Why Batteries, and how many?

It is possible to lower the cost of a SolarPV system considerably by just installing panels and an inverter with no batteries. Such a system may suit some users, but to make good use of such a system you have to be able to use the solar energy as it is produced. You CAN export excess electricity to the grid and then import some later instead - but the differential in export and import rates is generally pretty high and that is what usually makes your own local storage (i.e. batteries) worth having.

As to the size of battery to install, I found that the sales people did not have much expeience of homes with very large usage such as ours. I was advised that one 5.12kWh battery would be sufficient but I opted for two batteries because I had a feeling that I might need more. The fact that there is no VAT payable on batteries installed at the same time as the system also swayed me a bit.

The down side of bigger batteries is that you are, in effect, making a decision to buy more batteries in the future. Batteries are expected to last maybe ten years (compared with 25 years for the panels) so you can expect to buy more batteries before the panels are replaced. If you buy more or bigger batteries now you are likely to want to keep or increase the capacity when replacing them.

As I see it, there are two reasons for selecting a bigger battery. Firstly a bigger battery makes it more likely that you will be able to make it through any peak period you may be trying to avoid. Secondly, a bigger battery allows you to draw more instantaneous power without hammering the battery. My batteries can be discharged at 1.0C (10kW) but it is recommended (for best battery life) to limit it to 0.5C which is 5kW. Turning on multiple devices (ovens, microwave, water heating) we are fairly easily able to peak at 10 or 12kW so a bigger battery helps a lot. During the peak period we have adopted a habit of watching the smart meter display to avoid consuming power from the grid. When we turn on a oven, for example, if we see power being drawn from the grid then we turn it off immediately and just wait until something else is turned off.

The "C" rating of a battery is something that you will frequently come across. If 100AH battery is charged or discharged at 100A, then you are charging/discharging at 1.0C. 50A would be 0.5C. Looking at it a different way: for a 5kWh battery the 1.0C rate is 5kW, the 0.5C rate is 2.5kW. My batteries can be charged or discharged at 1.0C, but the life is specified at 0.5C. In other words, to get the rated life out of the battery you have to treat it relatively gently. If you look at your battery's specification you will find that the specified life is only guaranteed if you stick to a relativelyk low rate of charge, you do not discharge it fully, and you keep it within a certain temperature range. If your battery dies before its time, then you will probably be asked to prove that you met all these criteria. So collecting on the warranty could be hard - but then the battery company MAY not be around in ten years, so...

If you home and water with gas, oil or wood then you may well be able to meet all, or nearly all, your electricity requirements from an appropriately sized solarPV + battery set-up. For someone like me, who heats using a heat pump, this is much more difficult to do. Firstly, I would need room for a really large array of solar panels and secondly I would require a very expensive bank of batteries to carry me though the night hours. People do operate completely off-grid - but generally they do so with a much smaller electrical power budget.

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