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How to Calculate Battery Capacity for Solar System Off-grid
- Feb 24, 2018 -

How to calculate battery capacity for your solar system? You need two key pieces of information: energy consumption and days of autonomy

Follow these procedures when calculating the required battery capacity of your solar system.

Energy Consumption

The power consumption of electric appliances and equipment is generally measured in watts. To calculate total energy consumption, multiply the watts by the hours of use.
For example:

  • 20W TV on for 2 hours equals 40 watt-hours

  • 10W radio on for 5 hours equals 50 watt-hours

  • 20W water pump on for 20 minutes equals 6.66 watt-hours

  • 30W halogen light on for 3 hours equals 90 watt-hours

  • 70W laptop plugged into an outlet for 2 hours equals 140 watt-hours

Add up all the watt-hour values of every appliance in your home to know how much energy your household is consuming daily. You could also use your monthly electricity bill to estimate your energy consumption per day.

Days of Autonomy

How many days will your solar system run without sunshine?

If you live in an area where there are plenty of cloudy and rainy days, your battery must have enough capacity to power your solar panels until the sun comes out.

Two days of autonomy means that your batteries can supply continuous energy for two full days without charging. Standard days of autonomy are generally from two to five.

How much power can I take from a battery?

Unfortunately, you will never be able to take all the power from a battery. Each battery has a Depth of Discharge (DOD) rate to prevent them to be damaged.

For example

  • Lead acid GEL batteries will give you around 50% of their rated power. (i.e. a 100Ah battery has 50Ah of usable power)

  • LiFePO4 batteries will give you around 80% of their rated power. (i.e. a 100Ah battery has 80Ah of usable power)

How to calculate battery capacity for solar system

Now that you know that you know

  • how much power you need every day

  • how many days backup you need

  • the maximum power that a battery can give you

You can easily calculate how many batteries you need.

Batteries needed (Ah) = Daily consumption (Ah) * Backup days * Annual correction factor 1,15 / DOD (%)

Annual correction factory compensates the fact that batteries will not be new forever, but their maximum power will be less over the years. Like the battery of your mobile phone, it is not so powerful as the first days you have used it.
Let’s make an example.

You need 100Ah every day with 3 days backup time using LiFePO4 batteries that have 80% DOD.
Batteries needed = 100 * 3 * 1,15 / 0,8 = 431 Ah

As when you are preparing yourself for a trekking in the wilderness, it’s very important that you know how to calculate battery capacity for the solar system, you can ensure maximum power-generating efficiency from your solar panels.