Off-Grid Battery Bank Sizing Calculator

How to size a solar battery bank

Sizing a battery bank means answering one question precisely: how much usable energy must it hold to carry your loads for as long as you need, without being drained deeper than is healthy for the chemistry? Get it right and the system is reliable for years; undersize it and you'll hit frustrating blackouts on cloudy days; oversize it and you've spent money on capacity you never use. The maths is straightforward once you know four things — your daily energy use, how many days of backup you want, your battery's safe depth of discharge, and the losses in between.

The sizing formula

For example, a cabin using 3 kWh a day, wanting 2 days of autonomy, on LiFePO4 at 90% DoD and 90% inverter efficiency, needs (3 × 2) ÷ (0.9 × 0.9) ≈ 7.4 kWh of nominal capacity — and you'd add a 10–20% margin on top for ageing and cold. Notice how the two efficiency terms inflate the figure above the raw 6 kWh of energy you actually consume: those losses are real, and ignoring them is the most common reason a bank "doesn't last as long as it should."

Depth of discharge: the chemistry that changes everything

Depth of discharge is the share of capacity you routinely use. Lithium iron phosphate (LiFePO4) batteries tolerate 80–95% DoD, so most of their rated capacity is usable. Traditional lead-acid should only go to about 50%, or its life shortens dramatically. This is why a lead-acid bank must be roughly twice the rated capacity of a lithium bank to deliver the same usable energy — and, combined with lithium's far longer cycle life, why LiFePO4 usually wins on lifetime cost per kWh despite a higher upfront price. When you compare quotes, compare usable energy, not nameplate capacity.

Choosing the bank voltage

Banks are built at 12V, 24V or 48V. Higher voltage means lower current for the same power, which means thinner cable, lower losses and less heat — heating in the wiring rises with the square of the current, so a 12V system generates far more cable heat than a 48V one for the same energy. For anything beyond a small system (roughly over 2 kWh), 48V is the sounder choice for efficiency and safety. Small RV and portable setups stay at 12V for simplicity and compatibility.

Common battery-sizing mistakes

• Sizing to nameplate, not usable, capacity. A 10 kWh lead-acid bank only delivers ~5 kWh safely.
• Ignoring inverter and round-trip losses. You must store more than you consume.
• Forgetting cold weather. Capacity drops in the cold, exactly when you may need it most.
• No margin for ageing. Batteries lose capacity over years; size with headroom.

Cycle life and the real cost of storage

The sticker price of a battery tells you little about what it actually costs to use. What matters is the cost per usable kWh delivered over the battery's life, which depends on three things: its usable capacity (capacity times safe depth of discharge), its cycle life (how many charge-discharge cycles before it degrades to ~80% capacity), and how deeply you cycle it. LiFePO4 typically delivers several thousand cycles even at high depth of discharge, while lead-acid offers far fewer and is punished harder by deep cycling. A lead-acid bank that looks half the price can end up costing more per delivered kWh once you account for its shorter life and the larger nameplate capacity you must buy to get the same usable energy. When comparing options, divide the total lifetime cost by the total usable energy the battery will deliver over its rated cycles — that single number cuts through most marketing, and it's usually where lithium quietly wins despite the higher upfront figure.

Frequently asked questions

Start from the daily energy (in kWh) you want backed up — either your whole usage or just essential loads — then apply days of autonomy, your battery's depth of discharge and inverter efficiency using the formula above. Backing up essentials only is far cheaper than backing up the whole home, especially if you have large air-conditioning loads.

For off-grid homes, 2–5 days is typical, depending on how cloudy your area gets and whether you have a backup generator. More autonomy means more reliability but higher cost, so many off-grid systems pair a moderate bank with a generator for rare extended cloudy spells.

LiFePO4 costs more upfront but offers far higher usable depth of discharge and several times the cycle life, usually making it cheaper per kWh over its life. Lead-acid is cheapest to buy and can suit budget or rarely-cycled backup roles. Compare on lifetime cost and usable capacity, not sticker price.

Both, for different reasons. Battery capacity (kWh) decides how long appliances can run; the inverter rating (kW) and the battery's discharge current decide what you can run at once. Motors and compressors also draw a surge at startup, so check both energy and power.