Rule of thumb:
- 12V up to 1 kW.
- 24V up to 2 kW.
- 48V above 2 kW.
(where the power rating is the greater of the max charging power and the max load power).
This is because:
- Wiring losses (and approximate price of copper wire thick enough to keep them manageable) go up with the square of the current. Cabling and miscellaneous electrical devices and fittings are readily available for average currents up 100A, which corresponds to 1.2, 2.4, or 4.8 kW of raw DC power at the various nominal voltages. But above 100A they get harder to find and more expensive. Also: Working with wire suitable for more than 100A can be like wrestling full-grown anacondas. (I'm talking from personal experience here. I upgraded my townhouse to 200A service.)
- But once you go over 50V you get out of the "low voltage" part of the electrical code - and are approaching "touch it and maybe die" risks when working with or around the live equipment. So you're ahead, both financially and safety-wise, to stop at 48V even if it means using heavier wire and more expensive higher-current parts.
