How quickly can I discharge it? Does this matter?
There is no point having a large capacity battery unless it can deliver power fast enough to make it useful. Think of it as strength without agility…Basically, imagine that you have 10kWh stored but you can only access 1kW…that is potentially not that useful as it would take 10 hours to discharge (And charge) fully
What does this mean in practise? Think of your kettle, most people have a fast boil electric kettle. If you turn it over it will most likely say 2600w on the bottom of it. This means it will consume 2.6kWh of electricity if it runs for an hour. If you have a smaller battery that has a discharge rate of 2kW or less then the battery cannot cover the energy load of this boiling kettle even if the battery is full and working – it just cannot supply enough energy fast enough. So, if you want to be drawing power exclusively from your battery in the evening then you need a battery system that has a reasonable rate of discharge.
This data is made clear in the data sheet or marketing of the product under “technical” specs somewhere. It will most likely be termed “power” and, for example, say something along the lines of 7kW peak / 5kW continuous (in the case of the tesla Powerwall II). A smaller battery system may say 2kW or even 1kW. What this means is that the “rate” of the discharge e.g. how quickly the battery can supply power into your home is rated at 1kW – or one kilowatt hour per hour.
For those who want their house to be protected from Grid outages by their battery (to be able to run in the case of a power cut) this is even more important as you are reliant exclusively on this rate of discharge. Imagine having the telly on, a pc running and a couple of lights and then someone pops on the kettle? Not an unreasonable requirement, is it? If you cannot cover the load then there are going to be issues, particularly as you may not even notice that there has been a power cut with some battery systems…
My suggestion therefore is to decide what load you wish to cover and make sure your battery can deliver if this is a concern for you. If you are not worried and accept that topping up from the grid as needed is fine then a smaller discharge rate from the battery may be acceptable.
Written by John Rowlatt