How to Size a Battery for an Off Grid Solar System | Do Math

Two battery racks can look identical on paper and deliver very different runtimes, because the number that matters is usable energy rather than the figure printed on the box. Four inputs settle it — daily watt-hours, days of autonomy, usable depth of discharge, and an allowance for losses — and anyone who can add up an appliance list can work through how to size a battery for an off grid solar system in an afternoon.

Get the math wrong in either direction and the bill arrives later: a bank that is too small trips the inverter at 6 a.m., and one that is too large never gets refilled during a cloudy December.

Battery Sizing For An Off-Grid Solar System: What Decides The Number

Required battery kWh = daily load kWh × days of autonomy ÷ usable depth of discharge ÷ system efficiency. Each term is a decision rather than a guess, so work through them in order.

Audit your loads first. List every appliance, multiply its watts by hours of daily use, and add the results into one daily watt-hour total. A 10-watt Wi-Fi router running all day quietly contributes 240 Wh before you count the fridge, which is how always-on loads slip past most estimates.

Pick the chemistry, because it sets your depth of discharge. Lithium iron phosphate banks are commonly designed around 80% usable capacity, while AGM and other lead-acid banks are usually planned at 50%, so lead-acid needs nearly double the nameplate capacity to deliver the same energy. Chemistry is the fork in the road here, and our roundup of the best batteries for off-grid solar sorts the tested options by it before you lock in a size.

Leave room for losses. The inverter, the wiring, and the DC-to-AC conversion each take a cut, which is why the fuller version of the formula adds a system-efficiency divisor at the end. Skip it and the bank comes up short on exactly the days you were counting on.

A worked example: a cabin drawing 3 kWh a day, planned for 2 days of autonomy on lithium iron phosphate, needs 3 × 2 ÷ 0.8 = 7.5 kWh of usable storage.

How Many Days Of Autonomy Should You Plan For?

Plan for the longest run of bad weather you honestly expect, not your average sunny day: 2–3 days covers most households, a weekend cabin can live on 1 day, and remote or critical sites usually want 3–5 days. Unbound Solar’s battery bank sizing guidance puts the common target at 2–3 days and notes 1.5–3 days as workable for lighter setups.

Cold weather changes the target on its own. If the bank will sit below 10°C for much of the year, add 10–20% capacity on top of your calculation, since the cells give up usable capacity as they chill.

Size the architecture at the same time. Once daily use climbs past roughly 5 kWh, a 48V bank is the usual pick, because higher voltage drops the current and lets you run smaller, cheaper cable.

Then check peak current, a separate question from energy. Motors, pumps, compressors, and power tools pull a surge far above the bank’s steady draw, so confirm the inverter’s continuous and surge ratings and the battery’s own discharge limit before buying.

Will Your Panels Refill The Bank In The Worst Month?

Only if the array is sized against your worst-month peak sun hours rather than the annual average — a bank you cannot refill in the darkest stretch of the year just sinks deeper into deficit. Winter clouds cut panel output sharply across most of the country.

Oversizing the battery without enough array to refill it is the costliest version of this mistake, and it hides easily because the bank performs beautifully in July.

Two quieter errors show up just as often. Reading nameplate capacity as if it were usable capacity inflates your real budget, and forgetting small always-on loads — Wi-Fi gear, security cameras, phone chargers — adds hundreds of watt-hours a day.

WattSizing’s off-grid sizing calculator runs the worst-month recharge check in about a minute if you would rather not do the arithmetic by hand.

Run it in this order and the numbers hold together:

1. Total your daily load in watt-hours, always-on devices included.
2. Choose days of autonomy for your site and backup plan.
3. Divide by usable depth of discharge, then by system efficiency.
4. Confirm the inverter’s continuous and surge ratings match your heaviest motor.
5. Verify the array refills the bank using worst-month peak sun hours.