# How to measure watts.

by Jimmy Snyder
Tags: measure, watts
 P: 1,231 in theory, 24HP * 746W/hp = 17,904W. Your biggest problem with measuring this much output power is finding a load this big. You'll need a *big* resistor bank which can be expensive. Aren't toasters about a 1000 watts-maybe 2 slice or 4. Plug in 1 and see if it pops. Plug in 2 and see it both pop. etc..... Until they don't pop no more, and that should be max output of the generator. Anbody like a lot of cinamon toast?
PF Gold
P: 701
 Quote by Jimmy Snyder I give up. I will ask elsewhere. Thanks, everyone, for your effort.
A 13,000 watt load on the generator would stall a 17.5 hp engine, the 6.5 hp extra is the amount needed to keep the engine in a safe work load range. A small load on the generator would indeed waste a lot of fuel.
I can't tell you how, but someone else can say how to check the ohm's of the generator wiring and get the exact load rate of the unit.

Ron
 Mentor P: 38,920 Jimmy took his generator and went home. Now what are we supposed to do with all these light bulbs and toasters?
Mentor
P: 11,966
I didn't realize people ever rated electrical outputs in horsepower -- I thought that was used solely for mechanical outputs, i.e. motors.

At any rate, 13 kW at 120V means drawing about 110 Amps of current. Or using the 1000W toaster approach, you'd need to hook up 13 toasters to this thing. And you'd be hard pressed to find any one single device that consumes more than 1800W, since that is the maximum power a single 15A, 120V household outlet can supply.

EDIT:
 Quote by russ_watters As said, the generator will be regulated, but to measure the watts if you just have a light bulb: 1. Buy one of these: http://reviews.homedepot.com/1999/10...ws/reviews.htm 2. Plug everything in. 3. Turn everything on. 4. Read the watts off the watt meter. Also, at low speed, the efficiency is certainly much worse than the 50% of peak your numbers imply. The generator probably runs at 10 or 20% of peak fuel flow just to keep it spinning.
But if he wants to test the generator anywhere near the 13 kW maximum, the Kill-A-Watt does not measure power levels that high. When I tried to use one to measure the power of my neighbor's air conditioning system a couple years ago, the reading was off-scale.
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P: 38,920
 Quote by Redbelly98 I didn't realize people ever rated electrical outputs in horsepower -- I thought that was used solely for mechanical outputs, i.e. motors.
The horsepower rating is for the gas-powered motor that drives the elecrical-output generator. That's where the 17kW --> 13kW decrease in efficiency comes from.

I miss Jimmy's generator...
P: 2,163
 Quote by Redbelly98 I didn't realize people ever rated electrical outputs in horsepower -- I thought that was used solely for mechanical outputs, i.e. motors.
Here is an advertisement for a 24 HP generator. I do not endorse it, it's just an example.

24 HP generator

24 HP is the mechanical input, not the electrical output. However, it is possible to provide less than 24 HP at the input. The thing will still work, but it won't output 13 kW. Apparently, you can't even count on the 13/18 ratio (24 HP is roughly 18 kW). Since I am providing only a small but unknown amount of power to the generator (perhaps as little as 200 W), it is of little interest to know what the full capacity of the generator is. What I want to know is what power is coming out of the output. Adding lots of toasters won't help. What I propose to do is to load the output with a 100W light bulb and then measure the wattage coming out of the generator. I assume that if the measured value is less than 100 W, then I have my answer. If it is just 100 W, then I need to load a second bulb and measure again. Is that correct? It is unlikely in the extreme that I am delivering more than 300 W to the input so 2 bulbs ought to do it. I still don't know how to measure. I looked at the kill-o-watt, but it doesn't look like the right tool. It seems to say it would measure the efficiency of the lamp that holds the bulb, but that is not what I am interested in. I want to know in units of watts, how much power is coming out of the generator.
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P: 11,966
Berkeman, Jimmy, thanks for clearing up my question on the horsepower rating.

