Our Solar PV Journey

UPDATE: I have made a cost calculator spreadsheet (see link at bottom of page). I have posted this graph elsewhere. It makes it easy to estimate a notional "cost per cycle" for a LiFePO 4 battery. It can be instructive to calculate what the "cost per cycle" means when viewed as a "per kWh" value. When you calculate your "cost per cycle per kWh" don't forget to include factors such as: Charge/discharge efficiency (say 87% to 90%) Depth of discharge (obviously reduces the effective capacity) Average capacity degradation over the life of the battery (say degradation to 80%-90% at "half spent" point) It is easy to see that the cost per kWh of passing energy through your batteries can easily be in the region of 10p-15p per kWh of "delayed consumption". Knowing that figure helps to put a priority on storing the various "types" of kWh that you choose to store. It may change your perspective on exporting ...

Using the Octopus Compare app, I noticed that Agile has been beating Cosy for our usage profile. This has been the case since March, despite our usage being tailored to the Cosy tariff. I have switched to the Agile tariff and we have made some minor changes to our usage to, potentially, save even more. Points to note: The cap on Agile is technically 100p per kWh but, in practice Octopus have been capping it at 32p. This should not be a problem as long as we keep an eye on the rates. The switch to Agile was initiated on a Web page and happened very quickly (less than 24 hours, and retrospective to midnight on the day the email was received). Octopus allow you to switch frequently, but impose a 28 day limit on frequency (which seems very reasonable). Agile would be cheaper than Cosy even if we don't change our usage. Right now, Agile has long periods well below 32p. Cosy had only two 3-hour periods. Cosy sits at 32p for long periods, Agile (for now, anyway) spends a...

It seems to me, reading the forums, that there are quite a lot of people who are charging their batteries on the cheapest electricity that they can buy and exporting it back to the grid when the export rate is highest. I can see how, in principle, one might make money doing this. However I do wonder whether people have really considered all of the costs. Using round figures to keep it simple, imagine that you buy a 5kWh battery for £3000. Depending whether you get 3000, 6000, or 9000, or 12000 useful cycles out of it, this battery is costing you between £0.25 and £1.00 per charge-discharge cycle. Each charge-discharge cycle of a 5kW battery will earn me five times the export price per kWh, minus five times the import price per kwh, minus the battery cost per charge-discharge cycle*. I think that I would want to be very sure which cost-per-cycle figure was real before embarking on this idea. And that is before considering things like the "Time Value of Money", TVM - the idea...

Well, March was a bit rubbish! The prediction was for 465.5kWh but we only generated 207.7kWh. Let's hope April is better. We are more than half way to the predicted 601kWh for April (on Apr 15th) so it does look closer than March. CORRECTION!!!! The March figure was only for Mar 18th onward - so not so bad after all.

I think that, for quite a lot of folks, battery life is a major consideration. I have posted this graph about lithium ferro-phosphate (LiFePO 4 ) battery life elsewhere, so apologies if you find this repetitive. The graph is from a particular manufacturer, but it is likely to be broadly representative of any brand. You can read the graph for yourself but, broadly speaking, what it is saying is that (for the right-hand half of the graph where most of us will be operating) discharging at 1/2C instead of 1C could reasonably be expected to increase your battery life by something in the region of 40%. Reducing DOD from 100% to 90% could increase the life by about one sixth. Going from 100% DOD to 80% DOD increases it by about one half. Going from 100% DOD to 70% DOD roughly doubles it. I would guess that other factors, such as temperature, would have significant effects on battery life. Some users will be less cost-sensitive than others. I imagine that a business in South Afric...