If you really want to be exact and have lots of time on your hands, then you could theoretically go to each and every fixture to find out what type of ballast you have. This would be more exact as then you could determine your ballast factor, type of ballast and other relevant information.
This way would require a tremendous amount of time and effort which isn't necessary since companies like General Electric and most utilities adopt averages for calculations.
Not So Complicated:
You don't have to be exact when determining your fixture wattage. You should be able to determine from a distance what the type of fixture is and then determine the wattage. From there you can use averages like the ones that we use when doing an energy efficiency assessment.
Here are the fixture wattages that we typically use:
89 Watts - 2'x2' -U-shape Lamp
140 Watts - 2'x4'(3 Lamp)
178 Watts - 2'x4'(4 Lamp)
158 Watts - 8'(2 Lamp)
178 Watts - 8'(4 - 4' Lamps)
Step 5 - Multiply Your Quantity of Fixtures by the Wattage of Your Existing Fixture.
Let's say you have 100 fixtures and they are 8' - 2 Bulb T-12's:
100 fixtures x 158 Watts = 15,800 Total System Watts (Assuming this is the only type of fixture you have...)
Want one more complicated?
Okay, how about you have an office with (20) 2' x 4' 4 bulb T-12's, a work area with (78) 8' - 2 Bulb T-12's:
20 fixtures x 178 Watts = 3,560 Total System Watts
78 fixtures x 158 Watts = 12,234 Total System Watts
3,560 Watts + 12,234 Watts = 15,884 Total System Watts
Step 6- Forget the Calculations and Call Hovey Electric.....
If your still confused about all of this, here are some resources for you to consider:
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