Note: this section is likely not appropriate for newbies; if the ideas are implemented incorrectly it could result in battery murder and disappointment.
Running slimmer-than-normal battery reserves saves space, money, and weight. Here we discuss some ideas about reducing calculated reserves. For purposes of illustration, we will assume the vandweller's daily power requirements are 100Ah, and that they are using lead chemistry batteries.
Days of reserve calculations usually assume poor solar harvest == zero solar harvest. In reality this is almost always incorrect; some solar power is harvested even under dark overcast or rainy conditions.
In light-colored full overcast I see yields 25% of usual. In dark full overcast, as when storming, I see yields 10% of usual. – secessus
A fractional reserve might be a half- or quarter day of battery reserve. This takes into account the (reduced) contributions from your solar, alternator, etc. It may also require going into voluntary power-rationing mode when solar output is very low.
In areas with abundant and consistent insolation like the desert southwest, you may be able to run with no reserves at all.