If you have a home with a swimming pool, your annual electricity consumption is likely to be in the range of 10-20 MW per year, or between 27 and 55 Kilowatt hours (KWh) a day, after your overall energy consumption has been reduced by a Pooled Energy system.  For estimating purposes, you can add about 25% more if you do not have a Pooled Energy system.

If you are interested in batteries for Grid back-up, the first thing you need to decide is how much ‘back-up’ energy you want.  Remember that the 27-55 KWh is an average over a whole year and your daily consumption may vary considerably with season and weather, especially if you have air-conditioning.  Assuming that the peaks in demand are, say 30% more than average demand, you are looking at a range of 35-70 kWh. 

With the average home battery capacity being about 6kWh when new, this means you may need 12 batteries such as the Tesla Powerwall to back-up the Grid for a day.

If you want less than a day of back-up, you need to consider that your energy use is ‘peaky’ throughout the day, with major use being around breakfast and dinner.  The instantaneous peak demand is important as battery packs and their inverters are limited in their instantaneous discharge rates (power), as well as on their storage capacities (energy). 

Instantaneous power limits are typically between 2KW and 4 KW per battery pack at the time of writing, but you should check the specifics on any battery pack you are considering. The battery pack needs to deal with these peak power demands. Consider some typical appliances and how many appliances you may wish to run simultaneously from your battery:-1. Electric Kettle    2.0  KW

Electric Kettle                   2.0  KW

Small blower Heater          2.4  KW

Toaster                            1.0  KW

Fridge                              0.5  KW

Computers in house          0.5  KW

Large TV                          0.7  KW

Electric Oven  up to         10.0  KW

Typical household lights    1-2   KW

Single Speed Pool Pump   1.5  KW

The size of battery you need for tariff optimisation, is arbitrary as it does not provide a financial return at this stage of the technology.   

Battery sizing is a complex design task dependent on each house’s living and electrical requirements, its physical capabilities, as well as economic factors.  It is best done by skilled designers in conjunction with the customer.