An off-grid installation is a budget with three incomes, and it fails in the month when all three are small at once.
Solar carries the days at rest
It works while nothing else is happening, which is exactly when a stationary van needs it. Its weakness is the season: daily yield in midwinter is a fraction of the midsummer figure at the same latitude, and no reasonable roof closes that gap.
The alternator carries the days that move
A DC-DC charger takes whatever a modern variable-output alternator offers and produces the profile the leisure bank needs, which a simple relay cannot. On a vehicle that drives most days, it is the largest single income and the one least affected by weather.
The mains charger carries the nights on a hookup
Occasional, predictable and fast. It is also the one that most often sets the pace of a recovery after a bad week, which is a reason to size it rather than treat it as an afterthought.
The fridge sets the outgoings
It is the largest consumer in nearly every installation and the one estimated worst. Measure it over a couple of days in realistic conditions, and design everything else around the result rather than around a label.
Monitor it, then adjust
A shunt monitor, configured with the bank’s real capacity, turns the whole thing from a theory into a daily reading. Most installations are adjusted in the first month once the numbers arrive, and the adjustment is usually a charging source rather than a bigger battery.
What people ask before they order
Which source should be sized largest?
The one available in the conditions you actually travel in. A van used in summer sizes solar; one used through winter sizes the DC-DC charger, because driving happens regardless of the sky.
Can the sources fight each other?
They can, if two chargers apply different profiles to the same bank at once. Devices designed to coexist say so, and several DC-DC units include a solar input specifically to arbitrate between the two.
Last reviewed 16 September 2026