How to size a caravan solar and battery system that actually works

Ninety-five percent of the solar and battery questions I get come down to the same thing: someone bought a system based on what the bloke at the dealership told them, and now they're waking up to a flat battery three days into the Birdsville Track. The system isn't faulty. It was just never sized for the job.

I've been living out of a 21-foot van for nine years. I've got this wrong myself — twice, actually — and I'll tell you exactly what I've learned the hard way.

Start with what you actually consume, not what you wish you consumed

Most sizing guides tell you to add up your appliance wattages and multiply by hours. That's the right framework, but the numbers people use are fantasy. A 12V compressor fridge doesn't draw 45W continuously. It cycles — typically running the compressor somewhere between 30 and 50 percent of the time depending on ambient temperature, how often you open it, and what it's set to. On a 35-degree day in the NT, expect that fridge to cycle harder. Budget 50–60Ah per 24 hours for a 60-litre unit in summer. Not 20Ah.

Here's a realistic daily load for a two-person van doing genuine free camping:

• Compressor fridge (60L) — 50–60Ah
• LED lighting (3 hours evening) — 5–8Ah
• Phone and tablet charging — 3–5Ah
• 12V water pump (short runs) — 2–3Ah
• CPAP machine (if applicable) — 20–30Ah
• Diesel heater fan (low setting, 4 hours) — 4–6Ah

No microwave, no air con, no inverter kettle. That's the easy version. Add those up and you're looking at 60–100Ah per day before you touch anything high-draw. Now you have a real number to work from.

How much solar do you actually need

Here's where the marketing numbers come unstuck. A 200W panel does not produce 200Wh per hour. It produces that at Standard Test Conditions — 25°C cell temperature, 1000W per square metre of irradiance, no shading, perfectly perpendicular to the sun. In the real world, mount that panel flat on a van roof in summer with the cells running at 50–60°C and you'll see a 15–25 percent reduction in output before you account for cable losses, MPPT efficiency, and the fact that most camps have a tree or two.

A practical rule I've used for years: expect 4–5 peak sun hours per day for most of inland Australia in the warmer months, 3–4 for southern coastal areas in winter. Multiply your usable panel wattage by those hours and apply a 0.75 system efficiency factor. So a 400W array in outback Queensland in April gives you roughly 400 × 5 × 0.75 = 1,500Wh, or about 125Ah at 12V. That covers our realistic 100Ah daily load with some left over for banking.

In my experience, 300–400W of panel is the minimum worth installing if you want genuine off-grid capability for two people. Less than that and you're relying heavily on your DC-DC charger to make up the gap — which is fine, but you need to know that going in.

Battery capacity: the two-day buffer rule

Size your battery bank to carry two days of consumption without any solar input. That's your insurance for overcast days, dense tree cover, and the days when you simply don't move. For a 100Ah daily load, you want at least 200Ah of usable capacity.

The word "usable" matters enormously here, and it's where the lithium vs AGM argument gets concrete rather than theoretical. A 200Ah AGM bank should only be discharged to 50 percent — so you have 100Ah to work with before you damage the batteries. A 200Ah lithium (LiFePO4) bank can be taken to 80–90 percent depth of discharge safely, giving you 160–180Ah of real headroom. To get equivalent usable capacity from AGM, you need double the rated amp-hours. The weight and space penalty of doing that is significant in a van where payload matters.

I'm not anti-AGM — they have their place, especially in older vans where the charging profile is already set up for them. But if you're building a system from scratch for remote travel, the numbers favour lithium pretty clearly once you look past the upfront cost.

The DC-DC charger is not optional

A lot of people treat DC-DC chargers as an add-on. They're not — they're the backbone of the system for anyone who moves regularly. Your alternator is the most reliable charge source you have. Solar is weather-dependent and time-dependent. The alternator runs every time the engine runs.

