How to size a 12V system: start with what you run, not what you buy

Short answer: work out how much power you use in a normal day, then pick a battery that can hold that (plus a day or two spare), then pick charging that can put it back. In that order. Most people do it backwards: buy a battery, add a panel that looked about right, and find out on day three whether it was enough.

The rest of this guide is just doing those three steps properly.

Why start with what you run?

Because the battery and the charging only exist to serve the stuff you plug in. Get the daily number right and the rest follows. Get it wrong and no amount of expensive gear fixes it. Victron make the same point in their Energy Unlimited book: it's the daily energy need that sets the size of everything else.

What's the difference between watts, watt-hours and amp-hours?

• Watts (W) is how hard something pulls right now. A kettle pulls a lot; a phone charger pulls a little.
• Watt-hours (Wh) is how much energy it uses over time: watts times hours. A battery stores energy, not power, so this is the number that matters.
• Amp-hours (Ah) is how 12V batteries are usually rated. Wh = Ah x V, so on a 12V system, Ah is roughly Wh divided by 12.

Time matters more than size. Victron's example: 2 kW for an hour drains about 167 Ah from a 12V battery, but 2 kW for one minute is only about 2.7 Ah. A big load for a short time can matter less than a small one that never switches off.

Step 1: what do you actually run, and for how long?

Grab a notepad and go through a normal day. For each item, write down its watts (it's on the label or in the manual) and how many hours a day it really runs. Watts x hours = watt-hours.

It helps to think in three groups:

• Always on: the fridge, a router, anything on standby.
• Long runs: lights, a diesel heater's fan, charging laptops.
• Short bursts: the water pump, a coffee machine through an inverter.

Almost everyone underestimates the first two groups and overestimates the third. The fridge eats more than you think and the kettle less.

The fridge deserves its own line. Victron's worked example is a compressor drawing about 50 W (4.2 A at 12V) while it runs. If it runs half the day, that's 50 Ah a day; a quarter of the day, 25 Ah. Hot weather, a load of warm drinks and opening the lid a lot all push it up.

A worked example (illustrative figures, not any particular product)

Round it up. Call it 60 Ah a day, and add a bit more if summer trips are the plan.

Step 2: what are the hidden loads?

The ones that catch people out sit there quietly, day and night:

• An inverter left on. Victron list the MultiPlus-II 12/3000 at 13 W with no load at all (datasheet). Over 24 hours that's about 312 Wh, or roughly 26 Ah, for doing nothing. Its search mode drops that to 3 W. If you have an inverter, switch it off when you're not using 230V or use its low-power mode. (More in our inverter guide.)
• Standby stuff: USB outlets, alarms, chargers left plugged in, monitors.

Small standing loads matter most when the rig is parked. Victron's lithium manual puts it bluntly: "a residual current of just 10mA can damage a 200Ah battery if the system is left discharged for more than 8 days." If the van sits in the shed between trips, fit an isolator or make sure something keeps the battery topped up.

Step 3: how big a battery?

Size it for the usable part, not the sticker. How much of the rated Ah you can use depends on the type.

• Lead-acid (AGM, gel, flooded): Victron say to limit discharge to 50% if you want it to last, so about half the sticker is usable. Their AGM datasheet shows why: 400 cycles if you take 80% out each time, 1500 cycles at 30%.
• Lithium (LiFePO4): Victron put usable capacity at 80-100%. Exotronic recommend running their Alpha batteries between 10 and 90% state of charge, which works out at about 80% usable. Victron's Lithium Battery Smart is rated at 2500 cycles at 80% depth of discharge and 5000 at 50%.

Then decide how many days you want to go without charging (cloudy days, a few days parked in the shade), and multiply.

Using the example: 60 Ah a day, two days without much charging = 120 Ah you need to be able to take out.

• Lithium at about 80% usable: 120 / 0.8 = 150 Ah, so a 150 Ah or 200 Ah lithium battery.
• AGM at 50% usable: 120 / 0.5 = 240 Ah of AGM, which is a lot of weight.

That gap is the main reason people go lithium. Victron's rule of thumb is a 400 Ah lead-acid bank to match the usable energy of a 200 Ah lithium one. It isn't the only thing to weigh up, though: our lithium vs AGM guide goes through the rest, including when AGM still makes sense.

Step 4: how much charging do you need?

A battery only buys you time. What keeps you going is putting back what you used. The sum is simple: hours of charging = daily use divided by what the charger puts in.

For our 60 Ah a day:

• A DC-DC charger putting in 25 A needs roughly 60 / 25 = about 2.5 hours of driving, at best. Chargers taper as the battery fills, so real life is longer.
• Solar depends on the panels, the season, where you are and how much shade you park in. Our guide on how much solar you need goes through it.
• Most touring setups use both: the car does the heavy lifting on travel days and the panels hold you when you stop. Whether you need one, the other or both is its own question: see DC-DC charger, solar controller or both?

Step 5: how do you know if you got it right?

Your load list is a best guess. A shunt-based battery monitor tells you what's really going in and out, so after a trip or two you'll know. Our battery monitor guide explains why a shunt beats reading the battery voltage. The Victron SmartShunt 300A is the simple Bluetooth one, and there are others in battery monitors.

Is bigger always better?

No.

• A battery twice the size you need costs more, weighs more and takes longer to charge back up from the same panels.
• If your setup is a fridge and some lights for weekends, you may not need much at all. A single 100 Ah lithium battery and a decent charger might be the whole job.
• If you live in it, you'll want more. The van we use runs three 300 Ah lithium batteries, which would be overkill for a weekender and about right for full-time use.

Do the sums first, then shop. You'll spend less, and you'll stop wondering whether the fridge will last the night.

Our batteries, DC-DC chargers and solar panels are grouped by type.