Solar and battery sizing without guesswork

Solar and battery sizing is arithmetic. The inputs are what your appliances draw, how many days you want to sit still, and how much sun the sky gives you where you are camped. Everything else is a preference.

What the register publishes

CampSpec records 1,555 models. Fixed solar watts are published for 86 of them, battery capacity in amp-hours for 49, and both for 47. Neither figure is published for 1,467 models. Battery chemistry is published for 9 models and inverter output for 5.

That gap is the first finding. Solar and battery fitment is the most heavily optioned part of a modern van, and most manufacturers publish it in the pack list rather than the specification table. The figures below describe the models that do publish, not the market.

Fixed solar, 86 models publishing

Array size	Models
100 W	1
101 to 400 W	19
401 to 600 W	35
601 to 800 W	19
801 to 1,000 W	4
Over 1,000 W	8

Median 600 W, quartiles 440 W and 800 W, range 100 W to 2,033 W. The most common single values are 600 W (30 models), 400 W (19) and 800 W (12).

Battery capacity, 49 models publishing

Bank size	Models
120 to 150 Ah	12
151 to 200 Ah	5
201 to 300 Ah	5
301 to 400 Ah	17
401 to 600 Ah	8
Over 600 Ah	2

Median 315 Ah, quartiles 200 Ah and 400 Ah, range 120 Ah to 1,200 Ah. Every model that publishes a chemistry publishes lithium.

By sub-category, where published

Sub-category	Models	Publishing solar	Median W	Publishing battery	Median Ah
Off-road	720	41	720	27	400
Hybrid	104	5	720	3	300
Semi off-road	150	4	400	3	200
On-road	271	13	400	10	135

The off-road and hybrid medians sit at roughly double the on-road figures on both panels and storage, which is what you would expect when the use case moves from powered sites to unpowered ones.

By price band, where published

Price from	Models	Publishing solar	Median W	Publishing battery	Median Ah
Under $60k	148	3	600	2	120
$60k to $100k	509	37	600	21	315
$100k to $150k	139	14	600	4	200
$150k and above	84	13	600	2	400

Median array size does not move with price across the bands that publish. Panel area on a caravan roof is limited by roof area, not budget. Battery capacity is where the money goes, and the sample publishing battery figures above $100k is too small to read as a trend.

Across the 47 models publishing both figures, the median ratio is 1.9 W of panel per Ah of storage, with quartiles at 1.5 and 2.7. That is a useful sanity check on a quote, not a target.

The sizing method

Five steps. Do them in order and the answer falls out.

1. Build a daily watt-hour budget

Work in watt-hours, not amp-hours, until the very end. Watt-hours survive the conversion between 12 V loads and 240 V inverter loads without confusion.

For each appliance: average watts multiplied by hours used per day. For anything that cycles, such as a compressor fridge, use average draw over 24 hours rather than nameplate watts.

Load	Basis	Wh per day
12 V compressor fridge, 90 to 130 L	45 W average, warm weather	600
LED lighting	6 lamps, 2.5 W, 4 h	60
Water pump	60 W, 30 min	30
Roof vent fan	15 W, 8 h	120
Phone and laptop charging	DC outlets	120
Satellite internet terminal	30 W average, 5 h	150
Diesel heater	30 W average, 7 h	200
Inverter loads (kettle or induction)	1,500 W, 10 min	250
Inverter standby	20 W, 6 h powered	120
Daily total		1,650

Notes on the harder numbers. A compressor fridge is the largest and most variable load: 300 to 500 Wh per day in mild weather, 600 to 900 Wh at 35 degrees or with the van in full sun. A satellite terminal draws 25 to 45 W on the compact units and 75 to 110 W on the full-size ones, so it is worth measuring rather than assuming. An inverter’s own standby draw is real and often forgotten: switch it off when it is not being used.

Air conditioning is deliberately absent. A rooftop unit runs 900 to 1,400 W while compressing. Running one off battery is a separate exercise, not a rounding error on this budget.

2. Choose autonomy days

Autonomy is how many days the bank must carry the load with no useful charging. It is the worst case, not the average: three consecutive overcast days in a gorge, or a shaded site under trees.

Two days is a common planning figure for coastal touring. Three is reasonable for remote free camping. Beyond four days the bank size usually becomes heavier and more expensive than adding panel area or a DC-DC charger and driving every few days.

3. Convert to usable, then nominal capacity

Batteries are not fully usable. The planning depth of discharge differs sharply by chemistry.

	LiFePO4	AGM
Planning depth of discharge	80%	50%
Usable Wh per 100 Ah	1,024 Wh (at 12.8 V nominal)	600 Wh (at 12 V nominal)
Typical mass per 100 Ah	11 to 13 kg	28 to 32 kg
Cycle life at planning DoD	3,000 to 6,000	400 to 600
Charge acceptance	High, accepts full charge current to near full	Tapers, needs long absorption

Two consequences follow. First, a 100 Ah lithium bank delivers roughly 1.7 times the usable energy of a 100 Ah AGM bank at about 40% of the mass, which matters against payload as much as against price. Second, AGM’s tapering charge acceptance means the last 20% of an AGM bank takes hours of absorption that a short winter solar day will not supply, so real-world AGM autonomy is often worse than the arithmetic suggests.

