How do I calculate how many watts of energy I need when camping?
Generators, fuel cells, solar panels and other power supplies tend to be sold with Watts as the energy value. Some of my electrical devices list amps, how do I convert amps to watts?
3 answers
First question : are you walking or driving and do you need to camp for any length of time ? For short stay driving you take a second car battery and Amps x 11 = Watts or Watts/11 = Amps. It is 11 not 12 because the "12V" battery runs down, wires have loss, and the Amp-hours rating of the battery is not exact.
For anything sustained for more than a weekend, check the forecast and time of year before buying solar panels. Enough solar panels to run a 40 Watt TV from sunrise to sunset on the gloomiest miserable grey winter day could be as many as goes on a whole house 4000 Watts(peak). To run the same 3.3 Amps 12 Volts load for a few hours on a summer day under fair sunshine, a single "100 Watt" panel could keep a car battery above 12V but nowadays you'll want a solar charge controller (">=10 Amps rating") so that you don't have to check it with a multimeter quite regularly.
I don't go camping to watch TV and surf the internet. Do you really need anything more than a flashlight and a box of matches ?
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Power (watts) = Voltage (volts) * Current (amps)
or
Current (amps) = Power (watts) / Voltage (volts)
So in order to determine the watts from amps, you also need to know the voltage of the devices. Since you are camping, and specifically "car camping" rather than backpacking, I would expect most of your devices will be 12V DC (ie, made to run off a car battery). However, if you were to get a generator or power inverter you could run standard 120V AC items as well. So this is where amperes may come into play. If you get a decent 400W inverter (and ignoring any losses, power factors and spikes), it could pull up to 33A constantly from your battery, but you could only plug in 120V AC devices that will draw 3.3A. A good inverter will shut down once your battery voltage drops below say 10V or if you draw too much current. Some have fuses as well to guard against large current spikes. A word of caution - if you had such a heavy load on an inverter you would need 8 gauge cables to safely connect it directly to your battery. You won't be using your cigarette lighter, which is typically rated for 10A (120W)!
The battery-powered items you have are likely listed as mAh or milliAmpere-hours, which is an approximation of the charge capacity of the battery. Most device batteries are in the 3.3-5V range but you don't know the current draw and it will fluctuate greatly anyway with temperature and usage of the device (ie, screen-on time, CPU load, radio usage, etc). FWIW, Lithium-ion batteries are the best for outdoor usage and that is what most devices use today anyway.
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In principle as a practical question there are several different numbers of relevances. As noted by others the simple formulae that relates the different measures is Watts = Volts x Amps.
Now when looking at a particular appliance there are two basic concerns.
a) Does my inverter supply enough power to allow this appliance to run.
b) How much of the batteries capacity will this appliance use over a period of time. As I under stand your question it is the former your are most concerned with.
Batteries typically are rated in amp_hours which in principle implies that the battery will be able to supply the rated number of amps for an hour.
However it is important to note that traditional car batteries and even cheaper deep cycle batteries are not designed to use all of the rated amp hours on a regular basis. If you were to run the battery flat the life of the battery would be very short.
So as a basic approach. Assume that you have a 100 amp hour battery (Typical of trucks and some big suvs, care are more likely to be 30-40 amp hours.)
Apply a factor to reflect safe charging so assume you can only use 30-40% of the rated amp hours. So assume you have 30 amp hours to play with.
The lets say you have a 20W appliance at 120v. Then using W=VxA you get that the appliance will draw A =W/V amps = 1/6 amps. Thus your appliance will draw one of your 30 amp hours for every 6 hours of use.
If the appliance has an amp rating then the calculation is unnecessary but be careful in both systems because many appliances don't draw current at a steady rate. A fridge for example will draw a lot of current to get the interior cold and then virtually no current for an extended period of time until the temp rises above a cut-off. Power tools draw 3-4 times the rated wattage initially to get the tool to speed and then operate close to the rate wattage unless the tool is being stressed.
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