How Many Battery Amp-Hours Do You Need for Off-Grid Power?

Battery capacity is where a lot of off-grid systems go wrong — too small and you're constantly low on power, too large and you've overspent on capacity you'll rarely use. Here's how to work out a sensible number.

The basic maths

Battery capacity is measured in amp-hours (Ah) at a given voltage. To get the usable energy in watt-hours, multiply Ah by voltage: a 100Ah 12V battery stores roughly 1,200Wh. That's the number you compare against your daily power use.

Step 1: Know your daily usage

If you've already worked out your daily Wh consumption (see our solar sizing guide for how), you have the starting point. If not, add up every appliance's wattage multiplied by hours used per day. A fridge, lighting, water pump and device charging for a couple typically lands between 500Wh and 1,200Wh/day; add an inverter running larger appliances and it climbs quickly from there.

Step 2: Decide how many days of autonomy you want

"Autonomy" is how many days you want to run on battery alone with no solar or charging input — useful for stretches of bad weather or overnight use before the sun's back up. One day of autonomy is the bare minimum for most setups; two to three days gives a more comfortable buffer, especially if you're relying partly on solar in a climate that isn't always sunny.

Step 3: Account for usable capacity, not total capacity

This is the step people miss most often. Lead-acid and AGM batteries shouldn't be regularly discharged below 50% without shortening their lifespan significantly — so a 100Ah AGM battery gives you roughly 50Ah of usable capacity in practice. Lithium (LiFePO4) batteries tolerate much deeper discharge, typically usable down to 10-20% remaining without meaningful damage, so a 100Ah lithium battery gives you closer to 80-90Ah of real-world usable capacity — and it'll do that for many more charge cycles over its lifetime than lead-acid or AGM.

Putting it together

Take your daily Wh use, multiply by your desired days of autonomy, then divide by your battery voltage and by your battery chemistry's safe depth of discharge. As a working example: 900Wh/day, 2 days autonomy, 12V lithium battery (90% usable) works out to roughly (900 × 2) ÷ 12 ÷ 0.9 ≈ 167Ah — so a 200Ah lithium battery, or two 100Ah units, would comfortably cover it with some margin.

Lithium vs lead-acid vs AGM, briefly

• Lithium (LiFePO4): lightest, deepest usable discharge, longest cycle life, higher upfront cost — the default choice for most new off-grid builds now
• AGM: sealed, maintenance-free, no ventilation needed, moderate cost, but heavier and needs more capacity headroom due to the 50% discharge limit
• Gel: similar profile to AGM, slightly better deep-cycle tolerance, generally slower to charge

Frequently asked questions

Can I mix a new lithium battery with an old lead-acid one?

Not recommended in the same bank — different chemistries have different charge voltages and characteristics, and mixing them typically means neither is charged or protected correctly. Keep battery banks single-chemistry.

Does cold weather affect battery capacity?

Yes, for all chemistries, though lithium batteries also need protection from charging (not just discharging) below freezing — look for a battery with a built-in heating function if you'll be charging in sub-zero conditions.

How big a battery monitor or BMS do I need?

Most lithium batteries have a built-in BMS (battery management system) handling protection automatically. A separate battery monitor with a shunt is still worth adding at the system level so you can see real-time state of charge rather than guessing.

Explore our battery range or talk to us about sizing a bank for your specific setup.