The arithmetic:

usable Wh required = daily Wh x autonomy days
nominal Wh required = usable Wh / depth of discharge
nominal Ah at 12 V = nominal Wh / 12.8 (lithium) or / 12 (lead)

4. Estimate solar harvest

A panel does not produce its rated watts. Rated output is measured at 25 degrees cell temperature with the panel square to the sun. A roof-mounted array in Australia is flat, hot, sometimes partly shaded, and feeding through a controller and cable that lose a few percent.

Use a derate factor of 0.7 for a flat roof-mounted array, 0.75 to 0.8 for a tilted or portable panel aimed at the sun.

daily harvest Wh = rated watts x peak sun hours x derate

Peak sun hours are the number of hours of 1,000 W/m2 equivalent on a horizontal plane. They vary enormously by latitude and season. These are conservative planning figures, not measurements:

Region	Winter PSH	Summer PSH
Tropical north (Darwin, Cairns)	5.5	5.0
Central (Alice Springs)	4.5	6.5
South west WA (Perth)	3.0	6.5
South east QLD (Brisbane)	3.5	5.5
Coastal NSW (Sydney)	3.0	5.5
Victoria (Melbourne)	2.0	5.5
Tasmania (Hobart)	1.5	5.5

Two things stand out. The tropical north is the only region where winter beats summer, because the wet season brings cloud. And a system sized on Alice Springs numbers will fall roughly 60% short on a Tasmanian winter day.

5. Size the bank to the deficit, not the whole load

If the array covers the daily load, the bank only needs to carry the overnight portion plus a margin for bad days. If the array does not cover it, the bank must carry the daily shortfall multiplied by autonomy days. Sizing the bank as though solar produced nothing is how people end up towing 200 kg of unnecessary battery.

Worked example: a couple free camping for three days

Inputs: the 1,650 Wh per day budget above, three days of autonomy, a 600 W roof array (the register median), LiFePO4 at 80% planning depth of discharge, camped in Victoria in winter.

Case A, assume no solar at all.

usable Wh = 1,650 x 3 = 4,950 Wh
nominal Wh = 4,950 / 0.8 = 6,188 Wh
nominal Ah = 6,188 / 12.8 = 483 Ah

Round to a 500 Ah lithium bank, roughly 60 kg. In AGM at 50% depth of discharge the same load needs 9,900 Wh nominal, 825 Ah, and roughly 245 kg. On most vans that is the whole payload.

Case B, the same trip with the 600 W array working.

harvest = 600 x 2.0 PSH x 0.7 = 840 Wh per day
deficit = 1,650 - 840 = 810 Wh per day
usable Wh over 3 days = 2,430 Wh
nominal Wh = 2,430 / 0.8 = 3,038 Wh
nominal Ah = 3,038 / 12.8 = 237 Ah

A 300 Ah lithium bank covers it with margin, at roughly 36 kg. The 600 W array has removed about 200 Ah of battery and 24 kg.

Case C, the same van and same array in central Australia in winter.

harvest = 600 x 4.5 x 0.7 = 1,890 Wh per day

The array now exceeds the 1,650 Wh load. The bank is only carrying overnight use and cloudy days, so 200 Ah is comfortable. The same hardware behaves completely differently 2,000 km north, which is why a single “how much solar do I need” answer does not exist.

Trimming the budget. Dropping the inverter kettle for a stovetop one and switching the inverter off when idle removes 370 Wh per day, or 22% of the load. In case B that cuts the required bank from 237 Ah to 180 Ah. Load reduction is almost always cheaper per watt-hour than either panels or batteries.

What the arithmetic does not cover

Charging from the vehicle. A 25 A or 40 A DC-DC charger delivers roughly 300 W or 500 W while the engine runs. Two hours of driving is a meaningful contribution, but only on days you drive. Do not put it in the autonomy calculation, which is by definition the days you sit still.

Controller type. MPPT recovers more from a cold or partly shaded array than PWM, particularly with higher-voltage panels. The derate factors above assume MPPT.

Shade. One shaded cell string can cut a series array’s output disproportionately. Panels wired in parallel, or with separate controllers, degrade more gracefully under the tree you actually parked under.

Cold and heat. LiFePO4 must not be charged below 0 degrees without low-temperature protection. Cell capacity also falls in the cold, which is the same trip on which solar harvest is lowest.

Water, not just power. Autonomy is usually limited by whichever tank empties first. Across the 692 models publishing a water capacity, the median is 190 L, with quartiles at 164 L and 220 L. At 15 L per person per day, 190 L is roughly six days for two people, so on most vans the power system is the shorter constraint on a three-day stay and the water tank on a week-long one.

Solar and battery figures on CampSpec are recorded only where the manufacturer publishes them, and are left empty rather than estimated where they do not. The full sourcing policy is on the methodology page.