Sizing is one area where the DIY installer often gets wrong. For a standalone power system you need to know how many days of autonomy you require, ie how many days your battery bank needs to provide energy for with little or not charging as can happen on very overcast days.
A commonly used figure is five days of autonomy though with the reduction in the cost of solar panels, now it’s more common to install an oversized solar array that can cope better with poor weather. This mean fewer days of autonomy may be required and three days can be a more realistic figure.
You also need to decide on the maximum depth of discharge you want the battery to receive. For instance lead acid batteries this is usually 50%, meaning that after five days the battery must still retain 50% of their capacity. Some battery manufacturers allow of a deeper discharge. This is especially the case for lithium batteries where a figure of 80% is typically used.
For example is a house expects to 5 kilowatt hours (kWh) per day, but there are also losses in the inverter to consider. If its assumed that there is an average loss of 10% then the actual energy taken from the battery bank is 5/0.9 or 5.56 kWh per day or 5560 watt-hours.
If the house uses a 48 volt battery bank then the battery capacity required per day is:
5560/48 = 115.8 amp-hours (Ah)
If this is rounded up to 120 makes sense, as usage can vary, adding an extra 10 to 20% for future energy growth would give a figure of 144Ah per day. This is using an extra 20% on top of the 120 Ah.
Now that the daily requirements have been calculated, the size of the battery bank capacity needed can be calculated by multiplying the daily energy required by the number of days reserve capacity required. Divide the maximum total DOD expressed as a decimal.
In the example we are using the calculation would be:
(144 x 5)/0.5 = 1440 Ah
Obviously battery banks can become quite large and expensive, but there are ways to reduce the costs. Installing more energy generation capacity usually in the form of more solar panels can reduce the required storage.
If a considerable about of household energy usage is during energy production then the size of the battery bank can be reduced.
With lithium batteries it is possible to use a smaller and cheaper battery bank and cycle the battery bank more deeply, provided of course you also have extra generation capacity to cover days of low energy production.
Generally sizing a battery bank requires experience and needs to be considered carefully in light of average daily energy requirement’s, peak energy requirements and energy generation capacity.