A campervan energy system is not defined by the kit you bolt in. It is defined by how you travel: how many days you sit parked, how long you drive between them, whether you plug in, how much sun the roof actually catches, and how cold it gets while you sleep. Two vans with identical hardware can behave completely differently because their owners travel differently. So this guide plans from travel states, not from a shopping list, and carries one small worked example all the way through so you can see how the numbers connect.

Two boundaries first. Everything here is a preliminary, educational estimate — a plan to reason about and refine, not an engineered installation. And the parts that make a van dangerous — vehicle electrical integration, shore-power wiring, mounting and weight, a battery in an occupied space — are deliberately left to the exact vehicle and equipment documentation and to qualified people. This is a higher-risk application than a stationary system; treat the stop conditions below as real.

Start with travel states, not hardware

Before any component, write down the conditions the system has to survive. A component list without them cannot tell you whether anything is big enough:

  • Daily load energy — what you actually use in a day, in watt-hours.
  • Continuous, simultaneous, and startup power — what runs at once, and what surges when it starts.
  • Days parked without driving, and driving hours when you do move.
  • Shore access — how often you can plug in, if ever.
  • Solar resource — season, latitude, weather, shading, roof orientation.
  • Autonomy target and reserve — how long you must last with no charging, and how much battery you refuse to touch.

Plan at least four states: a normal parked day, a driving day, a shore-connected day, and a poor-solar or cold day. The tightest one sizes the system. Building the daily figure is its own method — see how to build a daily energy load profile.

Worked travel-day scenario

Here is a deliberately small parked day, with every figure measured or bounded at the battery side (that is, energy actually drawn from the battery, with conversion losses already folded into each entry). These are illustrative numbers to show the method — not a "typical van," which does not exist:

Fan            20 W × 8 h                 = 160 Wh
Refrigeration  measured interval energy   = 480 Wh   (not nameplate × 24 h)
Lighting       15 W × 4 h                 =  60 Wh
Laptop         60 W × 2 h                 = 120 Wh
Controls/standby (monitor, parasitic)     = 100 Wh
                                   ─────────────────
Daily energy   E_day                      = 920 Wh/day  (0.92 kWh)

Sanity check: 920 Wh ÷ 24 h ≈ 38 W average. An average-power back-check like this is the quickest way to catch a hidden high-energy load — if the average looks implausibly low, something is missing. Note the refrigeration line: a compressor's nameplate wattage times 24 hours would be wildly wrong, because it cycles. Use measured or duty-cycle energy for anything that switches on and off.

This example leaves out water heating, cooking, space heating and cooling, and pumps on purpose. Add your real loads; the method is the same.

Turn the daily budget into an autonomy scenario

Storage is sized from daily energy, your no-charging autonomy, and the fraction of the battery you can actually use:

Nominal energy  E_nominal = E_day × days ÷ u

  E_day = 0.92 kWh/day        (from above, battery-side)
  days  = 2 days              (your choice: no charging for two days)
  u     = 0.80                (usable fraction, after depth-of-discharge + reserve)

  E_nominal = 0.92 × 2 ÷ 0.80 = 2.30 kWh nominal

Check it in reverse: 2.30 kWh × 0.80 = 1.84 kWh usable = 0.92 kWh × 2 days. The ledger closes.

The usable fraction u does the heavy lifting, and it is not a constant. It depends on chemistry, and temperature, aging, and your BMS reduce it further — a cold LiFePO₄ pack in winter delivers less than its summer self. Take u from your battery's data sheet and usable vs. nominal battery capacity, not from optimism: at u = 0.70 the same scenario needs 2.63 kWh instead of 2.30. This arithmetic sizes energy only — it does not choose your voltage, chemistry, pack, BMS, mounting, or protection.

Treat roof solar as a variable source

Solar on a van roof is a genuinely variable source, and its nameplate is a ceiling, not a daily yield. The Department of Energy lists the drivers plainly: location, season, time of day, landscape, and weather. Add the van-specific ones — limited roof area, fixed (usually flat) orientation, and shading from vents, racks, and wherever you park.

To see what array would replace a day's energy under one stated scenario:

Array watts  P_array = E_day ÷ (PSH × derate)

  E_day  = 920 Wh/day
  PSH    = 4.0 h        (equivalent peak-sun-hours for your location/season/orientation)
  derate = 0.70         (temperature, wiring, controller, soiling, availability)

  P_array = 920 ÷ (4.0 × 0.70) ≈ 330 W

Then treat that 330 W as a floor and test the bad days. At PSH = 2 h — a grey winter week or a shaded pitch — the same load needs about 660 W, nearly double. This is an energy-balance scenario, not a promise of daily harvest and not a roof-fit check; a general-purpose PV model does not know your van roof or whether the battery can even accept the energy. Size for the trips you actually take, and expect solar-only days to fall short in poor conditions.

