How Much Solar Do I Need For My Van?

Start with the energy your van uses in a day, then compare it with the energy solar could replace in the places and seasons you will travel. Roof space alone cannot answer the question.

Separate Watts, Watt-Hours And Stored Energy

Watts describe power at an instant; watt-hours describe energy over time. At a constant or representative average power, Wh = W × hours. Divide Wh by 1,000 to express the same energy in kWh. The U.S. Energy Information Administration explains power and energy units.

A panel's watt rating, a day's energy demand and a battery's stored energy answer different questions. Keep the units beside every number. If you record battery capacity in amp-hours, the voltage basis matters; do not compare an Ah number directly with a daily Wh total.

Build A Daily Load Record

List each device, the time it actually runs, and where its energy is measured. A refrigerator cycling through a day needs a representative daily measurement or supported duty-cycle assumption, rather than treating its maximum power as continuous. Separate measured values, manufacturer information and guesses in your notes.

Choose a consistent boundary for the budget, such as energy drawn from the house battery. An AC appliance measurement may omit inverter losses; a battery-side measurement may already include them. Avoid adding the same loss twice. Check the selected inverter's manual for operating modes and idle draw. For example, Victron's Inverter Smart manual distinguishes ordinary operation from ECO behavior at no load; it supplies no universal idle allowance for other products.

Every input above is invented to show the calculation. Replace all four rows with your equipment and usage evidence. For this example only, add a separately identified 100 Wh/day allowance for remaining uncounted use and losses. The planning demand is therefore 800 Wh/day. That allowance is a placeholder, not a recommended percentage or a validated loss model.

Decide What Solar Must Replace

Keep other charging sources in a separate record. Credit only the energy you can reasonably expect under the trip plan, measured or estimated at the same boundary as demand. A charger's power label alone does not tell you how much energy it will deliver during your actual driving or shore-power time.

For the worked example, assume no dependable other charging, so the ordinary solar replacement target stays at 800 Wh/day. If you can document 200 Wh/day from another source, the arithmetic target becomes 600 Wh/day. Do not count a possible drive or campsite connection as guaranteed energy.

Test Solar Resource And Loss Assumptions

A simple screening relationship is daily solar energy ≈ array watts × equivalent sun hours × retained fraction. Equivalent sun hours express solar energy as hours at a reference irradiance of 1 kW/m²; they are not the number of daylight hours. The retained fraction represents the losses between that idealized array-energy calculation and your chosen budget boundary.

Use location, month, mounting orientation and shade information when replacing the assumptions. PVWatts documentation distinguishes nameplate capacity, losses, orientation and monthly solar radiation. Its modeled AC output is not automatically energy stored in a van battery. The results cautions explain limits of modeled production. A moving van's changing parking conditions need their own assessment.

The two sun-hour values and both retained fractions are deliberately hypothetical. They are not local weather results, equipment efficiencies or recommended defaults. The spread shows why one confident watt number can hide important assumptions. Rounded results are not shopping sizes or promised yields. Do not apply this retained fraction again to an output estimate that already includes the same losses.

Plan For Shortfalls And Recovery

Replacing an average day's demand does not necessarily recover an earlier deficit. In a separate illustrative calculation, recovering 400 Wh over two days adds 200 Wh/day to the ordinary 800 Wh/day target, making 1,000 Wh/day for those recovery days. Record that temporary target explicitly rather than hiding it inside a normal-day estimate.

Also record what you would reduce, where another charging opportunity could come from, and how long you intend to remain parked. Keep battery usable-energy planning separate from array generation. A large array does not by itself establish battery capacity, charging acceptance or a reliable supply through consecutive poor-solar days.

Turn The Budget Into The Next Check

1. Replace fictional load values with dated measurements or documented assumptions.
2. Choose the places, seasons and parking conditions you actually need to cover.
3. Document which losses and other charging sources the calculation includes.
4. Compare normal-day, poor-solar and recovery scenarios.
5. Check roof layout, panel ratings and the selected equipment manuals before selecting components.

Use the solar-panel ratings reference for label terminology and the electrical and solar reading route for supporting material. Keep unresolved choices in the decision log. This energy budget does not size cables, protection, controller limits or structural mounts.

Use The Blank CSV

The blank energy-budget CSV records load name, measurement boundary, average power, hours, daily energy, evidence type, source/date and notes. Fill in a measured daily-energy value directly when that is your evidence; otherwise show the power-and-time calculation. It does not calculate or total entries automatically.

Keep separate notes for other charging, solar assumptions and recovery days. Update the budget when the equipment, season or travel pattern changes. Treat an unknown value as an open check, not zero consumption.