# How to measure power used

#### river251

Joined Jan 2, 2023
2
I've never posted here before, and I hope that this is not an inappropriate post.

I purchased an LED light for my dirt bike online. I don't have a lot of confidence in its 300 watt rating. I have a VOM; I don't know if it's safe to use it to measure amperage on this. Can anybody suggest how I can find out how much wattage or amperage this light actually draws? My motorcycle has a 140 watt stator. I thought about just connecting it to a car battery with my VOM in line with it. Thanks for any help.

#### Papabravo

Joined Feb 24, 2006
20,615
You need to be careful in taking measurements on powered circuits.
1. In general, the probes of a voltmeter go between two points of a circuit, one of which is usually GROUND.
2. To measure current the ammeter must be in SERIES with the circuit and the meter must be able to handle the maximum possible current.

#### geekoftheweek

Joined Oct 6, 2013
1,103
Many time LED lights are rated at their light output relative to an equivalent incandescent light current so 300 may or may not be the actual power used.

If it is in fact 300 watt it should blow the fuse in your VOM if you try to measure the current since it would be in the order of 21.4 amps (assuming 14V output). Even if your stator is only putting out 140 watts it may blow the fuse if it's a decent meter (assuming 14V again that would be right at 10 amps).

Unfortunately the safest bet is to measure across a shunt resistor, but I'm guessing since you are asking how to measure it you don't have one nearby. If you have an electric range one of the heating elements may work as one...

#### WBahn

Joined Mar 31, 2012
29,519
That 300 W is almost certainly the wattage of an incandescent that puts out (what they claim to be) an equivalent amount of light. Look on the package carefully and you should find what the actual power draw of the headlight is, probably in fine print somewhere underneath the big 300 W label.

#### dl324

Joined Mar 30, 2015
16,168
Welcome to AAC!
Can anybody suggest how I can find out how much wattage or amperage this light actually draws?
I doubt that it's actually 300W, maybe 300W equivalent. I have some LED replacements for 9005 headlights that are rated at 65W, but the LED bulbs only draw a few hundred mA, whereas the stock bulb would draw more like 5A.

I'd assume it was equivalent watts and put it on a bench supply with volt and amp meters. Set it to 12V and read the current.

#### river251

Joined Jan 2, 2023
2
Thank you very much for the replies. I'm afraid my electrical knowledge is not quite up to translating that into action. The motorcycle is dead so can't be involved. But I wanted to see if the light I got can be used with the bike when the stator puts out 140 Watts. I thought to test it by hooking up a vom in line when connecting the light to a car battery to see the amperage draw. But I was afraid the amperage from a car battery with its high cranking power might damage the vom. You're right, it will be protected by the fuse but I don't want to be replacing the fuse and I understand about putting resistance in to lower the amperage but don't have something like that handy although I could buy it. I do have a power supply I used to use to charge my RC airplane batteries. It is variable voltage and variable amperage. So if I set the voltage to 14, and crank up the amps so voltage times amps equals 140, can I then connect the LED light to it with the vom inline and successfully read the amperage draw that way? Does the vom need to be in line on the positive or negative side or does it matter? This LED light came from the Chinese website TAMU, in a plain brown box with no instructions or anything. So there is no information about the power or rating of the LED light. If you're familiar with TAMU you'll understand why I wouldn't trust the ratings anyway. I think the light cost \$12. Thank you all very much.

#### Ya’akov

Joined Jan 27, 2019
8,553
I've never posted here before, and I hope that this is not an inappropriate post.

I purchased an LED light for my dirt bike online. I don't have a lot of confidence in its 300 watt rating. I have a VOM; I don't know if it's safe to use it to measure amperage on this. Can anybody suggest how I can find out how much wattage or amperage this light actually draws? My motorcycle has a 140 watt stator. I thought about just connecting it to a car battery with my VOM in line with it. Thanks for any help.
Welcome to AAC.

It is very unlikely the light is actually drawing 300W. A typical LED has a radiation efficiency of about 80-100 Lumens per Watt*. So, if you actually need 300W of electrical power to drive it, it should be producing about 24,000-30,000 Lumens!

For comparison, a car headlight produces 3000-4000 Lumens. This means your light would be the equivalent of 8+ car headlights—this is very unlikely. Add to this the almost universal tendency for “discount” manufacturers to lie about the specification and performance of what they sell, LED-based products being a special favorite for this treatment, and I suspect you will find the light draws less than double digit amps, possibly quite a bit less.

The best bet is to pick up a cheapish clamp style meter that can measure DC current. That way you don’t have to worry about fuses and the like. Connect the light to a car battery and measure the current.

*This is just an estimate to work with, its relatively conservative for modern, white LEDs as used in lighting.

#### dl324

Joined Mar 30, 2015
16,168
I do have a power supply I used to use to charge my RC airplane batteries. It is variable voltage and variable amperage. So if I set the voltage to 14, and crank up the amps so voltage times amps equals 140, can I then connect the LED light to it with the vom inline and successfully read the amperage draw that way?
If your power supply has voltage and current meters, just set it to 14V, connect the bulb, and read the current. If it doesn't, you need to use a current meter, but it will affect the current reading (make it read low).
Does the vom need to be in line on the positive or negative side or does it matter?
It doesn't matter which side of the bulb you place the meter. Just make sure the polarity is correct and that the range is set on a sufficiently high scale.