Budget alternator charging from driving opportunities

Charging while you drive is valuable, but a modern alternator is not a fixed-voltage tap you can wire straight to the house battery. Manufacturer documentation — Victron's Orion XS DC-DC charger manual is one current example — describes exactly why: smart alternators vary or stop their output, so a controlled DC-DC charger is used to regulate current, keep the starter and service batteries separate, and charge only while the engine runs. It also notes behavior can differ even within one vehicle brand, which is the whole point: the exact vehicle and charger decide, and integration must be commissioned on your van.

What you can do at the planning stage is bound the energy a driving session adds:

Charge energy  E_charge = V_out × I_out × t

  V_out = 14 V        (average charger output voltage)
  I_out = 30 A        (average output current — illustrative, no specific product)
  t     = 1.5 h       (driving time at that average)

  E_charge = 14 × 30 × 1.5 = 630 Wh   (battery-side, upper bound)

That 630 Wh is an upper bound under the stated averages, not a guarantee. Real output is reduced by smart-alternator behavior, current limits, BMS control, temperature, charge taper, cabling, and the actual drive cycle. Compare the energy you observe per session against this figure rather than assuming nameplate current. Combining alternator charging with solar and shore onto one bank is its own topic — see combining solar, alternator, shore, and generator charging.

Stop — this is qualified work. All alternator and vehicle wiring, ignition, D+, CAN, factory-ground, and body-integration work belongs to people competent on your exact model-year vehicle. Get it wrong and you can overload the alternator, disable starting, disturb vehicle electronics, or start a fire. If the starter battery discharges unexpectedly, or you see a vehicle fault, heat, odor, or abnormal charging, stop and reassess.

Treat shore connection as a governed AC interface

Shore power is not just another watt-hour input. It introduces an AC supply, a charger, transfer or source-selection behavior, protective functions, earthing and bonding questions, connector and environment requirements, and local motor-caravan or campsite rules. Its installation is excluded from generic self-build guidance for good reason: it can put hazardous AC voltage onto the vehicle body if grounding, bonding, or transfer are wrong.

Plan what you want shore power to do — run AC loads, charge the battery, take over from the inverter — and route the how to a qualified installer and the rules that apply where you travel. Stop immediately on a damaged or wet inlet or cord, a hot or loose connection, unexplained protective-device trips, or any abnormal supply or polarity indication, and never improvise an adapter or a connection into premises wiring.

Mass and placement are system inputs

Every kilogram of battery, inverter, cable, and enclosure comes out of the vehicle's payload, and batteries are both heavy and mounted low. Manufacturer upfitter documentation — again the Sprinter guidelines are a current example — treats payload, permissible axle loads, gross vehicle mass, and centre of gravity as planning inputs, and points you to the exact model's plated figures rather than generic van numbers.

Build a weight budget alongside the energy budget: equipment, mounting, cables, enclosures, fluids, plus passengers, water, fuel, and luggage. Then verify gross mass, individual axle loads, and centre of gravity against your exact base vehicle and local rules, and weigh the finished conversion where required. Overloading the vehicle or an axle is a road-safety problem, not just a paperwork one.

Design for motion, climate, occupation, and service

A van's environment can invalidate assumptions that hold fine in a shed. Movement, vibration, and impact fatigue connections and mounts; heat, cold, condensation, dust, and water attack equipment rated for gentler conditions; and the whole system usually shares an occupied sleeping space. Mounting and environmental protection are therefore functional and safety requirements, not finishing touches: equipment needs restraint, clearance, cooling or ventilation, an appropriate environmental rating, and enough access to service it.

Stop conditions for the battery. A battery or converter fault in a sleeping space can expose occupants to heat, smoke, gas, or fire with the exit blocked. Take the system out of service on any heat, odor, swelling, deformation, leaking, hissing, popping, smoke, flame, water exposure, or collision damage, and get qualified help for battery location and enclosure design and for assessing any battery after an impact. Do not judge a battery safe from its voltage alone.

Draw the measurement boundary before trusting the trip log

A shunt-based battery monitor only counts what flows through the shunt. Victron's SmartShunt manual states it directly: any load or source that bypasses the shunt is excluded from its current and state-of-charge reading. On a van that matters, because vehicle-factory circuits and the starter battery often sit on separate paths. If your fridge, a DC socket, or an alternator feed bypasses the sensor, your trip log will quietly drift from reality — and you will size the next system from bad data. Decide what the monitor is meant to measure, and put every relevant service load and charge source on the battery side of it. A monitor's state of charge is an estimate, not a direct measurement of battery health.

Key takeaways

  • Plan from travel states — parked, driving, shore, poor-solar — not from a hardware list; the tightest state sizes the system.
  • Work at the battery-side boundary, use measured energy for cycling loads like refrigeration, and back-check with average power.
  • Storage: E_nominal = E_day × days ÷ u; the usable fraction u (chemistry, temperature, aging, BMS) drives the result.
  • Solar nameplate and alternator/shore current are ceilings, not daily energy; convert each through real availability and test the bad days.
  • Vehicle wiring, shore power, mounting/weight, and a battery in a sleeping space are qualified, higher-risk work — respect the stop conditions.
  • Every figure here is preliminary; confirm the exact vehicle, equipment, and local rules, and get specialist review before building.

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