How to Size an Off-Grid Battery Bank (Step by Step)

The battery bank is the most expensive and the least forgiving part of an off-grid solar system. Undersize it and you will run out of power on the second cloudy day or wear the batteries out in a couple of years. Oversize it and you have spent money on capacity your solar array can never refill. This guide walks through the same method the 6th Star calculator uses, so you can understand, check, and adjust every number.

Step 1: Audit your daily energy use

Everything starts with how many watt-hours (Wh) you use per day. For each appliance, multiply its power in watts by the hours it runs per day:

Daily energy (Wh) = Power (W) × Hours per day

Use real numbers where you can. The label on a device shows its maximum power, but many appliances cycle on and off. A compressor fridge rated at 60 W might only run 35–50% of the time, so a plug-in energy meter over 24 hours is far more accurate than the label. Phones, laptops, and routers are small individually but run for many hours, so they add up.

Two corrections matter:

• AC loads pass through an inverter. Inverters are typically 85–95% efficient, so divide AC energy by the efficiency. The calculator uses 90%: a laptop using 375 Wh of AC energy costs about 417 Wh from the battery.
• The inverter itself uses power. A typical 2–3 kW inverter draws 10–40 W just by being switched on. Left on 24 hours, a 25 W idle draw is 600 Wh per day, which can be more than your fridge. Add it as its own line in your audit if the inverter stays on.

Step 2: Choose your days of autonomy

Days of autonomy is how many days the battery must carry your loads with little or no solar input. It is a judgment call that depends on your climate, whether you have a backup generator, and how critical the loads are.

Situation	Typical autonomy
Weekend camper van with alternator charging	1 day
Cabin with a backup generator	1.5–2 days
Full-time home in a sunny climate	2–3 days
Full-time home, cloudy winters, no generator	3–5 days

Beyond about three days, extra battery usually becomes poor value: the array often cannot refill a very large bank quickly after a long storm, so a small generator is the cheaper insurance.

Step 3: Apply the depth of discharge limit

You cannot use 100% of a battery's rated capacity without shortening its life. The safe depth of discharge (DoD) depends on chemistry:

• LiFePO4 (lithium iron phosphate): routinely cycled to 80–90% DoD. The calculator uses 85%.
• AGM, gel, and flooded lead-acid: limited to about 50% DoD for a reasonable cycle life. Deeper cycling sharply reduces the number of cycles.

Combining the three steps gives the required nominal storage:

Battery storage (Wh) = Daily energy (Wh) × Days of autonomy ÷ DoD

Step 4: Convert to amp-hours at your system voltage

Battery capacity is usually sold in amp-hours (Ah) at a nominal voltage. Divide watt-hours by the system voltage:

Battery capacity (Ah) = Battery storage (Wh) ÷ System voltage (V)

The same stored energy needs half the amp-hours at 24 V and a quarter at 48 V. Higher voltage also means lower currents, thinner cables, and smaller fuses, which is why systems above roughly 2–3 kWh per day are usually built at 24 V or 48 V. See the wire sizing guide for why this matters.

Worked example: a weekend cabin

A cabin uses 2,000 Wh per day after inverter losses. The owner wants 2 days of autonomy and is choosing between LiFePO4 and AGM at 24 V.

	LiFePO4	AGM
Energy over autonomy period	4,000 Wh	4,000 Wh
Depth of discharge	85%	50%
Required storage	4,706 Wh	8,000 Wh
Capacity at 24 V	196 Ah	333 Ah
Practical choice	24 V 200 Ah	24 V 350–400 Ah

The lead-acid bank must be about 70% larger in rated capacity for the same usable energy, and it would weigh roughly four to five times as much. The LiFePO4 vs AGM guide compares lifetime cost.

Step 5: Correct for temperature and ageing

Ratings are given at about 25 °C (77 °F). In the cold, available capacity drops. Lead-acid batteries deliver roughly 80% of rated capacity near 0 °C and much less in deep frost. LiFePO4 batteries discharge well in the cold but must not be charged below 0 °C unless the battery has built-in heating or a BMS that blocks cold charging.

All batteries also lose capacity with age. Many designers add 10–20% so the bank still meets the target near the end of its life. If your bank sits in an unheated shed in a cold climate, add a further 15–25% or, better, insulate the battery enclosure.

Step 6: Check charge and discharge rates

A bank must also handle your peak current. Divide your largest continuous inverter load by the battery voltage and check it against the battery's or BMS's maximum continuous discharge current. A 3,000 W inverter at 24 V can draw about 140 A, which is too much for a single 24 V 100 Ah lithium battery with a 100 A BMS even though its energy capacity might be enough. On the charging side, lead-acid batteries prefer charging currents of roughly 10–20% of their Ah rating; LiFePO4 accepts much more, often 50% or higher.

Common mistakes

• Using nameplate wattage for cycling appliances such as fridges, which overstates use, or forgetting standby loads, which understates it.
• Ignoring inverter idle consumption.
• Sizing lead-acid as if the full rating were usable.
• Mixing old and new batteries, or batteries of different capacities, in the same bank.
• Building a huge bank that the solar array cannot refill. Size the array next using the peak sun hours guide.