The true cost of battery storage?
I instinctively try to use as much of my solar energy at the time that it is generated and try to export as little as possible. But, at the risk of being controversial, I would like to propose the idea that it is not necessarily a bad idea to export a kWh for 15p and then later re-import it (or one very much like it!) at 20p or even 25p.
When calculating the cost of using batteries there are factors that, I think, we are inclined to overlook.
When you calculate your "cost per cycle per kWh" you could include factors such as:
o Charge/discharge efficiency (say 87% in total for charge then discharge)
o Depth of discharge (increases life cycles but, obviously, reduces the effective capacity)
o Average capacity degradation over the life of the battery (say degradation to 80%-90% at "half spent" point - probably not linear, though)
It is easy to see that the cost per kWh of passing energy through batteries can easily be in the region of 10p-15p per kWh of "delayed consumption" averaged over the life of the battery. The "cost of using your battery" is, at best, a useful concept rather than an experienced expense. After all, battery replacement may be many years away - but I think that it helps to put things in perspective a bit. BTW, I'd love to be proven wrong on the 10p-15p figure if anyone can refute it. The battery would be a lot more useful if that figure were a lot lower! If you choose DoD and charge/discharge rate for >9000 cycles, you may not need to replace the batteries in the lifetime of the panels and this all becomes academic (for 1 cycle per day). However, there are processes involved that mean that batteries also undergo "calendar ageing" independent of cycles of usage - so treating your batteries very gently will not necessarily make them last forever!
Prototype calculator at: bit.ly/3LDCBCA