Updated 10/21/2019
The following entry is a very basic explanation of Watts and Watt-hours to help new RVers understand the terms and what it means to them when calculating power needs. Many more pages can and have been written on this subject and I encourage RVers to take the time to understand the electrical needs of both 12-volt and 120-volt of their RV.
I recently wrote a post about using a back-up UPS as a convenient source of low wattage shore power when dry camping. From some of the comments I received to that post and comments I see online regarding 120-volt power needs when dry camping, there seems to be a mystery on what a watt is and how you can use this unit of energy measurement when calculating power consumption.
As an RVer, here is what you need to know about Watts and its cousin Watt-hours. Watts are a unit of power at a moment in time while Watt-hours (Wh) measures amounts of energy for a specific period of time (an hour). As the number of watts increase, so do the power requirements (i.e. a 100-watt bulb consumes more energy than a 60-watt bulb).
Power consumption – Photo via iRV2 Forums
Example: A 100-watt light bulb requires 100 watts of power to light it for a moment. A 100-watt light bulb burning for one hour will consume 100 watt-hours. It doesn’t matter if the power source is 12 volts or 120 volts, the consumption, as expressed in watt-hours, remains the same.
Most 120 volt (shore power) appliances list the watts (many times designated by a “W”) they consume either on the back of the appliance or in the owner’s manual.
One of the comments I received on my previous blog post is shown below.
Sadly, I added to the confusion when I initially responded to Bob as I had interpreted the information on my UPS unit as being able to deliver 480 Watt-hours of power, which would have been 480 watts over one hour of time. Had that been true, my answers below (slightly revised for clarity) would have been as follows.
Answering both parts of this question is quite simple. If the RVer looks up the power consumption (in watts) of his satellite receiver and flat screen, he will have everything he needs to figure out if the device will power the two items and for how long. Let’s say the flat screen draws 175 watts and the receiver 25 watts for a total of 200 watts. Had the UPS unit I blogged about been rated to deliver 480 watt-hours, the answer to the first question is “yes” as the power consumption (200 watts) is less than the power source (480 watts).
Now if you will remember watt-hours is the consumption of energy over time (one hour). To answer the second part of this question, you just take the available power 480 watt-hours and divide by the load 200 watts which will give you a rough number of hours the power source will power the load, which is 2.4 hours.
Resistance, inefficiencies, and other variables may impact the results for the example given above and other applications, but it is a good rule of thumb for newbies. No need to fill the comment box pointing this out!
In my next article, I will explain how to determine watts when it is not listed on the appliance and how to convert watts to amp hours when calculating how long your house batteries will last under a given load. Doing the math, just another adventure in RVing!
See also: This Office Product Will Enhance Your Dry Camping Experience
Dave Helgeson’s many roles in the RV industry started before he even had a driver’s license. His grandparents and father owned an RV dealership before the term “RV” had been coined, and Dave played a pivotal role in nearly every position of an RV dealership. He and his wife Cheri launched their own RV dealership in the Pacific Northwest. The duo also spent 29 years overseeing regional RV shows. Dave has also served as President of a local chapter of the Recreational Vehicle Dealers Association (RVDA), worked on the board of advisors for the RV Technician Program of a local technical college, and served as a board member of the Manufactured Home and RV Association. Dave’s reputation earned him the title of “The foremost expert on boondocking,” bestowed by RV industry icon, the late Gary Bunzer (The RV Doctor). When he’s not out boondocking, you’ll find Dave in the spotlight at RV shows across the country, giving seminars about all things RVing. He and Cheri currently roam in their fifth travel trailer, with Dave doing all the service, repair and modifications to his own unit.
Dave,
Thanks for helping us/me understand things like this…. taking the mystery out helps a great deal to us RVing newbies!!!
I understand Backpacking…. not RV electronics!!!
Hello Dave,
I have a 1990 36′ winnebago chieftain.
I have been hooked up at my friends in the country for almost a year now. I am not hooked up by a conventional rv electrical plug setup. I bought some 3-wire elec cord, hard wire and not stranded wire. I am 100′ from his elec box. I purchased an elec box to put next to my motorhome so I could hook up. I run my 13,500 btu a/c for lets say 24 hours a day. I think my electrical charge is too high. So I would like to know if because I am 100′ away from his box does my unit pull more electricity because of the distance that my motorhome is from his elec box? And if so, do you have any estimation of the percentage of increase? He is charging me $250/month just for electrical usage, I have never paid this much for one a/c before.
