Battery bank size calculator
Enter what the battery has to supply — a day’s watt-hours, or an inverter load for a set time — and the battery type. You get the bank size in amp-hours and how many 100 Ah-class batteries make it up.
Enter your measurements
More options: DoD · Battery temp
- Made up of
- 2 × 100 Ah 12.8 V in parallel
- Batteries
- 2 batteries · LiFePO4
- Energy the bank must deliver
- 2,000 Wh
- Rated energy of that bank
- 2,560 Wh
2,000 Wh × 1 day = 2,000 Wh
2,000 Wh ÷ 80% usable ÷ 12.8 V = 195 Ah = 195 Ah
195 Ah ÷ 100 Ah per battery → 2 × 100 Ah 12.8 V in parallel
Quick answers for common sizes
| Example | Answer |
|---|---|
| What size battery bank for a 3000W inverter | 326 Ah of 12 V LiFePO4 for an hour — 4 × 100 Ah |
| What size battery bank for a 2000W inverter | 217 Ah of 12 V LiFePO4 for an hour — 3 × 100 Ah |
Bank size for other days of autonomy
| Days of autonomy | Battery bank needed | Batteries |
|---|---|---|
| 0.5 | 98 Ah | 1 battery |
| 1 | 195 Ah | 2 batteries |
| 2 | 391 Ah | 4 batteries |
| 3 | 586 Ah | 6 batteries |
| 5 | 977 Ah | 10 batteries |
Sizing for an inverter rather than a day’s use? Worked examples: what size battery bank for a 3000W inverter and what size battery bank for a 2000W inverter.
How the battery bank is sized
energy needed (Wh) = Wh per day × days of autonomy (or inverter watts ÷ efficiency × hours) bank Ah = energy needed ÷ usable depth of discharge ÷ bank voltage batteries = (system V ÷ 12) in series × (bank Ah ÷ Ah per battery) in parallel
The depth of discharge is what turns a lithium bank and a lead-acid bank of the same label size into very different things. Victron rates its LiFePO4 batteries for 2,500 cycles at 80% depth of discharge and 5,000 at 50%; Trojan recommends discharging lead-acid only 20–50% for best life. So, by default, this calculator plans to use 80% of a lithium bank and 50% of a lead-acid one — a lead-acid bank has to be about 1.6 times bigger in amp-hours for the same job.
Days of autonomy
Autonomy is how many days the bank covers with no charging at all — cloudy days, a generator that won’t start. More days means a much bigger, more expensive bank, so many systems pair one or two days of battery with enough solar to refill it and a generator or charger for long cloudy spells.
| Daily use | 1 day, LiFePO4 12.8 V | 2 days, LiFePO4 12.8 V | 1 day, lead-acid 12 V |
|---|---|---|---|
| 500 Wh | 49 Ah | 98 Ah | 83 Ah |
| 1,000 Wh | 98 Ah | 195 Ah | 167 Ah |
| 2,000 Wh | 195 Ah | 391 Ah | 333 Ah |
| 3,000 Wh | 293 Ah | 586 Ah | 500 Ah |
| 5,000 Wh | 488 Ah | 977 Ah | 833 Ah |
Big inverters: current can matter more than energy
A large inverter on a 12 V bank pulls hundreds of amps. Victron recommends keeping continuous discharge on its 12.8 V 100 Ah LiFePO4 at or below 100 A (1C), with a 200 A maximum. So a 3,000 W inverter at full load — about 260 A from 12.8 V — needs at least three such batteries in parallel even if one would hold enough energy for the job. The inverter-mode result checks this and raises the battery count when current, not watt-hours, is the limit.
Series and parallel
- Series adds voltage. Two 12 V batteries in series make 24 V at the same amp-hours; four make 48 V.
- Parallel adds amp-hours. Two 100 Ah batteries in parallel make 200 Ah at 12 V.
- Match them. Same make, model, capacity and age. Mixing old and new, or lithium and lead-acid, makes some batteries work harder than others.
- Lithium limits. Check the battery manual for the maximum number in series and parallel; it depends on the battery’s management system.
Battery sizes by group number — 24, 27, 31 — and their amp-hours are in the battery group size chart; the chemistry trade-offs are in LiFePO4 vs lead-acid.
CalculatorSolar Panel Calculator →Now size the panels that refill this bank every day.What you’ll need
- 12 V 100 Ah LiFePO4 batteryPremium price
About 1.28 kWh rated in a box roughly the size of a group 24–31 lead-acid battery at around half the weight. Check the BMS discharge limit and whether it can charge below freezing.
See options on Amazon (opens in a new tab) - 12 V deep cycle AGM batteryMid-range price
Sealed lead-acid with no watering. Cheaper up front than lithium, but plan to use only about half its rating and expect it to weigh roughly twice as much.
See options on Amazon (opens in a new tab) - Battery monitor with shuntMid-range price
Counts amp-hours in and out, so you see true state of charge instead of guessing from voltage — which barely moves on a LiFePO4 battery until it is nearly empty.
See options on Amazon (opens in a new tab) - Vented battery boxBudget price
Covers the terminals so a dropped tool can’t short them and keeps a lead-acid battery’s spills contained. Flooded batteries need the vent.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many batteries do I need for solar?
Divide your daily watt-hours by the usable energy per battery. A 12.8 V 100 Ah LiFePO4 battery gives about 1,024 Wh usable; a 12 V 100 Ah lead-acid about 600 Wh. So 2,000 Wh a day for one day takes two lithium or four lead-acid batteries.
What size battery bank do I need for a 3000W inverter?
At full load for an hour, about 325 Ah at 12.8 V of LiFePO4 — four 100 Ah batteries — and at least three in parallel just to carry the current. See what size battery bank for a 3000W inverter.
Should I build a 12 V, 24 V or 48 V battery bank?
Twelve volts suits vans and small RVs with 12 V appliances. Above a couple of kilowatts of inverter, 24 V or 48 V halves or quarters the current, which makes cabling, fusing and battery current limits far easier to live with.
How many days of autonomy do I need?
There is no single answer: one day for a van that drives and charges, two or three for a cabin that relies on solar alone, plus a generator or charger for long cloudy spells. Every extra day adds a full day’s worth of battery.
Related guides
Sources
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
- Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.
- Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
- Crown Battery — Deep Cycle Battery Product Specifications (effective Oct 1, 2024) (opens in a new tab) — 20-hour capacities by BCI group for flooded, AGM and lithium 12 V batteries.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.