flex rate circa 7p and export at the peak at 41p, then your 8kW battery would make you £870 pa, probably a good 10% return, given the right software. You would still use the battery as a buffer for the solar (see below) but making sure it was charged fully by 4pm.
What you aren't doing in your maths is decoupling the battery and solar element paybacks. On most tariffs batteries increase the payback time compared to solar alone and a decent export price. The complication is for the brief periods you draw high current, when you are still importing at 30p/unit and then the kettle boils or the hot water switches off and you are back to exporting. In these periods the battery decouples you from the supply so you consume more export rate energy then without it.
Paul