RvBob, Thanks
RvBob,
Question 1 -So I would like to know if because I am 100′ away from his box does my unit pull more electricity because of the distance that my motorhome is from his elec box? Answer – It would help to know what size wire you used to extend the circuit. If you are using undersized wire then yes you could be consuming more power than you should be as under size wire can’t properly handle the load you are putting upon it. Undersized wire can result in heat and as we all know heat is created by the consumption of energy. My biggest concern would be low voltage damaging the compressor in your air conditioner. I suggest you read this article: https://sciencing.com/cable-length-vs-power-drop-12184174.html
Question 2 – And if so, do you have any estimation of the percentage of increase? Best guess 5%, but it is only a guess.
Rough math if you are running your AC 24 hours a day at an average of 1,200 watts / hour that is almost 30 kilowatts a day or 864 kilowatts per month. If the cost per kilowatt is $.29 per kilowatt then $250 per month is about right.
My 2019 LG Electronics 32LK610BPUA 32″ flatscreen
720p draws only 18 watts in a lower brightness “economy” setting. I don’t need better than 720p since we watch the TV from about 12′ away and the human eye can’t decern between 1080p and 720p at that distance. The economy setting is bright enough for me. Next brighter setting is 25 watts, which is more bright than I need. The DTV receiver also draws 18 watts. I replaced my 2010 Visio 32″ flatscreen because it drew 89 watts and we dry camp a lot.
Dave, You need to be clearer and more careful with your terms. The WATT is unit of electrical power. Power is a RATE of energy delivery or consumption. A watt-hour is an amount of energy delivered or consumed (total energy over time). They are not interchangeable yet you have used them interchangeably. If you truly want to help people understand energy consumption, start by explaining and using the correct terminology.
It is great that you wrote this article as an attempt to help educate people but PLEASE know what you are talking about before you write and confuse people.
Watt is a unit of POWER. Watt-Hour (Wh) is a unit of ENERGY. Using a 100 W bulb for one hour consumes 100 Wh. The power company bills you for ENERGY. How fast you use that energy is the POWER. A battery holds so many Wh of energy. The inverter and loads use so many W.
To use the hydraulic analogy, a bucket holds so many gallons (analogous to Wh). A hole in the bucket will leak so many gallons per minute (analogous to W). How long before you run out of water is Wh divided by W.
The author seems to be confusing Watts (power) with Watt-hours (energy). A 100 watt bulb would consume 100 Watt-hours in one hour. An AVRG900U Cyberpower UPS, for example, is capable of 480 watts of power, but for only about 2 minutes. (At lower Wattage drain they are more efficient due to battery chemistry.) The example that a 200 Watt TV plus Sat Receiver drawing 200 Watts total, would probably run for 10-15 minutes before draining the internal sealed lead-acid battery down completely (not recommended!), not 2.4 hours as the author suggests. Running the UPS’s battery below 50 percent of its capacity would greatly shorten its life, just as in an RV house battery bank.
UPSs are not intended for powering appliances, but holding up computers long enough in a power interruption to permit safe and systematic shutdowns. UPSs should not be used for the purpose the author states.
I feel that a UPS in strategic locations, throughout the RV can be a positive energy saver and money saver. Distance of wire, charging from shore-like sources and embedded control has factors to be recognized.
Unfortunately, AC power is not as simple as DC power. The true power equation is Volts X Amps X Power Factor = Watts. In DC, the PF (power factor) is always 1.000000. In AC it can vary from 0 to 1. The significance is that wiring losses and some other losses vary with the current, not the actual power. If you have the power requirement for an AC appliance, you CANNOT figure out the current draw by changing the equation to Amps = Watts / Volts and for the most part you care about the Amps for wire sizes, fuse or circuit breaker settings, how many appliances can you run at once.
Dear RVBob,
The statement “For example, a 100-watt light bulb will consume 100 watts of electrical energy in one hour”, is not quite right and illustrates what most people misunderstand. A watt is not a unit of energy that is used over any period of time. A Watt is power and specifically it is a Joule/sec, which is a rate, not an amount. Put differently, a watt-sec is a joule and a kilowatt hour (the units of energy that you get billed for by the power company) is 1000 watt-hr, or multiply by 60sec and you have 60,000 joules. Sorry to be pedantic – it’s a pet peeve of mine.
RG
“From some of the comments I received to that post and comments I see online regarding 120-volt power needs when dry camping, there seems to be a mystery on what a watt is and how you can use this unit of energy measurement when calculating power consumption.
As an RVer, all you need to know about watts is that they are a unit of energy consumed/supplied over time. For example, a 100-watt light bulb will consume 100 watts of electrical energy in one hour.”
Maybe the “mystery” is caused by the fact that very often folks who write about electrical subjects themselves use terms incorrectly.
