What is the formula for sizing a solar battery bank?

The standard off-grid engineering formula is: Required Ah = (Daily Wh × Autonomy days × Temperature factor) ÷ (System voltage × DoD fraction × Round-trip efficiency). Every term matters: forgetting the efficiency factor systematically undersizes the bank, while ignoring temperature derating causes surprise capacity loss in cold weather.

What is depth of discharge (DoD) and why does it matter?

DoD is the maximum percentage of stored energy you should draw before recharging. Drawing beyond the recommended DoD dramatically shortens battery life. Flooded lead-acid and AGM should not exceed 50% DoD; LiFePO4 can safely reach 80%; lithium-ion NMC/NCA also targets 80% but degrades faster when cycled hard. The calculator adds the DoD into the denominator of the sizing formula, which means a lead-acid bank must be roughly twice the size of an LFP bank for the same usable energy.

Why does round-trip efficiency increase the required bank size?

Round-trip efficiency accounts for the energy lost as heat during both charging and discharging. A lead-acid battery is typically 80% efficient, meaning for every 100 Wh you put in you only get 80 Wh back out. To deliver a target of, say, 1,500 Wh you therefore need to store about 1,875 Wh. LiFePO4 at 97% efficiency loses almost nothing, so it requires a smaller nominal bank for the same usable output.

How do I work out series and parallel cell configuration?

Cells in series add their voltages; cells in parallel add their capacities. To reach a 24 V system from 3.2 V LFP cells you need 8 in series (8 × 3.2 = 25.6 V, rounded up). If each cell is 100 Ah and you need 300 Ah nominal, you need 3 strings in parallel. Total cells = 8 × 3 = 24. The calculator does this arithmetic automatically and rounds up to whole cells so your configured capacity always meets or exceeds the requirement.

What is a temperature derating factor?

Battery capacity falls in cold conditions. A lead-acid bank at 0 °C delivers roughly 80% of its rated capacity; at −20 °C it may deliver only 50%. LFP degrades less steeply but is still affected. Enter the percentage capacity loss expected at your installation temperature (for example 20 for a cold garage or outdoor shed) and the calculator inflates the required nominal capacity by that factor so the bank still meets your load after the derating.

How many sun-days does it take to recharge a depleted battery bank?

The recharge time in hours equals the capacity to refill (configured Ah × DoD) divided by the panel current into the bus (panel watts × charge-controller efficiency ÷ system voltage). Dividing by your daily peak sun hours converts that to sun-days. A 400 W panel at 95% MPPT efficiency into a 24 V bus provides about 15.8 A; recharging a 300 Ah bank from 80% DoD takes about 15.2 hours, or roughly 4 sun-days at 3.8 h peak sun. If that is too slow, increase panel wattage or add a second panel string.

Solar Battery Bank Calculator

A solar battery bank calculator that applies the standard off-grid engineering formula to size a storage bank precisely — covering daily load, days of autonomy, chemistry depth of discharge, round-trip efficiency, temperature derating, and series-parallel cell configuration in a single interactive tool. Whether you are sizing a rooftop backup system, an off-grid cabin, a narrowboat or a motorhome, the calculation chain is the same, and the calculator walks through every step so you can see exactly why each number changes when you adjust an input.

How it works

The sizing engine is built around the canonical battery bank formula used throughout off-grid solar engineering:

Required nominal Ah = (Daily Wh × Autonomy days × Temperature factor) ÷ (System voltage × DoD × Round-trip efficiency)

Four things divide into the denominator, each one inflating the bank size you need:

System voltage converts energy (Wh) into capacity (Ah). A 24 V system needs half the Ah of an equivalent 12 V system for the same stored energy.
Depth of discharge (DoD) is the maximum safe draw fraction. Lead-acid batteries are limited to 50% DoD; LiFePO4 (LFP) safely reaches 80%. Because only the usable slice of the bank matters, the total nominal capacity must be scaled up by 1/DoD.
Round-trip efficiency captures heat losses during both charge and discharge. Flooded lead-acid is around 80%, AGM around 85%, LFP around 97%, and lithium-ion NMC around 95%. Every 1 Wh of load actually demands 1/efficiency Wh of stored capacity.
Temperature factor inflates the result to compensate for cold-weather capacity loss — a battery in a 0 °C shed delivers meaningfully less than its nameplate Ah.

The cell configuration step resolves the raw Ah figure into a real wiring layout. Cells in series add voltages; cells in parallel add capacities. The calculator computes series count as ⌈system voltage ÷ cell voltage⌉ (ceiling division, so voltage always meets the target), and parallel count as ⌈required Ah ÷ cell Ah⌉. The product of the two is the total cell count. The configured capacity will always equal or slightly exceed the requirement because whole cells are indivisible.

The solar charging section then asks a separate but equally important question: can your panel array replenish the bank each day, and how long does a full recharge take from empty? Panel charge current is panel watts × controller efficiency ÷ bus voltage; recharge hours are configured Ah × DoD ÷ that current; sun-days are recharge hours ÷ peak sun hours per day.

Worked example

A cabin uses a 200 W fridge for 24 hours, a 60 W LED lighting circuit for 5 hours, and a 30 W phone/laptop charger for 4 hours per day:

Appliance	Watts	Hours	Wh/day
Fridge	200	24	4,800
Lighting	60	5	300
Chargers	30	4	120
Total			5,220 Wh

With 3 days autonomy, a 24 V LFP bank (80% DoD, 97% efficiency, no temperature derating):

Required Ah = (5,220 × 3 × 1.0) ÷ (24 × 0.80 × 0.97) = 840 Ah nominal (≈ 20.2 kWh)

Using 100 Ah / 3.2 V LFP cells: 8 in series for 24 V, ⌈840 ÷ 100⌉ = 9 parallel strings — 72 cells total giving 900 Ah / 21.6 kWh configured.

Chemistry	DoD	Eff.	Required Ah	Required kWh
LFP	80%	97%	841 Ah	20.2 kWh
Lead-acid	50%	80%	1,631 Ah	39.1 kWh
AGM	50%	85%	1,535 Ah	36.8 kWh

The table shows why LFP dominates modern off-grid installs despite higher upfront cost — the bank is roughly half the size of a lead-acid bank for identical usable energy. A 1,000 W panel array at 95% MPPT into a 24 V bus provides 39.6 A and recharges the LFP bank (900 Ah × 80% = 720 Ah to refill) in about 18.2 hours — roughly 5 sun-days at 3.8 h peak sun. Doubling the panel to 2 kW cuts that to 2.5 days.

Every figure recomputes instantly in your browser; nothing is uploaded or stored externally.