A DC-DC charger (also called a battery-to-battery charger, or B2B) takes voltage from your vehicle's starting battery and delivers a proper multi-stage charge profile to your house battery. The old-style Anderson plug direct connection doesn't do this — it just passes voltage through, which means your house battery gets a surface charge at best and can actually cause problems with smart alternators in modern vehicles, which reduce output when they detect a low-resistance load.

For most touring setups with 100–200Ah of lithium, a 40A DC-DC charger is the standard. At 40A input you're putting roughly 480W into the battery bank while driving — that's equivalent to having another 100W solar panel running at peak, all day, regardless of cloud cover. On a driving day from the Birdsville Track up to Mungerannie, you can leave camp with a 50 percent battery and arrive at camp with a full one. The DC-DC charger makes that possible; the solar maintained it through the day.

Size the DC-DC to roughly 20–30 percent of your battery bank in amp-hours. So a 200Ah bank suits a 40–60A charger. Going bigger than that rarely helps — you hit diminishing returns and the acceptance rate of the battery becomes the limiting factor anyway.

Common sizing mistakes I see constantly

The single most common mistake is underestimating fridge load. People read the sticker on the fridge door, see "45W", and budget for a fraction of what it actually draws. Measure it with a clamp meter or a 12V monitor like a Victron BMV-712 for a full 24 hours before you commit to a battery size. The BMV-712 is one of the genuinely useful pieces of gear on my van — not because it's glamorous, but because real consumption data is worth more than any online calculator.

The second mistake is ignoring cable losses. A 6mm² cable run over 5 metres can drop enough voltage under load to measurably reduce charging efficiency. Run the thickest cable the terminals will accept for high-current runs, and keep runs as short as possible. It's not exciting, but neither is diagnosing a mystery underperformance issue at a remote camp.

Third — and this one surprises people — is mixing old AGM batteries with a new lithium-ready charger profile. If you've upgraded to lithium but your solar controller or DC-DC charger is still set to an AGM profile, you're both undercharging the lithium and potentially stressing it. LiFePO4 wants a charge voltage of around 14.4–14.6V and a float of 13.5–13.8V. AGM profiles typically float higher or cut off differently. Check the settings. This is documented in the Victron and Redarc technical manuals, and I'd trust those over the generic advice you'll find on forums.

What a practical remote touring system looks like

For a two-person van doing genuine outback travel — the kind of trip where you're three days from a town with a proper hardware store — here's the minimum system I'd recommend based on experience:

• 200Ah LiFePO4 battery (one 200Ah unit, or two 100Ah in parallel)
• 300–400W of solar (two 150–200W panels in series where possible)
• 40A MPPT solar controller (Victron SmartSolar or equivalent)
• 40A DC-DC charger
• Battery monitor showing state of charge, current, and cumulative Ah consumed

That system, properly cabled and configured, will run a fridge, LED lighting, device charging, and a CPAP indefinitely in good sun. Add a diesel heater to the mix in winter and you might want to bump the battery to 300Ah or add a third panel.

If you're heading somewhere like the Gunbarrel Highway or the Anne Beadell, where driving days are long and camps are in open desert, the DC-DC charger will carry you through any overcast period without drama. If you're camped under the canopy on the Gibb River Road for several days, lean on that solar harder and expect to run the engine for 30–40 minutes if you hit a run of heavy cloud.

One honest opinion on all-in-one systems

The market is full of "integrated" solar/battery management systems that promise a single box solution. Some are genuinely well-engineered. But I've watched enough of them fail in the field — usually the charge controller side, or the bluetooth module that's needed to adjust settings — to be sceptical of buying a system where one failure takes out multiple functions. Separate, quality components from reputable brands mean a fault in one doesn't strand the whole setup. A blown MPPT controller is a $200–400 fix if you have a standalone unit. It's a more serious problem if it's integrated into a $2,000 combo unit that's no longer supported.

Do the load calculation, match the panel array to your location and usage, size the battery for two days of autonomy, fit a DC-DC charger, and monitor what's actually happening. That's the whole system, honestly. Everything else is details.

— Marto Kelleher, Founder & Touring Editor