4 Easy steps to calculate battery capacity for an off-grid solar system

Calculating the approximate battery capacity for a solar system correctly is important when you want to ensure that your appliances work the for the intended duration without damaging the battery.

To workout the battery capacity you need:

the combined appliance’s power consumption in watts that needs to be powered by the battery

how long the appliances will be powered by the battery.
battery type: AGM or Lithium
battery system voltage: 12V or 24V

If you size your battery too small then your appliances will not run for the desired duration. On the other hand if your battery capacity is larger than necessary, you’ll have invested in used battery capacity.

Let’s look at the steps for calculating the battery size for an off-grid solar system..

Step 1. Combined appliances power consumption (W)

Workout the combined appliances power consumption (W) by adding the power consumption values of all the appliances the battery will power

Step 2. Desired battery run-time (hours)

Decide on the desired runtime of the battery before the next charge. The longer the runtime is, the bigger the battery size in Ah.

Step 3. Battery chemistry

Decide on the battery chemistry – AGM or Lithium. Lithium battery types (Lithium ion) are lighter, last longer but are more expensive than AGM type batteries. To maximize battery life, AGM batteries should discharged at most (depth of discharge) to 50% compared to 90% for Lithium battery types.

Step 4. Use the battery capacity formula
Using the combined appliance power consumption, runtime and depth of discharge from steps 1, 2 and 3 respectively, in the formula to get the indicative battery capacity.

With the information obtained from steps 1-4, use the formula below to calculate the solar system battery size.

Formula for calculating an off-grid solar system battery size:

Battery capacity (Ah) = (Combined appliance power consumption (W) x battery runtime (h)) / (battery system voltage x depth of discharge)

Note: Depth of Discharge for AGM is 0.5 (50%) while for Lithium, it is 0.9 (90%).

Lastly, choose the nearest available standard battery capacity to the calculated battery capacity. For example, if your calculated battery capacity is 173Ah, a 200 Ah battery can be used.

Worked example showing steps to calculate the battery capacity for an off-grid solar system.

Lets say that after working through steps 1-4 for your solar system , you have the system information as follows:

Step 1. 200W

Step 2. 5 hours

Step 3. Lithium

Using the formula, the calculated battery capacity for a combined load of 200W running for 5 hours a day on connected to a 12V Lithium battery system is:

200 x 5/(12 x 0.9) = 92Ah. A 100 Ah or 120Ah battery – typically the nearest standard sizes can work well.

Using the online calculator to work-out the battery capacity for a solar system

Alternatively, you can also use the free online battery capacity calculator to calculate battery capacity for solar system

In summary, you can calculate an approximate battery capacity for a solar system once you know combined appliance power consumption, battery runtime and depth of discharge.