Let’s start by determining how many Amp Hours are being drawn by your DC appliances such as the lights and radio that come installed in your RV. To determine how many Amp Hours a DC appliance is going to consume, you start by determining its Amperage rating. The Amperage rating is usually stamped onto the appliance somewhere or can be found in the owner’s manual. If all else fails, you can usually find the information in technical specification sheets available on the web through the manufacturer. With a 12V DC appliance, figuring out Amp Hours is simple. All you have to do is multiply the amperage by the number of hours you plan to use it.
If, for example, you have a car stereo in your RV that draws 1 Amp of power and you plan to use it 1 hour per day, then that appliance will consume 1 Amp Hour (1 Amp x 1 Hr = 1 Amp Hour). If you have a DVD player that consumes 2.2 amps and you plan to use it for 2 hours each day to entertain your children, then it will consume 4.4 Amp Hours (2.2 Amps x 2 hours = 4.4 Amp Hours).
Converting AC (110V) Appliance Usage into DC Amp Hours:
If you have an inverter in your RV so that you can use standard household appliances and you know the amperage rating of the appliance, then you only need to take one extra step in order to convert that to its equivalent DC amperage rating. In order to convert AC Amps to DC Amps, all you need to do is multiply the amperage by 10. Say you have a TV that consumes 1.5 amps of AC current and you are running it off your batteries through an inverter. In order to power that TV, you are actually using 15 Amps of DC current (1.5 Amps (AC) x 10 = 15 Amps (DC)). From here you can do the same calculation as you did above to determine total Amp Hours. Therefore, if you plan to watch TV 3 hours a day, then you will consume 45 Amp Hours total (15 Amps (DC) x 3 hours = 45 Amp Hours) .
What if Your Appliance Does Not List Its Amperage Rating:
If your appliance does not list its specific amperage rating, then chances are it will give you other information such as its wattage. If you know the wattage, then you can easily determine its amperage rating. In order to accomplish this you need to be aware of three simple formulas:
Amps = watts/volts
Volts = watts/amps
Watts = volts x amps
Since we know the amount of watts and we already know the amount of volts (12V for DC items and 110V for AC items), it is easy to determine the amperage based on the formulas above. Say for instance that you have a light bulb that runs off of your RV’s 12 Volt electrical system and you do not know its amperage rating. If you look closely at the bulb, chances are good that it is going to state how many watts it uses. Let’s say 25 watts for this example. Since we know that it is using 12 Volts with a 25 watt light bulb we can easily determine its amperage by using the Amps = watts/volts formula.
Amps = 25 Watts/12 Volts = 2.08 Amps.
Now that you know the amperage rating, you can easily determine how many Amp Hours this light bulb will consume based on how long you plan to use it each day. Let’s assume that you are going to use this light bulb for 3 hours each night to read. Therefore, you are going to consume 6.24 Amp Hours per day using this bulb (2.08 Amps x 3 hours = 6.24 Amp Hours).
Now let’s do the same calculations but this time with a standard AC lamp that we brought from home that is using a 60 Watt light bulb. The first thing that you need to do is determine its Amperage. Using the formula above:
Amps = 60 Watts/110 Volts = .55 Amps.
But remember, since this is an AC appliance you need to take one more step to convert it to DC amperage by multiplying that number by 10.
.55 Amps (AC) x 10 = 5.5 Amps (DC)
Now you can use that number to determine how many Amp Hours it will draw from your batteries. Let’s assume that we are going to use this lamp for 3 hours each night to read. Therefore, you are going to consume 16.5 Amp Hours (5.5 Amps x 3 Hours = 16.5 Amp Hours).