 Quote by Jimmy I looked at the kill-o-watt, but it doesn't look like the right tool.
The kill-o-watt will tell you the voltage, current, and power (in Watts) of whatever device is plugged into it. I know this because I own one myself and have used it. You plug the kill-o-watt into any standard outlet (as are found on the generator), and you plug the device of interest (lamp, toaster, etc.) into the kill-o-watt.

Assuming the kill-o-watt consumes negligible power itself, it will tell you how much electrical power the generator is producing. It will provide the measurement you are looking for.

 It seems to say it would measure the efficiency of the lamp that holds the bulb
Well, to do that it would need to measure the power of the light being emitted by the bulb, which it can not do.
$\text{Efficiency} = \frac{\text{Light power output of lamp}}{\text{Input power to lamp}}$
The kill-o-watt will only tell you the input power to the lamp which, as I mentioned before, is essentially equal to the output power of the generator, the quantity you are after.
 P: 135 Here is a more practical answer, Your friend has a 17 KW generator for something, that size is what would run a whole house, say around 2500 Square feet, including AC. If it is for a backup, hook up some stuff and see. An electric clothes dryer makes a VERY GOOD load test, but you would need 3 of them to get to 17KW. Also Dryers do not have the same start load as a 5 ton AC unit. If your friend is thinking about running computer equipment with this generator, also think about a big UPS to condition and clean up the power.
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P: 11,966
Welcome to Physics Forums, johnbbahm.

I think Jimmy is interested in learning and understanding the details about the generator's capability. Just hooking stuff up to it and seeing if they work is not what he is after here, he is interested in measuring the actual wattage out of the generator.

Also, the electrical output turns out to be 12 to 13 kW, not 17 kW. The 24 hp (= 17.9 kW) figure is the rating of the motor output that would drive the generator.

