Overland Power for Beginners: Where to Start

A 12V house system is a second battery, separate from the one that starts the engine, with its own charging path and its own fused circuits. That is the whole idea. Everything else in this guide is arithmetic about how big that battery should be and how you put energy back into it. The single most useful thing a beginner can do is stop thinking in products and start thinking in amp-hours per day, because that one number decides the battery, the charger, the array and the wire gauge all at once.

The reason a separate battery exists at all is that a starter battery and a house battery are asked to do opposite jobs. A starter battery delivers several hundred amps for about three seconds and is then immediately refilled by the alternator. It spends its life at close to full charge and is damaged by being drawn down. A house battery is drawn down slowly over many hours and refilled once a day, which is a completely different duty cycle. Running a fridge from the starter battery gives you both a worn out battery and, eventually, a vehicle that will not start at a trailhead with no phone signal.

How much power does an overland setup actually use?

Less than most people fear and more than the labels suggest. The label on an appliance is usually its peak draw, but what empties a battery is duty cycle: the fraction of each hour the thing is actually running. A compressor fridge is the clearest example. It draws roughly 2.5A at 12V while the compressor runs, but in mild weather it only runs about a third of the time, so the real daily figure is closer to 21 amp-hours than the 60 you would get by multiplying 2.5 by 24.

Add the mild-weather fridge, the lights, the fan, the phones and the pump and you land at about 35 amp-hours a day. Swap in the hot-weather fridge figure and add a laptop and you are at roughly 59. Round the working number to 55 amp-hours a day for two people with a fridge, and you will not be far wrong until you add an inverter and start cooking with electricity. The power consumption calculator does this sum with your own appliance list rather than this one, and it is worth ten minutes before you spend anything.

Two loads dominate almost every build. The fridge is the constant one, running every hour of every day whether you are awake or not. Cooking with electricity is the spiky one: an induction burner or a kettle through an inverter can pull 100A or more from a 12V battery for the few minutes it runs, which is a wire and fuse problem long before it is a capacity problem. Everything else, the lights, the phones, the water pump, is rounding error by comparison.

How big a battery do I need?

Depth of discharge is the percentage of a battery's rated capacity you can actually use before recharging. It is the reason two batteries with the same number on the label are not the same battery. Lead acid and AGM are safely usable to about 50 percent, so a 100Ah AGM gives you 50 usable amp-hours. LiFePO4 is usable to 80 to 90 percent, so a 100Ah lithium battery gives you 80 to 90. That ratio is why lithium at roughly twice the purchase price is often cheaper per usable amp-hour, and it is before you count the cycle life difference or the 60 lb of weight you save.

So the sizing rule for a beginner is: take your daily consumption, decide how many days you want to sit still without charging, and divide by 0.85 for lithium or 0.5 for AGM. Fifty-five amp-hours a day for one and a half days is about 82 usable amp-hours, which a single 100Ah lithium battery covers. Two full days with margin is 110 usable amp-hours, which means 200Ah of lithium or, more sensibly, the same 100Ah battery plus a charging source that runs while you are parked.

For a first battery the Lithova 12V 100Ah LiFePO4 Battery (Group 24, 100A BMS) is the cheapest honest way into lithium, and the Renogy 12V 100Ah Self-Heating LiFePO4 Battery (DuoHeat, Mini) is the one to buy if you will ever camp below freezing, because LiFePO4 cannot safely accept a charge at sub-freezing cell temperatures without a built-in heater. That single limitation catches more people than any other spec on the datasheet. The battery bank calculator turns your daily figure into a bank size, and lithium versus AGM works through the cost per usable amp-hour properly if you want to see the arithmetic.

How do I put the energy back in?

There are three charging paths and most builds eventually use two of them. Alternator charging through a DC-DC charger is the fastest and the cheapest per amp-hour, because the engine is running anyway. Solar is slower but works while the vehicle is parked. Shore power charging from a wall outlet is the one that matters least on the road and most in a driveway between trips.

A DC-DC charger sits between the starter battery and the house battery and converts the alternator's output into a proper multi-stage charge profile for lithium. It also protects the vehicle, because it limits how much current it will pull and it will not run the starter battery flat. Connecting a lithium battery straight to an alternator through a simple relay is the mistake to avoid: a lithium battery will accept everything the alternator can give, which on a modern smart-regulated vehicle is a good way to cook the alternator. The Renogy Smart 50A DC-DC MPPT Battery Charger 12V (Dual Input) at 50A is the size most builds settle on, roughly 600W of charging, which puts about 50 amp-hours into the bank per hour of driving. The smaller Renogy REGO 12V 30A DC-DC Charger with MPPT (Dual Input) at 30A is sensible for a single 100Ah battery and costs less.

