![]() It just about keeps the fridge running without any battery depletion. That’s actually pretty sad, but also quite accurate in our experience. So, we can replace about 500/12 = 41 amp hours of battery capacity from our massive solar array. In the Pacific NorthWest during winter, the NREL site says this gives 15kWh energy per month. That’s a much larger system than most people use. We have just under 800W of solar panels on our van roof. The battery amp-hours that will be replaced is your daily kWh figure divided by 12 (DC system, remember). It’s up to you whether you choose an average figure, or the most pessimistic (winter) figure as your baseline. Note the difference between summer and winter figures. Your daily charge capacity will be around 1/30 of these figures. This gives you kWh values for your location for each month. 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). 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. Panel wattages are calculated for an “ideal” situation that you’ll never see in real life. Solar charging isn’t as efficient as it says on the panels. Some people add a second alternator just for charging their house batteries. Overall you might have as little as 40 amps available for charging your house battery, or as much as 100 amps. 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. 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. 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. Overall though, having a rough idea of your daily amp-hour usage lets you calculate your overall battery needs.įor my hypothetical list of items above, the daily figure is just under 300 amp-hours. In the winter, you might use more lights but also less fridge and fan time. Obviously that figure will change from day to day. Now you know how much power you’ll be using on an average day. ![]() Sum up the amp-hours for all your devices. You’ll end up with a list something like this: Multiply the amp consumption for each device by how many hours you’ll use it every day. It has a “duty cycle” that ranges from 25% to 50% depending on how hard it has to work to keep things cool. For instance, a fridge is plugged in all the time, but it isn’t running all the time. Work out how much of the time each device will be on. Actually, because the inverter isn’t 100% efficient, it’s better to multiply by 13. So you need to multiply the amp consumption of those devices by 10 to see how much DC power they’d take. 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. Watts = amps of current * volts, so you divide the Watts by 12 (for 12v DC devices) to get amps. The power draw may be listed in Watts rather than amps. Read the label or check the device’s manual or specification document. ![]() You need to know how many amps each device consumes so you can calculate how many amp-hours of battery capacity you need. Those work through an inverter, which converts 12v DC to 120v AC to power household devices. You might also have 120v appliances like a microwave oven or induction burner. 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.įancier vans might have diesel heaters (these also need electricity to power their igniters and fans), sound systems, air compressors, and other gadgets. Make a list of all your electrical devices If nothing else, doing the sums means you’ll be aware of which items in the van are “costing” you the most electricity. ![]() 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. It’s pretty easy to calculate the load you’ll put on the battery, and from that how much power storage you’ll need. How much battery power you need depends on the number of things you want to power, and how often you need to use them.
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