In our last article, we looked at watts along with watt-hours and what they mean to an RVer. In this post, we will look at how to determine the watt rating of an appliance when it isn’t listed or you want to know what a particular appliance consumes over the course of a set period of time (watt-hours).

The first thing to know is that watts are the product of voltage times amps, typically expressed as (watts = amps X volts) which can also be expressed as (volts X amps = watts) or (amps X volts = watts). Therefore, if you know the operating voltage and amp draw of an appliance, you can determine watts.

**Examples:**

- A 120-volt toaster has an amperage draw of 9 amps. (120 volts x 9 amps = 1,080 watts)
- A 120-volt microwave with an amperage draw of 12.5 amps (120 volts x 12.5 amps = 1,500 watts)
- Don’t forget to include your 12-volt appliances: A 12-volt RV furnace (circuit board and blower motor) with an amperage draw of 7 amps (12 volts X 7 amps = 84 watts)

With some appliances, the amperage draw varies depending on how you use it. For example, you may have a flat screen TV that allows adjustment of the brightness of the display along with a volume control which in turn will affect the amount of amps required to power it.

This amount may be considerably less than the amount posted on the back of the appliance. Maybe you have an appliance that cycles on and off like a refrigerator and want to know what the power consumption is over the course of a set period of time (watt-hours). For the above situations, consider purchasing a watt meter.

By plugging a 120-volt appliance into a watt meter (or your entire RV) over a set period of time you can measure watt-hours consumed. Even if the appliance has a constant current draw when in use (like a 120-volt lamp), you can still use a watt meter to calculate the power requirements in watts rather than doing the math (amps X volts) like the examples given above or to verify the stated manufacturer’s ratings. Never exceed the rated amp capacity of the meter.

Once you have determined the requirements of each of your RV’s appliances (don’t forget the converter), just add up the ones likely to be running at the same time and you will know what your shore power needs are expressed as watts. The amperage needed to start the compressor on an air conditioner requires a short burst of excessive current to get things turning which needs to be taken into account.

Now that we know how to calculate watts and watt hours, what are you going to do with this information? There are several useful applications for knowing your shore power (120-volt) needs in watts:

- If you dry camp often, this information will help you in choosing the correct-sized generator for the power needs of your RV.
- If you are living in your RV and always hooked up to shore power for extended periods of time you can determine your watt-hours consumption. This will help you determine if it is more economical to pay a set monthly fee for electricity or ask for a campsite with an electrical meter and only pay for what electricity you actually consume.
- When pulling into an RV park that only offers 20 and 30-amp electrical outlets, you only need to divide your watts requirement by 120 volts to determine the minimum amp circuit you can comfortably hook up to.

Remember: Watts = volts X amps which can also be expressed as watts/volts = amps. Therefore, if you have determined your RV typically doesn’t need more than 2,100 peak watts of shore power, you can plug into a 20-amp electrical outlet without worry (2,100 watts / 120 volts = 17.5 amps). 17.5 amp draw is less than 20 amps available, so you can have a comfortable margin of “extra” power.

Conversely, you can think of a 120-volt 20-amp outlet as a 2,400-watt outlet (120 volts x 20 amps = 2,400) or a 120-volt 30-amp outlet as a 3,600-watt outlet (120 volts x 30 amps = 3,600).

Note: 50-amp outlets contain two 50-amp legs of 120-volt power for a total of 100-amps or 12,000 watts. Check out this page if you need more information on 50-amp RV outlets.

In our next post, we will look at the electrical draw of 12-volt appliances and how to determine how long your house batteries can run them before becoming depleted.

Learning how to perform basic electrical math, just another adventure in RVing!

Kennyboy says

Very practical informative article. Looming forward to your future articles.