Let’s go camping with solar

March 17, 2010
Let us consider a possible scenario. You want to go camping and want to use solar power to charge a 12V battery connected to a 120VAC inverter to power your stuff. Calculating battery size and solar panel power is important if you don’t want to be left without power. Amp hours (Ah) work much the same way as watt hours (Wh). Multiply battery voltage times the Ah rating and you get Wh (more or less), but there are also other things to consider.

Let’s setup a possible scenario

Let’s say you would like to take a primitive camping trip. You want to make sure you have enough power to occasionally run your laptop and charge batteries for your camera. You would like to run your laptop for 4 hours a day and charge 4 AA NiMH batteries a day. Let’s also assume you’ll be gone for 5 days.

As we go through this scenario, keep in mind that this same deductive reasoning can be applied to many different scenarios. Maybe you want to power:
  • something during an outage
  • your RV’s appliances
  • your laptop through a 14 hour flight using a few parallel sets of 16 NiMH batteries in series
  • a remote pond water pump
Also keep in mind that this is only a general guide. It does not consider discharge rate, temperature, or various other factors. For details, check out the following:

How much power am I using?

To know how much power you need, you first need to know how much you’re using. If you can’t measure the current your laptop uses, check the power supply rating. Also, your NiMH batteries will need to have at least as many watt hours as they store. Don’t forget to compensate for conversion efficiency.

Estimating Laptop Power Usage

Let’s say your laptop power supply puts out 19V at 4A – that’s 19V * 4A = 76W. However, it may only use that much power powering your laptop and charging its battery. If the battery is charged, assume 76 watts anyway. If your laptop actually used 60 watts, given a conversion efficiency of 85%, the actual power consumed would be (100% / 85%) * 60W = 70.6W. Running for 4 hours, that would be 76W * 4hrs = 304Wh.

Estimating Battery Charger Power Usage

If you have 4 AA NiMH batteries with a capacity of 2500mAh (2.5Ah), then 1.2V x 2.5Ah x 4 = 12Wh needs to go into those batteries. Again assuming a conversion efficiency of 85% for the battery charger, let’s say we need (100% / 85%) * 12Wh = 14Wh.

Total Usage

So, all together you’ll be using about 304 + 14 = ~320Wh a day. To err on the side of caution, let’s say 350Wh.

Where will I get my power?

It’s unlikely that you’ll have a MPPT solar power converter that will work directly for your laptop or batteries. (Although you might be able to charge 10 AA NiMH batteries in series with a 12V charge controller.) So you’ll likely need a 12V SLA (sealed lead acid) battery and a 120VAC inverter. If you bring a big enough battery, you won’t even need a solar panel, but that would be quite a heavy battery.

Estimating Battery Size

Assuming you’re using a 12V battery and your inverter has a conversion efficiency of 85%. To have enough power for one day the battery would need a capacity of (350Wh / 12V) * (100% / 85%) = 34Ah. For five days you would need 5 of these batteries or one 170Ah battery (34Ah * 5 day). But that’s about 140lbs of weight and it doesn’t take advantage of the sun, so let’s use only one with a solar panel.

Estimating Solar Panel Needed

The battery we chose can store about 34Ah * 12V = 408Wh. Assuming a charge controller efficiency of 90%, we need about (100% / 90%) * 408Wh = ~450Wh from the sun. Let’s also assume the sun will shine adequately for about 6 hours (or a fourth of the day). You’d need a 450Wh / 6hrs = 75W solar panel, or an 80W panel which is more common.