How to Size a Lithium Battery Bank for Touring

How to Size a Lithium Battery Bank for Off-Grid Touring

Ask ten people how to size a lithium battery bank for touring and you’ll get ten different answers, most of them a guess dressed up as advice. “Just get a 200Ah” is the kind of thing that gets repeated around a campfire, but it ignores the one thing that actually determines the right size: how much power you’re using in a day.

Get the sizing wrong in one direction and you’re carrying weight and cost you didn’t need. Get it wrong the other way and you’re rationing torch batteries by day three because the fridge ate through everything overnight. The good news is that sizing a battery bank properly isn’t complicated — it just needs to start in the right place.

Start with your daily power draw, not the battery size

Before you look at any battery, work out what you’re actually running and how much energy it uses in a 24-hour period. This is your daily energy budget, usually expressed in amp-hours (Ah) or watt-hours (Wh).

A 12V compressor fridge is usually the single biggest draw in a touring setup, and it’s worth budgeting generously for it — daily consumption commonly sits anywhere from around 20Ah up to 50Ah depending on the fridge size, ambient temperature and how often the door’s opened. Hot weather and a warm engine bay can roughly double what the same fridge draws on a mild day, so it pays to size around the higher end rather than the manufacturer’s best-case figure.

Beyond the fridge, add up everything else that draws from the battery: LED lighting (small, but it adds up over an evening), a 12V water pump, phone and laptop charging, a UHF radio left on, and anything running through an inverter — camera batteries, power tools, a coffee machine, whatever’s relevant to how you actually travel. None of these individually move the needle much, but together they can easily add another 10-20Ah to your daily total.

How to size a lithium battery bank: the formula

Once you’ve got a realistic daily energy figure, working out how to size a lithium battery bank against it is straightforward maths:

Total daily Wh ÷ battery voltage (12V) = daily Ah required

If your fridge, lighting, water pump and charging add up to roughly 50Ah a day, that’s your baseline. From there, decide how many days of autonomy you want — meaning how many days you need to run without meaningful solar input or driving time to recharge, for overcast weather or a stint camped in heavy shade.

Two days of autonomy on a 50Ah/day budget means you want 100Ah of *usable* capacity in reserve. This is where lithium’s depth of discharge advantage matters directly to sizing: a LiFePO4 battery can typically be discharged to 80-100% of its rated capacity without shortening its working life — see [how depth of discharge affects lithium battery lifespan](https://www.batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries/) for the underlying chemistry — so a 100Ah lithium battery gets you close to the full 100Ah you calculated. An AGM battery kept to a safe 50% depth of discharge would need to be rated at roughly double that to deliver the same usable energy — which is a large part of why lithium setups can be physically smaller and lighter for the same real-world runtime.

Matching the battery to your charging sources

Sizing the battery is only half the job — it needs to be matched to how you’re actually going to recharge it, or you’ll spend the trip watching the state of charge slide backwards regardless of how big the battery is.

If solar is your main charging source, work out your panel’s realistic daily output (not its rated peak — actual output is lower once you account for cloud, panel angle and time of year) and check it comfortably exceeds your daily draw, with margin for a run of overcast days. If you’re relying on driving time and a DC-DC charger, be realistic about how many hours you’re actually behind the wheel each day versus parked at camp — a DC-DC charger only tops up the battery while the vehicle’s running and the input voltage is high enough.

A battery bank that’s sized generously but paired with undersized charging just means you run flat more slowly. The battery and the charging source need to be sized together, not separately.

Common mistakes worth avoiding

The most common sizing mistake isn’t getting the battery wrong — it’s forgetting to budget for inverter loads properly. Running a device through an inverter draws more from the battery than the device’s own rating suggests, since some energy is lost as heat in the conversion, and larger appliances draw a genuine surge on startup that’s worth checking against the inverter’s rating.

The second common mistake is sizing for a best-case scenario rather than a realistic one — assuming full sun every day, minimal fridge use, and nobody leaving the lights on. It’s worth building in a genuine buffer rather than sizing to the exact number on your first calculation, particularly if your trips regularly extend past what you originally planned.

Getting the number right before you buy

The most useful thing you can do before choosing a battery size isn’t reading more comparison charts — it’s tracking your actual usage on a trip or two, even roughly. A cheap inline battery monitor will show you real daily Ah consumption far more accurately than any generic estimate, and it takes the guesswork out of what’s otherwise a fairly abstract calculation.

Invicta Lithium’s range is built around the kind of realistic touring loads this sizing process points to — batteries with a BMS built in as standard, clearly published Ah ratings and usable capacity, so the number on the spec sheet is the number you can actually plan around. At its core, how to size a lithium battery bank comes down to working out your genuine daily draw first and letting the right capacity follow from that, rather than reaching for any single “just get a bigger one” rule of thumb.

https://www.batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries/
https://www.invictalithium.com.au/accessories/invicta-touch-interface/