How the solar battery calculator works
This tool estimates how much battery storage you need to back up your loads for a set number of hours, then converts that into a real-world battery count based on your chosen battery model and system voltage.
You enter:
- Daily energy usage in kWh per day (from a utility bill or your appliance list)
- Backup hours — how long the battery bank must carry that load
- System voltage — 12 V, 24 V, or 48 V
- Battery type — lithium (LiFePO4) or lead-acid/AGM/gel, which sets a sensible default depth of discharge
- Depth of discharge (DoD) — editable if your battery datasheet differs from the default
- Inverter efficiency — defaults to 90 percent, tucked under advanced options
- Single battery capacity and voltage — used to work out how many physical batteries you need
The math, in plain terms
The calculation runs in five steps:
- Average load:
Average load (kW) = daily kWh ÷ 24 - Backup energy:
Backup energy (Wh) = average load × backup hours × 1000 - Required nominal capacity:
Required Wh = backup energy ÷ depth of discharge ÷ inverter efficiency - Required amp-hours:
Capacity (Ah) = Wh ÷ voltage - Battery count: batteries in series = system voltage ÷ battery voltage; parallel strings = required Ah ÷ battery Ah, rounded up; total batteries = series × parallel
The result is shown as both usable kWh (what you can actually draw) and nominal kWh (the battery's rated capacity), because those two numbers are not the same once depth of discharge and inverter losses are factored in.
Why depth of discharge matters
Depth of discharge is the percentage of a battery's rated capacity you are allowed to use before recharging. Push a battery past its safe DoD too often and you shorten its life dramatically.
- Lithium (LiFePO4): typically safe at 80 to 100 percent DoD, with 3,000 to 6,000+ cycles at 80 percent DoD depending on the cell quality
- Lead-acid, AGM, gel: usually limited to 50 percent DoD, with 300 to 700 cycles at that depth before capacity fades significantly
A shallower DoD means you need more nominal (rated) capacity to deliver the same usable energy, which is why lead-acid banks end up physically much larger than an equivalent lithium bank.
Series vs parallel wiring
Batteries are combined two ways:
- Series — connects batteries end to end to raise voltage (for example, four 12 V batteries in series make 48 V). Capacity in Ah stays the same as a single battery.
- Parallel — connects batteries side by side to raise capacity while voltage stays the same. Ah values add together.
Most home systems use a mix: a series "string" to reach the target system voltage, then multiple strings in parallel to reach the target capacity.
Sizing for one night vs multi-day autonomy
A battery sized for a single overnight cycle is the cheapest option and works well in sunny climates where the panels reliably refill the bank the next day. If your location has long overcast stretches, or you're backing up something critical like refrigeration or medical equipment, add a autonomy buffer of one to three extra days of storage. Just remember that oversizing a lead-acid bank for rarely-used deep backup wastes money on cycle life you'll never spend, whereas lithium tolerates being lightly cycled far better.
Temperature and C-rate notes
- Cold weather: Lead-acid capacity drops as temperature falls. Most lithium batteries refuse to charge below freezing without an internal heater, so batteries in unheated sheds or garages need either a heated enclosure or a bigger margin.
- C-rate: This is the battery's maximum safe charge or discharge rate relative to its capacity. A 100 Ah battery rated at 1C can safely discharge 100 A continuously; a 0.5C battery can only manage 50 A. Check your battery's peak discharge rating against your inverter's peak draw, especially for motor-starting loads.
Typical battery module sizes worldwide
| Module type | Voltage | Capacity | Common region |
|---|---|---|---|
| Flooded/AGM lead-acid | 12 V | 100 Ah (~1.2 kWh nominal) | Global, especially budget off-grid |
| LiFePO4 "drop-in" | 12.8 V | 100 Ah (~1.28 kWh nominal) | US, Europe, Australia RV/off-grid |
| LiFePO4 rack module | 48 V | 5 kWh nominal | Commercial and larger residential |
| LiFePO4 wall unit | 48 V (internal) | 10–13.5 kWh nominal | Grid-tied home backup, most regions |
Lithium vs lead-acid comparison
| Factor | Lithium (LiFePO4) | Lead-acid / AGM / gel |
|---|---|---|
| Usable DoD | 80–100% | 50% |
| Cycle life at rated DoD | 3,000–6,000+ | 300–700 |
| Round-trip efficiency | 95–98% | 80–85% |
| Weight per usable kWh | Low | 3–4× heavier |
| Maintenance | None to minimal | Periodic watering (flooded), terminal checks |
| Cost per usable kWh over life | Lower long-term | Lower upfront, higher long-term |
Upfront lithium pricing is higher, but because you can use more of the rated capacity and get many more cycles, the effective cost per usable kWh over the battery's lifetime is often lower. Exact pricing varies a lot by region and supplier — treat any number as a rough guide, not a quote.
Limitations to keep in mind
This calculator gives a sizing estimate, not an engineering design. It does not account for charge controller current limits, inverter surge ratings, battery management system (BMS) settings, or local fire and electrical codes. Always check the manufacturer's datasheet for your specific battery and consult a qualified installer before wiring a bank together, especially for anything above 48 V or with parallel strings.