Battery capacity is the specification everyone shops on and the one that explains the least. A bank size only becomes meaningful in relation to three other numbers: daily consumption, peak current, and how much energy you can put back each day.
This page compares every common bank size sold for Australian boats. Use it to place your own load audit on the scale — not to pick a size and work backwards.
Convert amp-hours to watt-hours first
A bank marked in amp-hours cannot be compared across voltages. A 12.8V 400Ah bank and a 25.6V 200Ah bank store the same energy:
12.8 V x 400 Ah = 5,120 Wh = 5.12 kWh
25.6 V x 200 Ah = 5,120 Wh = 5.12 kWh
Every figure in the table below is therefore given in kWh as well.
Bank sizes compared
Nominal energy assumes a 12.8V nominal cell voltage for amp-hour ratings. Daily-use and solar figures are starting ranges to investigate, not recommendations.
| Bank | Nominal energy | Typical daily use | Solar to investigate | Typically suits |
|---|---|---|---|---|
| 100Ah | 1.28 kWh | 0.3–0.7 kWh | 100–300W | Day sailing, fishing electronics, lighting, VHF, modest refrigeration |
| 200Ah | 2.56 kWh | 0.8–1.4 kWh | 200–450W | Weekend cruiser with fridge, electronics, autopilot, laptops |
| 300Ah | 3.84 kWh | 1.5–2.5 kWh | 300–700W | Small coastal cruiser with refrigeration and moderate autopilot use |
| 400Ah | 5.12 kWh | 2.0–3.5 kWh | 500–1,000W | Coastal cruising with Starlink and a freezer |
| 500Ah | 6.4 kWh | 2.5–4.5 kWh | 700–1,200W | Extended coastal cruising, light inverter use |
| 600Ah | 7.68 kWh | 3.0–5.0 kWh | 800–1,400W | Liveaboard without electric cooking or air conditioning |
| 800Ah | 10.24 kWh | 4–7 kWh | 1,000–1,800W | Full liveaboard, some electric galley use |
| 1000Ah | 12.8 kWh | 5–9 kWh | 1,200–2,200W | Liveaboard with electric cooking and laundry |
| 10 kWh | 10 kWh | 4–7 kWh | 1,000–1,800W | As 800Ah, usually at 24V or 48V |
| 15 kWh | 15 kWh | 6–11 kWh | 1,500–2,500W | Large catamaran or motor yacht, high hotel load |
| 20 kWh | 20 kWh | 8–14 kWh | 2,000–3,000W | Liveaboard with limited air conditioning |
| 30 kWh | 30 kWh | 12–20+ kWh | 2,500–4,000W+ | Overnight air conditioning, full electric galley, microgrid design |
Worked examples
A day boat on 100Ah
- navigation electronics: 150Wh
- small fridge use: 130Wh
- VHF and AIS: 60Wh
- phone charging: 40Wh
- lights: 30Wh
- pumps and miscellaneous: 20Wh
Total: roughly 430Wh/day. A 1.28kWh bank covers this with useful reserve, provided the charging path can replace it.
A weekend cruiser on 200Ah
- fridge: 380Wh
- navigation electronics: 240Wh
- autopilot: 160Wh
- laptop: 140Wh
- phones: 90Wh
- lights: 80Wh
- pumps: 60Wh
Total: approximately 1.15 kWh/day, comfortably inside a 2.56kWh bank.
Capacity is not current
The bank size tells you how long the system may run. It does not tell you how much current the battery can safely supply at one moment. A large bank can still trip if the inverter, windlass support load or combined DC demand exceeds the BMS continuous or peak rating.
Whether a bank can run a washing machine, induction cooktop or air conditioner depends more on instantaneous current, inverter size, BMS limits and appliance duty cycle than on capacity alone. Check the BMS continuous rating and the BMS peak rating separately — they are different specifications and are often quoted interchangeably in marketing material.
When to change system voltage
Larger banks push 12V systems into impractical currents. As a rough guide, once the bank passes roughly 10kWh, or the inverter passes about 3,000W, compare a 24V design against 12V; past roughly 20kWh or 5,000W, evaluate 48V. Higher voltage reduces current for the same power, which reduces cable size, busbar rating, fuse cost and voltage drop.
A better sizing test than picking a size
- Add all normal 24-hour loads in Wh.
- Add inverter conversion loss for AC appliances.
- Decide how much poor-weather reserve you want.
- Define the minimum state of charge you are willing to reach.
- Check whether solar and other charging sources can replace an average day of use.
- Check maximum simultaneous DC current against the BMS, cabling, busbar, fuse and battery ratings.
Solar and recharge planning
To replace a given daily consumption from solar, divide the daily energy by the effective peak-sun-hours and a planning efficiency. For 4kWh/day at 4.5 effective peak-sun-hours and 80% planning efficiency:
4,000 Wh / (4.5 x 0.80) = 1,111 W of solar
That is a planning calculation, not a guarantee of daily yield. Cloud, panel temperature, shading, orientation, controller limits and seasonal sun all reduce production, and Australian yield varies substantially between Darwin and Hobart.
Do not oversize the bank without sizing charging
A larger bank that cannot be recharged is worse than a smaller bank that can. If the charging system cannot replace an average day's consumption, adding capacity only extends the time before the problem appears.