• Days of autonomy (or reserve) is the number of days of power one stores for periods of reduced or non-existent charging.
• A system whose capacity == daily power requirements has one day of autonomy; 24 hours after a full charge there will be no usable capacity left.
• Depending on charging sources we might think in whole or fractional days

about these summaries

• the decision on how much capacity to purchase is economic as well as technical

These charging sources lend themselves to planning in whole days because they are either ON or OFF, available or unavailable:

• shore power charging with a converter or other charger
• no field charging at all (charging at home only, no charging over weekend outings)

Joe Campervan requires 1,000Wh (1kWh) per day. He will fully charge his bank at home before leaving on a 2-day campout.

• his total requirements are 2,000Wh (2 days x 1,000Wh).
• the bank capacity required depends on the usable capacity percentage of his chosen battery chemistry, ~50% for lead and ~80% for lithium
  • lead: 2,000Wh / 0.5 = 4,000Wh of capacity required. At 12v nominal this would mean a ~333Ah bank.
  • lithium: 2,000Wh / 0.8 = 2,500Wh of capacity required. At 12.8v nominal this would mean a ~195Ah bank.

If he is on shore power at an RV park for one of those days then (from a power perspective) his off-grid is only 1 day, so his total requirements and required capacity is halved. This is true because shore power is effectively limitless; you are not offgrid.

Solar and alternator charging are different.

Alternator charging is a bit more arithmetically complex because it involves both time the engine is running and the bank's current acceptance in Amps from the alternator. Let's assume Joe is driving 1 hour per day and is running a a 30A DC-DC charger to charge his Li bank. The DC-DC will produce ~400Wh of charging daily¹⁾. So now the bank only has to cover 1,200Wh on the outing rather than the whole 2,000Wh. (2,000Wh - (2 days x 400Wh charging)). So now he only needs ~117Ah of Li (1,200Wh / 0.8 usable / 12.8v nominal).

; in any given 24hr period the setup will make at least //some// power no matter the conditions or actions of the owner.

• on average, the setup will make the power your wattage and insolation models predict
• on clear, sunny days you will exceed the average²⁾
• on overcast days you might get 1/3rd your average harvest

Solar harvest is highly variable based on time of year and geographical location.

The decision between larger bank vs smaller bank + field charging is up to Joe. The frequency and duration of his outings will affect the cost/benefit analysis.

If his requirements are modest and outings are short and infrequent he might buy enough Wh to make it through his campouts with zero charging.

If his requirements are heavier or outings longer, more frequent (or even full-time) then it is no longer practical to buy enough Wh to cover the outing; he will need some amount of field charging. If he is overlanding/roadtripping he might lean into alternator charging. If he will be camping in place he might invest more in solar.