Sizing your battery bank

Your battery bank is the heart of an off-grid system: it stores the energy your panels make during the day so you can use it after dark. Size it too small and you run flat overnight; size it too large and you spend money (and carry weight) you do not need.

The honest way to size a bank is to start from your loads, not from a number that "feels" big enough. Add up what you run each day, then decide how many days of bad weather you want covered.

Step 1 — Add up your daily energy

For every load, multiply its power draw (watts) by how many hours a day it runs. A 45 W fridge that cycles for roughly 10 effective hours uses about 450 watt-hours (Wh) per day. Do this for lights, pump, fans, chargers and any inverter loads, then add them all up.

The Costs and editor views do this roll-up for you as you place components — but doing one load by hand first builds the intuition.

Step 2 — Choose your autonomy

Autonomy is how many days the bank should run your loads with no charging at all. One day is tight; two to three days is a comfortable target for a campervan; cabins that sit unused often want more.

Multiply your daily Wh by the number of autonomy days to get the energy your bank must hold.

Step 3 — Allow for depth of discharge

You should not drain a battery to empty. Lithium (LiFePO₄) banks are happy down to about 80–90% depth of discharge; lead-acid and AGM want to stop near 50% to last. Divide your required energy by that usable fraction to get the rated capacity.

Convert to amp-hours by dividing watt-hours by the system voltage. For example, 1,000 Wh of usable energy on a 12 V system is roughly 83 Ah; on 24 V it is half that current for the same energy.

A worked example

Say your loads total 600 Wh/day and you want two days of autonomy: that is 1,200 Wh. On a LiFePO₄ bank at 90% usable, you need about 1,333 Wh rated. At 12 V that is roughly 110 Ah — so a single 120 Ah lithium battery comfortably covers it, with headroom for the inevitable extra gadget.