Battery Bank Sizing: Amp-Hours, Autonomy and DoD
A battery bank is sized by three things: how much energy you use, how many days you want to ride through without charging, and how deeply you can safely discharge the chemistry you choose.
Amp-hours, watt-hours and why the difference matters
Batteries are rated in amp-hours (Ah), but energy is measured in watt-hours (Wh). The link is the voltage: Wh = V × Ah. A 100 Ah battery at 12 V stores 1,200 Wh; the same 100 Ah at 24 V stores 2,400 Wh. This is why amp-hours alone are meaningless without the voltage — always think in watt-hours when comparing banks, and convert with the Watt-Hours calculator.
Depth of discharge: the usable fraction
You cannot use a battery’s full nameplate capacity without shortening its life. Depth of discharge (DoD) is the fraction you may safely draw before recharging. Flooded and AGM lead-acid batteries last far longer if you keep DoD around 0.5 (use only half). Lithium iron phosphate (LiFePO4) tolerates about 0.8 or more. So a 100 Ah lead-acid battery gives you about 50 usable Ah, while a 100 Ah lithium gives about 80 usable Ah — a big practical difference that often makes lithium cheaper per usable watt-hour despite the higher sticker price.
The sizing formula
Combine energy, autonomy and DoD:
battery Ah = (Wh/day × days of autonomy) / (system V × DoD)
and the stored energy is:
kWh = Ah × V / 1,000
Days of autonomy is how long the bank must carry your loads with no meaningful charging — through a stretch of cloudy days off-grid, for example. Two to three days is common for solar; critical systems use more.
Worked example: 1,200 Wh/day, 3 days, 12 V lead-acid
Take a modest cabin using 1,200 Wh/day, wanting 3 days of autonomy, on a 12 V flooded lead-acid bank (DoD 0.5):
Ah = (1,200 × 3) / (12 × 0.5) = 3,600 / 6 = 600 Ah.
The stored energy is 600 × 12 / 1,000 = 7.2 kWh. Switch to lithium at DoD 0.8 and the same job needs only (1,200 × 3) / (12 × 0.8) = 375 Ah — about 38% less amp-hours for the same usable energy. The Battery Bank calculator compares chemistries instantly.
Series and parallel: building the bank
Few banks are a single battery. Wiring batteries in series adds their voltages (four 12 V batteries in series make 48 V); wiring in parallel adds their amp-hours. A 2-series, 2-parallel arrangement of 12 V 100 Ah cells yields 24 V at 200 Ah. Match capacity and age within a bank so cells share load evenly. The Series / Parallel calculator works out the pack voltage and amp-hours for any arrangement.
Temperature and real capacity
Rated capacity assumes moderate temperature and a slow discharge. Cold reduces lead-acid capacity sharply, and very high discharge currents reduce delivered amp-hours (the Peukert effect). Lithium is more stable but should not be charged below freezing. Build in margin rather than sizing to the exact nameplate, and remember the numbers here are planning estimates: real capacity depends on temperature, age and discharge rate.
How runtime ties back in
Once the bank exists, the reverse question — how long will it run a given load — uses the same usable-energy idea: runtime h = (Ah × V × DoD) / load W. A 100 Ah 12 V lithium bank at DoD 0.8 runs a 120 W load for (100 × 12 × 0.8) / 120 = 8.0 hours. The Battery Runtime calculator handles that direction, which is especially useful for RV and van builds.
Cycle life and the real cost of a bank
The sticker price of a battery is only part of its cost; what matters off-grid is the price per usable kilowatt-hour delivered over its life. A battery is rated for a number of charge-discharge cycles, and that number depends heavily on how deeply you cycle it. A lead-acid battery cycled gently to thirty percent depth may last several times as many cycles as one routinely drained to its limit, which is why the conservative half-depth guideline exists. Lithium iron phosphate tolerates deeper cycling and offers far more cycles to begin with, so even though it costs more up front, its cost per usable kilowatt-hour over a decade is often lower. Sizing a bank generously so it cycles shallowly is therefore not just kinder to the batteries; it can be the cheaper choice over the life of the system.
Charging current and bank size
A battery bank is not only an energy store; it also accepts charge at a limited rate, and that rate is tied to its capacity. Lead-acid banks generally want a charge current somewhere around ten to twenty percent of their amp-hour capacity, while lithium accepts much higher rates. This couples the bank size to the array and charger: a bank that is too small for a large array cannot absorb the midday current and forces the controller to throttle, wasting harvest, while a bank that is large relative to its charging sources may never reach a full charge, which is hard on lead-acid in particular. A balanced design matches the array, the controller and the bank so the battery can be charged fully and at a healthy rate.
Keeping cells balanced
When batteries are combined into a bank, they should share the load and charge evenly, and small inequalities grow over time into real problems. Mixing old and new batteries, or different capacities, lets the weaker cells be over-worked while the stronger ones loaf, dragging the whole bank down to the weakest member. Good practice is to build a bank from identical batteries of the same age and to wire it symmetrically so each battery sees the same resistance to the load. Lithium banks rely on a battery management system to keep cells balanced and to protect against over-charge and over-discharge, which is one more reason the chemistry behaves so predictably compared with a string of lead-acid batteries left to fend for themselves.
Round-trip efficiency
No battery returns all the energy you put in. Some is lost as heat on the way in and out, an effect captured by round-trip efficiency. Lead-acid typically returns around eighty to eighty-five percent of the energy it stores, while lithium does better, often above ninety percent. This loss is separate from depth of discharge and stacks with it, which is part of why the overall system derate sits near three quarters. When you size an array to refill the bank, you must replace not just the energy you used but also the energy lost in storing it, so a less efficient chemistry quietly demands a slightly larger array as well as more amp-hours.
Battery sizing rests on stable values (DoD ranges, voltage, your measured consumption), so it is maintenance-free to compute — but verify the chemistry’s recommended DoD with the manufacturer and design conservatively.