Calculating the cost of using your batteries

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 LiFePO4 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 a kWh for 15p and then later re-importing it (or one very much like it!) at 20p or 25p.

Certainly, although storing 0p solar energy for later use will probably always be worthwhile, storage of 20p (or even 15p) electricity for later use may be a false economy.

In reality, battery aging is a bit more complicated than I have implied here. A LiFePO4/Carbon battery exhibits "calendar aging" (i.e. aging regardeless of whether it is charged and discharged) on top of there being a limit on the number of charge-discharge cycles. Also, the capacity does not necessarilly degrade linearly with the number of cycles. All of this makes it difficult to tell exactly how long the life is going to be (and hence the cost per cycle). However, that does not mean that you should not try to estimate it - especially if you are considering buying more batteries for your system.

UPDATE: you might find this cost calculator spreadsheet useful.

Note: If you choose DoD and charge-discharge rate such that the estimated life of your batteries are >9000 cycles, you may not care about all this. The batteries are likely to outlast the solar panels. This all becomes a bit academic!

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