How much battery power you need depends on the number of things you want to power, and how often you need to use them. It’s pretty easy to calculate the load you’ll put on the battery, and from that how much power storage you’ll need.
It’s hard to know exactly how much you’ll use each device before you build the electrical system for your van, but it’s worth making a guess. If nothing else, doing the sums means you’ll be aware of which items in the van are “costing” you the most electricity.
Make a list of all your electrical devices
What electrical items will you have in your build? Often people have a fan, lights, a fridge, a water pump and laptop and cell phone chargers.
Fancier vans might have diesel heaters (these also need electricity to power their igniters and fans), sound systems, air compressors, and other gadgets.
You might also have 120v appliances like a microwave oven or induction burner. Those work through an inverter, which converts 12v DC to 120v AC to power household devices.
Work out the power draw of each item
You need to know how many amps each device consumes so you can calculate how many amp-hours of battery capacity you need.
Read the label or check the device’s manual or specification document. The power draw may be listed in Watts rather than amps. Watts = amps of current * volts, so you divide the Watts by 12 (for 12v DC devices) to get amps.
If you’re using an inverter to power some 120v appliances like a microwave or induction burner it takes 10 times the 12v power to make the 120v power. So you need to multiply the amp consumption of those devices by 10 to see how much DC power they’d take. Actually, because the inverter isn’t 100% efficient, it’s better to multiply by 13.
Work out how much of the time each device will be on. For instance, a fridge is plugged in all the time, but it isn’t running all the time. It has a “duty cycle” that ranges from 25% to 50% depending on how hard it has to work to keep things cool. Your lights aren’t on all the time. Maybe just a few of hours at night.
Multiply the amp consumption for each device by how many hours you’ll use it every day. That gives you amp-hours.
You’ll end up with a list something like this:
Get your total amp-hour figure
Sum up the amp-hours for all your devices. Now you know how much power you’ll be using on an average day. Obviously that figure will change from day to day. In the winter, you might use more lights but also less fridge and fan time. Overall though, having a rough idea of your daily amp-hour usage lets you calculate your overall battery needs.
For my hypothetical list of items above, the daily figure is just under 300 amp-hours.
Think about how you’ll recharge
How many days do your batteries need to last for? If you will be camping in wilderness areas for 5 days at a time, you’ll need a bigger capacity than if you can plug in to the mains each night.
If you have solar power or an engine alternator charging system, you’ll also be replacing some of this capacity just by parking in the sun or by driving.
The stock V6 engine alternator provides 220 amps, so every hour of driving gives you 220 amp-hours of capacity. However, that alternator is recharging your engine battery and powering all the other vehicle electrical systems, and it only gives its rated power at high revs, not just when you’re parked and idling the engine. Overall you might have as little as 40 amps available for charging your house battery, or as much as 100 amps. Some people add a second alternator just for charging their house batteries.
Solar charging isn’t as efficient as it says on the panels. Panel wattages are calculated for an “ideal” situation that you’ll never see in real life. To get a realistic understanding of the amp-hours you’ll get from a solar panel, go to the National Renewable Energy Laboratory site and enter your location. On the next page, enter your panel size (remember you’ll have a smaller system than they expect; 100W = 0.1kW) and set tilt to 0 (you have the panels flat on the van roof). This gives you kWh values for your location for each month. Your daily charge capacity will be around 1/30 of these figures. Note the difference between summer and winter figures. It’s up to you whether you choose an average figure, or the most pessimistic (winter) figure as your baseline. The battery amp-hours that will be replaced is your daily kWh figure divided by 12 (DC system, remember).
We have just under 800W of solar panels on our van roof. That’s a much larger system than most people use. In the Pacific NorthWest during winter, the NREL site says this gives 15kWh energy per month. 15000/30 = 500Wh per day. So, we can replace about 500/12 = 41 amp hours of battery capacity from our massive solar array. That’s actually pretty sad, but also quite accurate in our experience. It just about keeps the fridge running without any battery depletion. If we visited Phoenix, AZ in May though, we’d see almost 400 amp hours of battery charge from solar every day.
Another thing to consider is that parking in the sun to use solar recharging means your van will get hot. If you park in the shade, you won’t see anything like these figures from your panels, but you might stay cooler!
Size your batteries
Now you know how many amp-hours you’ll use in a day, how much you can recharge every day, and how many days you want to use the van for before you have to do a major recharge from the grid.
Multiplying daily amp-hour use by number of days off-grid gives you your battery storage requirement. For my example, I have 300 amp-hours demand every day, but I can replace perhaps 150 amp-hours with solar, so I actually have 150 amp-hours daily requirement. If I want to camp for five days without plugging in to recharge, that’s 750 amp-hours storage capacity.
Batteries can’t be drained completely. The general rule of thumb is that lead-based batteries can be safely discharged to 50%, lithium to 70%. So battery size = storage requirement x 100/usable capacity%.
If I was thinking of using absorbed glass mat lead-acid batteries, I’d need 750 * 100/50 = 1500 amp hours total battery capacity. That’s six massive 8D sized batteries, costing about $3900 and weighing 960lbs.
Even with a lithium system that I can run down to 30%, I would need a battery that holds 750 * 100/70 = 1071 amp hours of total capacity. Yikes! After doing those sums, maybe I would start thinking about using a propane stove rather than my induction burner, or heading back in to town after three days rather than five.
Consider what size inverter you need
It’s tempting to just get a big 2kW or even 3kW inverter to turn the 12v battery power into 120v mains power. But there’s a problem with that. Lead-acid batteries have an amp-hour rating based on a certain, pretty low, rate of discharge.
In other words, a lead acid battery will easily last for the given number of amp-hours if you are just running your LED lights and a fan from it. As soon as you start pulling very large amounts of power from it, the amp-hour rating goes down.
A 2kW inverter is pulling 2000W / 12v = 166 Amps from your battery. A 200 Amp-hour rated battery might only provide 50 or 70 Amp-hours when it’s being asked to provide power at that rate. That means it will discharge very quickly, get very hot, and also deposit sulphates on the lead plates which can damage the battery longer term.
So, look at the power draw of your 120v devices. Consider what the smallest size inverter is that will let you use them, or even reconsider whether you actually need them at all.
If you do decide that you need a 2kW inverter, remember to size your battery appropriately. As a rule of thumb, you probably want no less than a 400 Amp-hour lead-acid battery system, or a 300 Amp-hour lithium system (lithium can handle the large power draw better than lead acid).
It’s worth doing the sums
You can see how doing the calculations makes it clear which of your electrical items is consuming the most power. Knowing this means you can either change your plans for what devices to install, or make adjustments to your device use depending on how long you plan on being away from a recharge. It also explains why RVs with Keurig coffee makers, satellite dishes, and large-screen TVs need to plug in to a power source each night!