Why 12 Volts LED lights are not reported in Watts.

I get asked the watts of a 12 VDC (Volt Direct Current) light on a pad or cluster bulb all the time and find that people are puzzled when I can't give them an answer in watts that help them decide what light to buy. The old rule of Watts = Amps x Volts applies here, as well as in house current. If you know two of the "Watts = Amps x Volts" equation, you can figure out the 3rd part. In house current VAC ( Volts Alternating Current) there is a greater consistency. The amps and the volts are close enough to being the same, to where when a light bulb says it is a 100 watt bulb you know what to expect. In an RV it changes and thus you won't get a truthful idea of what a light will produce in wattage.
The reason is, a power converter in an RV "in charging  mode" ( or fast charge) is suppose to be 13.4 VDC. Once the battery reaches 13.4 to 13.6 VDC the smart charge is suppose to bump back to a float charge which in between 12.2 to 12.6 depending on who makes the converter. The converter is designed to do that, to keep from boiling the water out of your battery(s). This also points to why it can shorten the life of your converter if you run your converter with out a battery. The converter stays in fast charge and it works very hard.

The converter going from fast charge to slow (float) charge is enough of a difference to give a big difference in light out-put, or lumen s.
Click here to see Wikipedia entry for luminous flux. 

Below are a couple of pictures of our power supply and meter which are connected to a cluster bulb that puts out a 1/3 the amount of light of the bulb it replaces (close to the same size).
Click here for an explanation of why cluster bulbs don't work in certain applications. 

This cluster bulb is being driven at 12.5VDC which is what a fully charged battery puts out. Here you see .09 amps, or 90mA, and you get around 140 lumen s which is 1/3 that of a the 1141 bulb.

This shows how accurate our DC power converter is. It is
right on 90mA.

Below are 4 groups of two pictures that show the different power draw and light output of our Large Warm White 42 super flux 10 to 24VDC LED pad 1156p48x68WW  
This pad is driven, or powered by, our Mas Tech variable DC power supply. The 4 settings are 12VDC, 12.5VDC, 13.4 VDC and 18 VDC.

This picture (above) is showing a true 12 volts DC with a .2 AMP power draw. The (below) picture shows the lumen s in our testing environment. 
We celebrated our testing environment off of a 100 watt light bulb (1600 Lumen s) and this pad is 62 % as bright as a 100 watt bulb when tested with a clear lens.

The picture below is at 12.5 VDC which is what a fully charged battery puts out. It has a .22 amp draw.
At 12.5VDC it is 79 % as bright as a 100 watt bulb when tested with a clear lens.

The picture below is at 13.4 VDC which is what a properly functioning RV converter puts out on fast charge.

It has a power draw of .27 amps and is 81% as bright as a 100 watt light bulb.


And just to show what the power draw is, as well as the light out-put  we tested it a 18VDC. Most LEDs will not be able to handle this amount of voltage. This pad was designed to handle it.
It uses .53 amps and is 1.42 time brighter than a 100 watt bulb.

 If you are wondering what the lumen out put is, based on watts in an 110/120 VAC (which is house current) it breaks down like this

  • 40 watts=450 lumen s
  • 60 watts = 800 lumen s
  • 75 watts = 1100 lumen s
  • 100 watts= 1600 lumen s 

If a LED supplier is reporting that their lumen out put as 180 lumen. A fair question would be what was the voltage in-put. Is it a true 12 volt? Is it 12.5VDC,or 13.4VDC or even higher? There is a difference!!!

Further more, there are allot of suppliers who report lumen as a total count of the LEDs on the bulb. If the cluster bulb has 20 LEDs that put out 18 lumen each, they report 20x18 and they say it is 360 lumen. How much light are you getting from the LEDs on the backside of that cluster bulb? You may be getting some light as it reflects off of the reflection plate, but your not getting the full amount. The only way to get a truthful report is for the to be reported in the application it is used in, and to know the voltage that is driving the light.

If there is another thing you could take away from this post, other than why LEDs are not reported in watts, is that pads, with all their LEDs facing the same direction, are going to put out more light than a cluster bulb.

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