Calculating battery size

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.

Water pump electrical specs

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:

Power consumption spreadsheet

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!

8 Replies to “Calculating battery size”

  1. Hi Diesel. How many amp hours did you settle on with your lithium batteries? How many years have you had it going and would you look to making them smaller or larger in the future?

    1. Our battery is 700Ah. We’ve had it since mid-2015. We are based in the Pacific North West and in the winter we’ve found we need that capacity when we’re charging solely on solar power.

      But that’s our usage pattern with hot water and cooking all powered by electricity. If you were using propane for heating or cooking, then you could get away with a much smaller battery.

      If I was doing the build all over again, I might be tempted to go with a 400-500Ah battery from a cost and size perspective, but we have definitely made use of all the power we have available in ours.

      In the end, it’s down to your specific usage patterns, location, charging options, and ability to rough it if your battery gets low.

      1. Thanks that is good to know. I am going to take your electric approach (electric 750 W hot water tank and induction cook top) coupled with my 448 AH AGM batteries. I will carry a 1 lb propane cooker and a solar shower as back up. I am weekend warrior status so can heat the hotwater tank on shore power before I go and turn it on when I am traveling day 2 and 3 to let the alternator make some extra hot water as well. Thanks for all your great advice on this.

  2. Hi Dieselfumes, could you shed some light about your battery box? What material did you use? How did you secure it? Thanks

    1. Pujan, the battery box is made of 1/2″ ply, covered in rubber coin flooring on the outside. It holds the battery above the wheel well. The box is attached to/integral with the wall panel, which is also 1/2″ ply. The whole section from the back door to the front of the battery box is made in one piece. That includes the breaker panel, and the place where the solar charge controller and inverter are mounted.

      The wall panel is held in place by the L-track at the top, and using some bolts/rivnuts at the base. The base of the battery box rests on the metal floor of the van, but is not attached to it in any way. We cut the factory floor to fit around the battery box. In retrospect, that probably wasn’t necessary and the factory floor may even have given us some additional mounting points to hold the box in place. The battery is bolted into the box.

      What I will say is that the box is heavy. The battery weighs ~235 lbs. The wood construction is more than sufficient for that weight. If I was starting today, I’d probably create an 80/20 or welded aluminum frame for the battery, bolt that to the wall with some rivnuts, and just face it with 1/4″ or 1/8″ panels.

  3. Hi Dieselfumes. As most who read this amazing material, my husband and I are on the electrical portion of our build and trying to teach ourselves as we go. Oy! We have been very grateful for your wisdom and written guidance. I feel like I have such a stupid question but we cannot get our minds around our math to save our souls, so here goes: In your little table above (and I understand these are not your accurate numbers, just an example blah blah), why do you convert your AC amps (micro and stove) back into DC Amps? If “a watt is a watt”, and W/V gives me amps, isn’t that just the “amps” I use to multiply by my hours to get my total Amp hours? For instance, accordingly to my math, my induction stove at 1800 watts, if used generously for 2 hours a day, would cost me 30 Amp Hours (1800/120 = 15 x2 hours = 30 amp hours). But this is obviously missing a step from your math. What am I not understanding? Isn’t my inverter accounting for the AC/DC difference? And if it is, again, why do I need to account for going from “AC Amps” to “DC Amps”? I hope this question isn’t as muddled as it feels in my my head… Thanks so much for listening… the end.

    1. Hi Meghan,
      The main reason for showing Amps at 120V and Amps at 12V in the table is to show people that you can’t just look at the AC 120V amperage rating for an appliance and calculate your battery capacity drain based on that.

      In a perfect world, we’d all use Watt-hours as our unit of electricity consumption because that takes both voltage and current into account. The thing is, most 12V batteries are rated in Amp-hours, not in KiloWatt-hours. So, we need to look at the DC Amps and usage time (duty cycle) in order to work out how much juice each device is going to suck from the battery.

      A Watt is a Watt, and an Amp is an Amp, but the voltage is different between the AC and the DC supplies (obviously). Your inverter is running from a 12V supply. It is being asked to output 120V. If you try to make the 120V quantity of electricity using a 12V inverter, that quantity has to come from somewhere.

      Let’s just assume it’s a 10x increase in quantity (reality is messier, but we’re trying to keep our sums simple). So that means using 10x as much 12V battery power to create one unit of 120V electricity. The 12V electricity has to flow 10x as fast to keep up with what the 120V appliance wants to use. So… if the 12V electricity is flowing 10x as fast, what does that do to your Amps? Amps is a measure of rate of flow, so there are 10x as many Amps at 12v than at at 120V.

      Let’s walk that through with your induction stove example. You already worked out that 1800 W is 15A at 120V (1800/120=15). How many Amps is that at 12V? Well, 1800/12=150. Because your Voltage changed, your Amps also have to change in order to balance out the equation.

      Does that help at all, or leave you more confused?

      1. Wow Diesel! Thank you so much for taking the time to explain that to me. I really appreciate it. Definitely less confused so thank you 🙂 Safe travels, Meghan

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