Running a Troopy fridge, water pump and lights off one battery bank: the real numbers

Somewhere on the Anne Beadell a few years back, I sat at Ilkurlka with a cup of tea and did the sums on paper because I genuinely didn't trust my monitor. Three cloudy days, a fridge running harder than usual in 38 degree heat, and a battery that had dropped further than I expected. Turns out the maths was fine. My assumptions about cloudy-day solar output weren't.

Most single-battery setups aren't undersized because the owner did the sums wrong. They're undersized because nobody wrote the sums down in the first place, and everyone quietly assumed a good sunny day is the baseline rather than the exception. If you run one battery bank off a Troopy, ute canopy or slide-on camper, this is the budget I actually work from, with real numbers, not the rounded-up ones off a brochure.

Start with what actually draws power, not what you think draws power

The compressor fridge is the big one, and it's also the one people get most wrong because they quote the wattage on the sticker instead of the duty cycle. A 40-60 litre 12V compressor fridge (Engel, Waeco/Dometic, Evakool) typically draws somewhere around 0.8-1.2 amps at 12V while the compressor is actually running, but it doesn't run constantly. In mild weather it might cycle at 30-40% duty, in Kimberley-in-October heat closer to 60-70%. That's the difference between roughly 25 amp-hours a day and 45+ amp-hours a day off the exact same fridge.

Everything else is smaller but it adds up in a way people underestimate. A water pump might only draw 4-5 amps, but it's for seconds at a time, so daily use is negligible — a rounding error, not a line item. LED lighting for an evening is 1-2 amp-hours total if you're not silly about it. Laptop charging is 3-5 amp-hours for a full charge. UHF radio on standby all day is close to nothing; transmitting is more but you're not transmitting for hours. Phone charging is 1-2 amp-hours. None of these individually matter. Collectively, on top of the fridge, they can add another 15-20 amp-hours to a daily budget that people built around the fridge alone and nothing else.

I run a Redarc BMS1230 with a shunt, so I actually see amp-hours in and out rather than guessing off a voltage reading, and I'd say that's the single biggest upgrade for a solo one-battery rig. If you're still eyeballing state of charge off a voltmeter, have a read of our piece on battery monitors and shunts — a voltage number alone is close to useless once you're running lithium, because the voltage curve is flat for most of the discharge and then falls off a cliff.

A realistic daily budget for one person, one fridge

Here's roughly what a genuinely modest solo setup looks like across a day of free camping, no generator, no shore power:

• Fridge (40L compressor, moderate heat, well-insulated box): 35-45 amp-hours
• Lighting, phone, UHF, water pump combined: 10-15 amp-hours
• Laptop or tablet charging most days: 3-5 amp-hours
• Total daily draw: roughly 50-65 amp-hours

That's a single person keeping things simple. Add a second fridge, a CPAP machine, or a Starlink dish for remote work and you're easily doubling that number. If any of that applies to you, our piece on sizing a genset habit down covers where a battery bank alone stops being realistic.

What one battery actually gives you

A single 100Ah lithium (LiFePO4) battery gives you close to 100Ah of genuinely usable capacity if you're not fussy about pushing it right down, though I personally don't like running below about 20% as a habit — it's not that lithium chemistry demands it the way AGM does, it's that I want buffer for a run of bad weather, not a battery that's already near empty on day one of three cloudy days. Call it 80Ah of comfortable daily budget from a 100Ah lithium.

An AGM of the same 100Ah rating gives you roughly half that in practice, because you're not meant to discharge AGM much past 50% without shortening its life significantly. So a 100Ah AGM is really a 50Ah battery for planning purposes. This is the number people get burned by — they buy "100 amp-hours" and budget as if they have 100 amp-hours, then wonder why the battery's tired after eighteen months. We've written a fuller comparison of the two chemistries in lithium vs AGM: which caravan battery setup is right for you, but for a single-battery remote rig, the usable-capacity gap is the whole story.

Run the numbers against that 50-65Ah daily budget and a single 100Ah lithium covers you comfortably on one battery. A single 100Ah AGM does not — you're drawing 50-65 out of a battery that only wants to give you 50, which means you're either flattening it regularly or you need a bigger AGM bank than a lot of one-battery setups actually carry.

Solar has to replace what you used, not just what's sunny

This is where my Ilkurlka afternoon went sideways. A 150-200W fixed roof panel on a Troopy canopy, in genuinely good sun, angled reasonably well, might give you 40-60Ah back into the battery across a full day. That sounds like it comfortably covers a 50-65Ah daily draw. It does, on a clear day, with the panel not shaded by half the roof rack or angled flat against a low winter sun.

Cloud cuts that output by a third to a half. Smoke haze from a burn-off, which is common through the Top End dry season, does something similar. A flat roof-mounted panel angled wrong for the season can lose 20-30% compared with one that's tilted properly, which is exactly the trade-off covered in our article on solar panel angle and placement. Fixed roof panels are convenient and I run one myself, but I'd argue anyone doing genuinely remote touring — Anne Beadell, Gunbarrel, the back roads off the Gibb River Road — is better off adding a portable folding panel they can chase the sun with, rather than assuming the roof panel alone will always keep up.

The honest planning number, in my experience, is to size solar for roughly 130-150% of your daily amp-hour draw, not 100%. That buffer is what gets you through two or three overcast days without the battery just sliding flat, and it's the difference between free camping being relaxing and free camping being a low-grade stress every afternoon watching the sun disappear behind a ridge.

The DC-DC charger is your backup plan, not your primary plan

If you're driving most days, a DC-DC charger off the alternator is doing real work refilling the battery while you're on the move, and it's worth having even if your solar is well sized. A 20-25A DC-DC charger, run for two or three hours of driving, can put a genuinely useful chunk back into the battery — more, in cloudy weather, than the solar managed all day. For anyone still deciding what to fit, our explainer on DC-DC chargers covers sizing against your alternator and battery chemistry properly.

But I wouldn't build a power budget that relies on driving every day to stay topped up. Plenty of free camping is about staying put — three or four nights at a spot like Bullara Station or a quiet bush camp off the Gibb River Road — and on those days your DC-DC charger is doing nothing at all. Solar and battery capacity need to cover you on their own, with the DC-DC charger as the thing that tops you back up on driving days, not the thing holding the whole system together.

Where the sums actually fall apart

Nearly every flat battery I've heard about on the road, mine included, traces back to one of three things. Parasitic drains nobody accounted for — a fridge lid not quite sealing, a 12V socket with something plugged in and forgotten, a inverter left on standby — which is worth a proper read through if you haven't already, in our piece on parasitic drains nobody checks. Underestimating fridge duty cycle in real heat rather than showroom conditions. Or assuming solar output on the worst three days of a trip will match the best three days, when in reality it can be half.

None of this is complicated maths. It's mostly just being honest about worst-case rather than best-case numbers, and building in enough buffer that a run of bad luck is an inconvenience rather than a genuine problem four hundred kilometres from the nearest servo. Carry a means of checking your actual state of charge properly, not just a voltage reading, and always have a way to call for help if the system genuinely fails somewhere remote — a satellite phone or PLB, not just a UHF, once you're off the beaten track.

I still carry a paper notepad with rough daily amp-hour numbers scrawled in it from trip to trip. Old habit, and probably an unnecessary one now the shunt logs everything anyway. But it's saved me from a flat battery more than once, and there's something calming about doing the sums yourself rather than trusting a screen entirely.

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