Usable battery
Nominal Wh (V × Ah) × the usable fraction your chemistry allows. Protect lead-acid from deep cycling; let lithium off the leash.
01 / THE QUESTION
Independence is a number, not a vibe.
The Nebuchadnezzar carries the classic factory loadout: a 100 Ah leisure battery and a 110 W roof solar panel. The question every wild-ish itinerary hangs on — the CL nights, the far-north pitches, the quiet Brit Stops — is simple: how many nights does that actually buy before the battery hits its floor?
The answer isn't a single number. It's a budget with three lines: what the battery can genuinely give, what the van genuinely takes, and what the sky genuinely returns. Get those three honest and the nights fall out of the division.
02 / THE ENERGY BUDGET
Amp-hours lie politely. Watt-hours don't.
Convert everything to watt-hours and the fog clears. A 12 V, 100 Ah battery holds a nominal ~1,200 Wh — but nominal isn't usable. A conventional lead-acid leisure battery shouldn't be cycled much below half charge if you want it to live, so plan on roughly ~600 Wh usable. (A lithium upgrade changes this line dramatically — most of its nominal capacity is genuinely usable — which is exactly the kind of what-if the calculator is for.)
The spending side is where guesses go to die. The honest method is to list every load with its wattage and its hours of actual use — and where a device cycles, like the compressor fridge, to measure rather than assume. The 70 L fridge is the marathon runner of the budget: modest draw, but it runs day and night, and its duty cycle swings with ambient temperature and how often the door opens. In cold weather the heater's fan becomes the second marathon runner — the gas does the heating, but the blower does hours.
Daily demand
Fridge duty cycle first, heater fan in cold weather second, then lights, water pump, phone and device charging, and anything on an inverter — inverters tax every watt they pass.
Charge sources
Solar while pitched, the engine's charging while driving, and hook-up when you take it. Endurance is store ÷ (spend − income).
03 / SOLAR'S SHARE
What 110 W honestly returns.
A panel's wattage is a laboratory number: perpendicular midsummer sun, cool cell, clean glass. A flat-mounted van panel in Britain meets none of those conditions, so the working method is panel watts × effective sun-hours × system losses — and "effective sun-hours" is the brutally variable term. A bright, long June day on an open pitch can make a 110 W panel a genuine daily contributor; a grey November afternoon under Borrowdale's oaks returns a trickle that barely covers the fridge's thinking time.
The honest planning stance: treat summer solar as a meaningful extension of endurance, treat winter solar as a bonus you don't budget for, and remember shade is binary — a pitch under trees can cost more than the season does. The calculator lets you set the solar contribution per scenario instead of pretending one number covers April and December.
04 / A WORKED NIGHT
Illustrative numbers, real pattern.
Round figures for the shape of the thing — run your own loads through the tool for your answer.
Usable slice of the 100 Ah battery, protecting the plates from deep discharge.
Fridge around the clock, LED lights, pump, two phones and a tablet, no inverter luxuries.
An unshaded British summer day's plausible return from 110 W. Winter: budget near zero.
Summer: multi-night. Winter: nightly deficit.
In summer the panel can nearly balance a frugal day — endurance stretches to several nights. In winter the same van runs a deficit every night, and the battery floor arrives on a schedule you can compute in advance.
05 / STRETCHING IT
Doctrine for longer legs.
The archive's routes already encode the main trick: the two-night rhythm with transits between means the engine recharges the bank every move day, and a hook-up night every few pitches resets the budget entirely. Add fridge discipline (shade the van's fridge side, pre-chill hard on hook-up, open briefly), charge devices while driving rather than while parked, and be ruthless about inverter use — anything that can charge from 12 V should.
And know your floor before you need it: the far-north file's departure check says it plainly — run the off-grid calculator; the far north is where it earns its keep. The tool models battery chemistry, loads, solar scenario and driving days, saves locally in your browser, and pairs with the Payload Core — because every extra battery and panel you're tempted to fit costs kilograms the other calculator has to find.