A Watt is NOT a unit of electrical energy, it is a RATE of using electrical energy. It is analogus to gallons-per-minute in fluid flow.
A Watt-hour (Wh) is a unit of electrical energy. Since it is a relative small unit it is often listed in thousands of units, Kilowatt-hours (Kwh), the units you will find on your household electric bills.
As in fluid flow, to determine the amount delivered multiply the Rate times Time. A flow of 5 gallons-per-minute for 10 minutes will result in 50 gallons delivered. A 100 Watt light bulb burning for 2 hours will use 200 Watt-hours (.2 Kwh) of electricity. Note that during that 2 hours the bulb will be using the electricity at the Rate of 100 Watts. If the bulb is on for only one minute it will still be using energy at the rate of 100 Watts.
Some of the mystery may be that in electrical terms the Rate of use is a single word, Watt, which designates a quantity-unit-used-per-time-unit. Whereas in fluid delivery terms the Rate is spelled out as the quantity-unit-delivered-per-time-unit. That is, Watt is analogous to gallons-per-minute. Then the resultant quantity in electrical terms is noted as the Rate-times-time spelled out, whereas in fluid measurement terms a single word is used for the resultant quantity, gallons. That is, Watt-hours is analogous to gallons.
I hope that clears up some of the mystery about electrical terms.
By the way, I often see the terms Amp and Amp-hour used incorrectly in many RV articles about electrical subjects, also. But I’ll leave that for a reply to an article where that is done.
Wil
Fast simple rule to go by:
Volts X Amps = Watts
Coffee pot = 110 X 12 = 1320 Watts
Toaster = 110 X 10 = 1100 Watts
Therefore 12 amps + 10 amps = 22 amps on 15 amp breaker
results “ Marge you overloaded the circuit again, how many times do I have to tell you “ don’t do that. LOL
Electric space heater: 1500 watts………..Watts divided by Volts
equal Amps
Camping is great!
Well, you’ve mostly corrected most of the incorrect terminology, but there is still the “1,200 watts / hour” and “30 kilowatts a day” in your answer to RVBob’s AC question. There are no electrical units “watts / hour” or “kilowatts a day.” Watts and watt-hours are still confused in this example.
I think what you are trying to say is that if an AC draws 1,200 watts and runs 24 hours a day that amounts to an energy usage of about 30 Kwh (kilowatt-hours) a day. (actually 28.8 Kwh.)
In actuality, an AC turned on all day does not draw maximum wattage all day. That is the wattage used while the compressor is running. So, in order to determine the cost of running the AC all day you would have to know the wattage of the fan only and the duty cycle of the compressor, which turns on and off as the temperature changes. A somewhat more complicated calculation than your example, which would undoubtedly result in a considerably lower energy (Kwh) usage per day.
Thanks for your explanation Will. During the summer months down here in Central Texas it was very hot and my motorhome was in the direct sunlight. So my a/c compressor was running most of the time. I am in my unit most all day everyday and I have to run my a/c all the time in order to stay cool. My a/c compressor did not cycle very often within a 24 hour period. It was just so darn hot last summer. As he said that $250 was about right @ 29cents a kwh, I have never ran above $125 to $150. The electric company here “cps energy” charges at rate of 13 cents/kwh. The people I am staying with are charging me way too much and I wanted to find out what I was actually using. I finally found out that they were just ball parking the amount, when they said that they actually had the bills to compare the energy usage from the year and month before I got there but they were lying just as I suspected. When someone tells me that I can stay on their land for “bills only” and then hoses me, I get a little irritated. Hope you understand.
RvBob San Antonio, Texas
OK RVBob, understand. So if your compressor is on all day you are still getting hosed.
I once was staying long term in an RV park that metered each hook-up and charged what the electric company charged plus a surcharge. After a few complaints it was found that it is illegal in this state (Washington) add a surcharge on metered electricity usage. On a non-metered “estimate” I’m not sure.
You are right Wilyoung, I know that I am being
” hosed “. The owner of the property is a liar but hat is the way he is and he lies about everything. I have been living in rv’s for more than 20 years now and I pretty much know the costs incurred whether it be water, propane or electricity. I just wanted to know what you other rv er’s thought. I am glad that you agree with me. My thoughts were that during last summer my electric should have been from
$100 to $150 max. but like everyone else they are greedy and want the guy in the rv to pay their electric bill too! haha … haven’t moved yet but I am looking for another place.
Stay warm from RvBob … blessings!
Calculating RV power needs sometimes can be quite challenging for people, and I see you have simplified it very well. Thanks!