Solar panel battery: how to calculate how many Ah you need

A battery that is too small will be drained every day and won't last long; one that is too big makes the installation more expensive without adding anything. Here is how to calculate the right capacity.

Step 1: work out your daily consumption (Wh)

Multiply the power of each appliance by the hours it runs per day and add them up. Example: 5 LED bulbs of 10 W for 5 h (250 Wh) + a fridge averaging 60 W for 24 h (1,440 Wh) + another 300 Wh = 1,990 Wh/day.

Step 2: choose your days of autonomy

These are the days you want to have power without sun. An off-grid home usually allows 2-3 days; a weekend cabin, 1 day. With 2 days: 1,990 × 2 = 3,980 Wh.

Step 3: apply the depth of discharge

Lead batteries (AGM, GEL, OPzS) last much longer if they are not discharged below 50 %; lithium batteries allow 80-90 %. With lead: 3,980 / 0.5 = 7,960 Wh.

Step 4: divide by the system voltage

In a 24V system: 7,960 / 24 = 332 Ah. For example, you could fit two 12V 330-350 Ah batteries in series, or 12 OPzS 2V cells.

Check the discharge rate

Solar capacity is usually stated as C10, C20 or C100 (discharge over 10, 20 or 100 hours). The same battery has more Ah at C100 than at C10, so always compare at the same rate.

Tips to make your battery bank last

• Don't mix new and old batteries, or different models.
• Set the charge controller to the charging profile for your technology (AGM, GEL or lithium).
• Install them in a ventilated place away from extreme temperatures.

Find your battery in solar batteries and renewable energy.

Any questions? Call us on +34 627 45 96 87 and we'll give you the exact reference. All our prices include VAT and delivery to mainland Spain.