You would start by working out approximately how many lumens is being produced by your current lighting system.
To do that, determine the total wattage of your current system and multiply that value by the lumens per watt (LPW) for the type of lighting you are replacing from the second column in the table below.
Lumens per Watt (LPW)
50 - 100
|Metal Halide Lamp||
65 - 115
|High Pressure Sodium Lamp||
85 - 150
|Low Pressure Sodium Lamp||
100 - 200
Note that this table is for general reference only. The actual LPW rating for many types of light can vary by quite a bit. You may be able to find more accurate lumens ratings in your lighting documentation, or from the manufacturer of the light you are currently using.
Once you have the total lumens output of your current system you can then estimate how many LEDs you will need by simply dividing the lumens rating of the LED you will be using by the lumens generated by your current system.(W x LPW) / Ll = Ln
- W = Total wattage rating of the light being replaced.
- LPW = Lumens per Watt from the table above, or the manufacturer's documentation.
- Ll = Total lumens output of each LED or LED assembly you want to use.
- Ln = Total number of LEDs you will need to use to get an equal or better amount of light.
For example, if you are replacing a 40W Halogen lamp that has a LPW rating of 20 with a MR-WC100-20T that you will be powering at 700mA, then you will probably want to use 2 MR-WC100-20T to replace the halogen lamp.(40 x 20) / 540 = 1.48
This will only tell you how many LEDs you will need to generate an amount of light that is equal to your current lighting system. There are other factors that can effect lighting including color temperature, optics, etc.