How To Determine Your RV Power Needs
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).
What are watts?
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.
Monitor your power usage. Photo via Amazon
How do you measure watts?
By plugging a 120-volt appliance into a wattmeter (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.
Learning how to perform basic electrical math, just another adventure in RVing!
Read more: What’s A Watt? How To Calculate Your Power Needs
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Very practical informative article. Looming forward to your future articles.
You just missed one very important detail. Electrical service connections in RV parks are covered by the National Electrical Code (NEC). NEC requires that all circuit breakers trip at 80% of rated capacity. So a 30 amp outlet will only deliver 24 amps.
Gary, That is incorrect. Breakers trip at their rated capacity.
There might be two places where you are drawing your ‘80%’ concern.
1) In 210.23, it states that ‘any one cord and plug connected appliance shall not exceed 80% of the circuit’s rating’, That means the load must not demand, not that the breaker will interrupt it. BUT that rule applies to circuits with multiple receptacles which RV pedestals are not. Circuits with only one receptacle are allowed 100%.
2) There is another 80% rule; the design standard for breaker protected circuits. It requires that the ‘continuous duty’ (loads in service over 3 hours/day) design load on a circuit should not exceed 80% of the breaker capacity. The ‘intermittent duty’ load (loads in service less than 3 hours/day) is the other 20%. IF the single receptacle is INTENDED to convey 100% rated capacity over 3 hours per day, the wire gauge serving the receptacle should be increased to 125% per 210.19(A)(1). This reinforces the requirement to support the full rated breaker/receptacle capacity and SHOULD be applied to power pedestal installations in RV parks but likely will not be in residences.
Its very important to note that most people who plug into a ’20A/120VAC receptacle’ (aka NEMA 5-20R) do so with a 15A plug/cord (aka NEMA 5-15P). This is OK and will actually provide 20A presuming your cord and the rest of the ‘links in the chain’ are rated for 20A. Learn how to tell a 20A receptacle from a 15A by examining the shape of the left contact in the socket. It will have a shorter horizontal slot (kind of a sideways ‘T’). Here’s where lots of folks get into trouble; if a ‘thrifty’ repairman has replaced an old RV park 20A receptacle with what he has handy, a 15A receptacle (NEMA 5-15R), or if you are plugging into a 20A circuit at a residence which is almost always wired with multiple 15A receptacles, the receptacle will overheat if you draw over 15A. It will happen faster if either set of contacts are corroded (likely). The reason is, a 15A receptacle is less robust internally. It typically only has a metal contact inside on ONE side of the plug contact or small ones on both sides, to make it cheaper to manufacture. A 20A receptacle has large terminals on both sides of contact to carry the higher current (there is also a unique plug with the neutral terminal ‘sideways’, to prevent connecting a known 20A load to a 15A receptacle but they are exceeding rare). My point is, MAKE SURE YOU KNOW WHAT A 20A receptacle looks like and load your connection appropriately. Electrical power transmission is a matter of the ‘weakest link’, and just as a chain, a weak link, when overloaded will break.
For those who don’t know: Your article leaves another important point out that when combining all the loads to determine the total watts used in the RV, certain loads have to be derated because their likely use is not always at a 100% use. For example no one turns on the toaster all day. Nor do you use the electric heat/furnace element when you use the air conditioner. So a 30 Amp circuit may be (or may not) be enough to run all your appliances if you multiply the loads by a derating factor, which a licensed electrician or engineer would do. These derating factor can be found in the NEC code book.
In this article, you mentioned you could plug your entire RV into the Watt Meter. Where exactly in the chain would you put it, especially if you’re plugged in using your shore power cord?