 Quote by johnbbahm If your friend is thinking about running computer equipment with this generator, also think about a big UPS to condition and clean up the power.
Excellent idea, and definitely worth bringing up.
P: 303
 Quote by Redbelly98 Welcome to Physics Forums, johnbbahm. I think Jimmy is interested in learning and understanding the details about the generator's capability. Just hooking stuff up to it and seeing if they work is not what he is after here, he is interested in measuring the actual wattage out of the generator. Also, the electrical output turns out to be 12 to 13 kW, not 17 kW. The 24 hp (= 17.9 kW) figure is the rating of the motor output that would drive the generator. Excellent idea, and definitely worth bringing up.
yeah you're right about 24hp being the mechanical output of the generator. 13KW is the electrical power output.
PF Gold
P: 11,129
 Quote by Jimmy Snyder So, just because I think I am supplying 24 HP, doesn't meant that I actually am. If I load it with a 100 W light bulb I may only be supplying it with 200 W or less. Let's say, just for argument's sake that it takes 200 W input to get 100 W output. Let's also suppose that I am supplying it with 190 W, not enough for the bulb. I assume that the bulb will light up anyway, just not as brightly as designed. Am I right about that? So just because I see light coming from the 100 W bulb does not mean that I am actually getting 100 W out. How do I measure what I am actually getting? Think cook book, ingredients and procedures.
The electricity supply to your house will be fused to limit the power supplied to your house (maybe 20kW), That doesn't mean that you are using (or paying for) 20kW all the time.
The only difference between the mains supply and your generator is that you tend to pay the same rate for your mains electrical energy, whatever the power you happen to be using. As has been mentioned above, because the efficiency of your generator depends on the actual load, then you pay more per kWh, effectively (fuel costs), when using just one lamp and less per kWh when you are using it at full output.
 P: 2,432 I suspect alot of generators of this type fudge a bit on the ratings. For instance, I have a Coleman generator with I believe a 10 HP gas engine. But NO SINGLE OUTLET can supply its continous rated power. It has a duplex 120 volt outlet good for up to 20 amps. It has a single 120 volt outlet (Twist-lock) good for up to 30 amps. It has a single 240 volt outlet (also twist-lock) good for up to 20 amps. No single outlet will supply on its own the continuous rated power. I don't recall exact numbers, but I do know they fudge the specs. The duplex 120 volt outlet is on the same winding as the 240 volt outlet. Each half of it is technically good for 20 amps. - Jimmy, I think you've have been given enough to have a pretty good grasp of how to go about determining if the generator will do what it says. My original answer still stands. Put a load on it, see what happens. To calculate watts you need to measure the current as well as the voltage. To measure this kind of current you will need a clamp on type ammeter. Also, make sure the tractor driving it (I assume it is a tractor based on the Northern Tool ad) is running at the correct speed to get the required speed on the PTO shaft.
 P: 2,163 The ad for the Kill A Watt says you can use it to measure the efficiency of household appliances. Perhaps it was my own fault, I took that to mean that the Kill A Watt was not what I needed. I no longer think that. Here is how I understand it: 1. Plug the Kill A Watt into the output of the generator. 2. Plug the male end of an extension cord with 3 female receptacles into the Kill A Watt. 3. Plug a lamp with a 100 W bulb into one of the female receptacles. 4. Turn on the lamp, the generator, and the driver of the generator input shaft. 5. Read off the watts from the Kill A Watt. If the value is less than 100 W, then that is what the generator is putting out and the amount of power coming into the generator is some multiple of that, presumably greater than 18/13. 6. If the value is just 100 W, then plug a second lamp with a 100 W bulb into the extension cord, turn it on, and remeasure. If less than 200 W, then finished, otherwise, try a third lamp. I seriously doubt that there is 300 W going into the generator, let alone coming out of it, so 3 should suffice. But if not, I can buy a second extension cord and plug it into one of the receptacles of the other cord and so test up to 5 lamps & etc. The reason for all these questions is that I have an inventor friend who has a gizmo that drives the input shaft of the generator. I saw the generator itself today and turned the input shaft with my hand. It requires a lot of force to get it started turning, and impossible to keep it turning for more than a second or two and the rpms were about 20 or 30 at best. In order to run it at capacity it requires something like 3000 rpm, but he has a transmission attachment that allows him to run the transmission's drive shaft at 500 rpm and the transmission converts it to 3000 rpm at the generator's drive shaft. Since I was turning the transmission shaft, the speed provided was roughly 1/20th of what was needed. I asked him if he had a crank so I could spin it faster, but he did not. There was no load on the output. His invention that delivers power to the input shaft was not set up. He told me that he intended to have it set up 'real soon now' and when he does, I can see his gizmo in action. He said that he is going to make a more expensive version ($2000) of his invention and I suggested the$35 Kill A Watt to him so that he could measure the output of the generator with his contraption in place before sinking that much money into it. He said that he would and that I could see that running too. I think he has an unrealistic view of how much power his 'invention' is able to provide to the drive shaft. He thinks it will deliver 5 kW. My suggestion to him was that 750 W (~1HP) at the output was a good cutoff point. In other words, if the Kill A Watt measures 750 W coming out of the generator, then he could sink the extra \$2000 into an improved version, but if less, then he should work on design improvements rather than construction improvements. The 750 W figure was arrived at by considering a different way of getting electrical current that was cheaper than his method and could provide roughly 1500 W. He knows that he must reach the 1500 W mark and surpass it and that if he can't even get 750 W then it was unlikely his invention would work no matter how it was tweaked. I didn't tell him so, but in my opinion, he won't give 300 W to the input of the generator and that the output of course will be even less. I actually watched his setup running an air conditioner a few years ago. However, in addition to the generator, he also had a UPS attached so I couldn't tell how much he was getting from the generator and how much from the UPS, and I had no clue how many watts were being consumed.