12V 200Ah or a 24V 200Ah battery for an off-grid cabin?

Direct answer: They are not the same size in energy. The 12V 200Ah stores about 2.5 kWh and the 24V 200Ah about 5.12 kWh according to the pages, because energy is voltage × Ah. For a cabin with an inverter of more than about 1,000–1,500W, a 24V system usually means half the current for the same load, smaller cables and lower losses. Choose 12V if most loads are 12V DC and the inverter is small; choose 24V if the inverter is larger, cable runs are long, or you need more energy.

Page figures. 12V 200Ah: 12.8V, 200Ah, 2,500 Wh, 501 × 185 × 242 mm, 18 kg, 14.6V charge, 10V cut-off, versions at 100A, 150A or 200A continuous. 24V 200Ah: 25.6V, 200Ah, 5,120 Wh, 523 × 269 × 218 mm (BCI Group 8D), 40 kg, 29.2V charge, 20V cut-off, same current versions. The 24V 100Ah (Ultra) lists 2,560 Wh, about the same energy as the 12V 200Ah at twice the voltage.

Current and cable. Our arithmetic: a 1,500W load draws about 1,500 ÷ 12 ≈ 125A at 12V and about 62A at 24V, before inverter losses. Cable losses rise with the square of current, so for the same cable the 24V system loses roughly a quarter as much. That matters for long runs from a battery shed to the cabin. Conductor sizes and overcurrent protection must follow your electrical code (NFPA 70 in many US jurisdictions) and the inverter manual.

Equipment. The inverter and the solar charge controller must match the battery voltage: a 12V inverter will not run from 24V. If you have 12V DC loads such as lights or a pump on a 24V system, you need a 24V-to-12V converter. Check that the controller supports 24V lithium settings (29.2V maximum on the page).

Expansion. The 12V 200Ah lists up to four in series and four in parallel; the 24V 200Ah lists up to four in parallel and no series connection. Plan the final size first; mixing old and new units later needs care.

Rule of thumb, not a requirement: small cabins with mostly 12V loads and a small inverter can stay at 12V; anything with a larger inverter, induction cooking, a well pump on an inverter or long cable runs points toward 24V. For a related case, see two 12V batteries in series vs one 24V battery and, for the cable side, cable and fuse sizing at 12V.