cost to operate formula

[pscobb]

Mike had given us a formula for calculating the cost to operate an electrical device based on the KWH cost of the utility . I'm trying to show a customer the savings of using a MH fixture vs. a quartz halogen over the period of a year to justify the cost difference in the fixtures. Thanks.

 

[dbuckley]

The fastest way is calculate the watts saved by using MH rather than incandescent, and then calculate the money saved on the saved wattage. eg 70W MH vs 500W incandescent, watts saved is 430W. Calculate money saved by what that 430W would have cost.

(saved watts / 1000) = KW per hour.
Multiply that KW by burning hours
Multiply that by cost per KWH.

Also, a MH will last loads longer that a incandescent, so you need to factor up brurning hours and replacement lamp cost to get the full picture.

 

[__dan]

dimensional analysis

dimensional analysis

dimensional analysis

operating cost = (cost per kwh) x (load kw) x (runtime in hours)

The metal halide is roughly 5 times more light at the same wattage (number pulled from arse). The color is a lot worse, bluish.

So, for the same light, you would swap a 500 quartz lamp with a 100 watt MH for roughly the same lumens. Check a lamp catalog for tighter numbers.

annual operating cost quartz at 4 hours daily = (.12 dollars / kwh) x (.5 kW) x (4 hours x 365 days) = $87.60

annual operating cost MH at 4 hours daily = (.12 dollars / kwh) x (.1 kW) x (4 hours x 365 days) = 17.52

In your specific example I would not sell MH over quartz in residential based solely on energy savings. A 100 watt MH miniflood can throw light 150 ft out and sideways from the fixture. You can get in a lot of trouble with glare and light trespass in residential. Do they want to light a yard or a walkway, will the light keep the neighbors up at night.

After trying to put up 75 watt hps minifloods and miniwallpacks, lots of times the safer bet is two 150 watt PAR halogen floodlamps. My concern would be lamp lifetime. The quartz lamp is 300 hours rated, the MH lamp is 18,000 hours, the incandescent PAR halogen lamp is 2000 hours. When LEDs drop in cost, that will clearly be the way to go.

Trying to sell the customer on numbers for energy savings or lamps lifetime, lamp changes every three months (quartz), every two years (PAR halogen), or every eight years (MH), you would have to think you've lost the customer in the first two seconds. The customer wants aesthetics, something they can brag about.

The best customer is the idiot with money, the guy who has hot air heat and single pane windows in his house but hot water radiant heat under the driveway paving to melt the snow. He wants to look out the window and see snow melting off the driveway, the poor sap across the street shoveling snowing, and say "I can burn oil to melt snow and I have money left over".

Trust me the system installer was likely also an idiot, the snow melter control was a cheapo, turned on and never shuts off, runs 7-24.

This is your customer. Now what product would you like to sell him.

 

[weressl]

Mike had given us a formula for calculating the cost to operate an electrical device based on the KWH cost of the utility . I'm trying to show a customer the savings of using a MH fixture vs. a quartz halogen over the period of a year to justify the cost difference in the fixtures. Thanks.

MH will have a higher initial cost, especially if you include a higher quality fixture where you can select the ballast efficency and longer service life. Color corrected MH are available, they have a somewhat shorter life, but still significantly longer than quarz.

If you have dawn-to-dusk operation then your total length of operation will vary based on your location, eg. cost wil change from Anchorage to Miami, but it is not significant. You yearly dawn to dusk is about 4450 hours.

What is significant though is the cost of the bulb and the cost to replace the bulb. MH will lasts anywhere between 10-15 times longer than quarz, especially the ones made in China seem to have very short life. If you include the replacement labor cost, it is a no brainer.