How to size a van electrical system without guessing

Start with a daily load table, not a battery size. Add the amp-hours each device draws in 24 hours: a 55L compressor fridge is about 30Ah in summer, LED lighting 5Ah, a diesel heater 10Ah on a cold night, laptop and phone charging 20Ah, water pump 3Ah, fan 15Ah.

That example totals roughly 83Ah per day. Lithium can be discharged to about 20 percent, so a 200Ah LiFePO4 bank gives you about two days of autonomy and a 300Ah bank gives you three.

Solar replaces roughly 30 to 35 amp-hours per 100 watts of panel on a good summer day and half that in winter. Two 200W panels are the realistic ceiling on a 148-inch Transit roof once you fit a fan and a rack.

The honest answer for winter and cloudy weeks is a DC-to-DC charger off the alternator. A 50A unit puts 50Ah into the bank for every hour of driving, which makes travel days into charging days and lets you carry less panel.

Fuse everything within seven inches of the battery positive terminal, size wire for a 3 percent voltage drop rather than by ampacity alone, and use a Class T fuse on any lithium bank over 100Ah.