Solar output follows a simple model: daily watt-hours equal panel watts multiplied by peak sun hours multiplied by a system derate, which is 0.75 for a flat roof-mounted array with an MPPT controller and 0.6 with a cheaper PWM controller. A Renogy 200W 12V Solar Panel (N-Type, 16BB) at five peak sun hours returns roughly 750 watt-hours, which is about 59 amp-hours at 12.8V. That is one fridge day, which is exactly why 200W is the standard first array. A single Renogy 100W 12V Monocrystalline Solar Panel (N-Type, 16BB) returns closer to 25 to 35 amp-hours a day and fills the gap on a small roof.

Should I buy a power station instead?

For a first season, often yes. A power station like the EcoFlow Delta 2 (1024Wh, 1800W) is a battery, an inverter, a solar charge controller and a fuse panel in one box that needs no installation, no crimping and no holes in the vehicle. It also resells well, which matters more than people admit when they are not yet sure whether they will keep doing this.

What you give up is charging speed from the vehicle, cost per usable amp-hour, and the ability to run 12V loads without an inverter in the path. Most power stations accept a fairly modest DC input from the vehicle, so a two hour drive does much less for a station than the same drive does for a wired bank behind a 50A DC-DC charger. The power station versus DIY bank comparison lays out where each one wins. A middle path exists too: a Dometic PLB15 Portable Lithium Battery lives next to the fridge, charges from the vehicle while you drive and needs no install at all.

What does a beginner build actually cost?

Those figures include the parts nobody photographs, and that is deliberate. Cable, lugs, heat shrink, a fuse at the battery, a busbar and a disconnect switch add up to a few hundred dollars on any build and they are not optional. A Blue Sea Systems 5191 MRBF Terminal Fuse Block (30 to 300A) bolted directly to the battery post is the single most important part in the whole list and costs about ten dollars. A FASTSTORM 12 Way Blade Fuse Block with LED Indicators (12V) gives every branch circuit its own fuse sized to its own wire. A pair of Joinfworld 12V 250A Busbar Power Distribution Block (4 x 3/8 in studs) turns a battery terminal with six cables stacked on it into something that can actually be torqued properly. The beginner build list prices a complete working system with every one of those parts included.

What should I buy first?

Buy the fridge first. It is the appliance that changes how you travel, and it is also the load that sets every other number in the system. A BougeRV CRPRO 30 Quart 12V Refrigerator is a genuine compressor fridge at roughly the price of two seasons of ice, and the ICECO VL45 45L Portable Refrigerator (Secop Compressor) is the better long-term buy if the budget stretches, because a better compressor and better insulation directly reduce the battery you have to carry.

Buy the measurement tool second. A Victron SmartShunt 500A Battery Monitor (Bluetooth) counts amp-hours in and out through a shunt, which is the only honest state of charge reading on a lithium battery, because the voltage curve is nearly flat between 20 and 80 percent. A AstroAI Digital Clamp Meter, AC/DC Current and Voltage (4000 count) tells you what an appliance really draws instead of what its label claims. Both are cheap, and both stop you buying capacity you do not need.

Buy the battery third, the charger fourth and the solar last. That order is the opposite of how most people shop, and it is the order that produces the fewest wasted purchases. A MAXXAIR MaxxFan Deluxe Roof Vent Fan (10 Speed, Reversible) deserves a mention here too, because a roof fan on low draws under an amp and does more for comfort in heat than any amount of battery capacity spent on cooling.

What do beginners get wrong most often?

Undersizing wire and oversizing the fuse. The fuse protects the cable, not the appliance, so it is always sized at or below the ampacity of the smallest conductor in the circuit. Fitting a 150A fuse on 10 AWG wire, which is rated for 60A, means the wire becomes the fuse. Every circuit is fused at the source of power, within a few inches of the battery terminal, before it goes anywhere. The 12V wiring and fusing guide has the full ABYC ampacity table and the voltage drop arithmetic.

Forgetting weight. A 100Ah lithium battery, a 20 gallon water tank and a rooftop tent together can account for most of a mid-size SUV's payload before you have loaded a person or a bag. Water alone weighs 8.34 lb per gallon, so 20 gallons is 167 lb.

Assuming solar covers everything. Roof solar in winter, at northern latitudes, under trees, or on a vehicle you park in the shade on purpose because it is hot, produces a fraction of its summer figure. Alternator charging does not care about any of that. Build the system so that a two hour drive resets it, and treat solar as the thing that lets you stay put longer.

Finally, assuming the whole system can be commissioned by eye. A lithium installation should be inspected by a qualified installer before it carries load, particularly the fusing, the terminal torque and the cable routing. The figures on this page are researched guidance drawn from published standards and manufacturer documentation, not an electrical certification.