How to calculate watts/$?

[Larry Grenier]

I guess that should be $/watt.

 

I'm trying to make a case for upgrading to LEDs

 

I know it depends on the county/provider. I'm in Prince William county VA using VEPCO I think.

Edited October 21, 2010 by Larry Grenier

[iangibby]

The vepco website will tell you what they charge per kwh. 1000 watts per hour. I suggest getting a plug that tells you how much your light draws and then do the math. In a year or two I think youll recoup the cost of the led from electric savings.

[Chad]

electricity is sold in the kw-hr, and I think the average in this area ia 12 or 13 cents per kw-hr.

 

Here is how I would do it:

 

Determine the inital cost of the LEDs:

easy part, it is just the up-front investment

 

Determine the cost to operate:

I would check your power bill to get an exact rate, but if not, the .12 or .13 that I gave will be adequate for a SWAG.

Find out the power usage of the fixture you are looking for. I am sure the number will be in watts, so divide by 1000 to put the number in kilowatts (kw), let's use 250 watts as an example.

Determine the length of time that you will be running the fixture a day, let's use 10 hours as an example.

Now determine the cost per day to operate

cost per day = power usage x length of time operating, in my example, this would be (250 watts / 1000) kw * 10 hours/day * .12 dollars/kw-hr = 1.74 dollars per day

[Chad]

Since I am a geek and kind of dig numbers I will take this a step further.

 

If I was going to make a "business case" for switching, I would do the daily cost calculation calculation for both your current lighting system and the LED system. Then I would take the difference and figure out how long it would take to make up the difference or

 

number of days = (up front investment of new system) / (operating cost difference per day)

 

This is a pretty simple thing to throw into excel and graph up to find the cross over points and all that on.

 

Have fun!

[hypertech]

electricity is sold in the kw-hr, and I think the average in this area ia 12 or 13 cents per kw-hr.

 

Here is how I would do it:

 

Determine the inital cost of the LEDs:

easy part, it is just the up-front investment

 

Determine the cost to operate:

I would check your power bill to get an exact rate, but if not, the .12 or .13 that I gave will be adequate for a SWAG.

Find out the power usage of the fixture you are looking for. I am sure the number will be in watts, so divide by 1000 to put the number in kilowatts (kw), let's use 250 watts as an example.

Determine the length of time that you will be running the fixture a day, let's use 10 hours as an example.

Now determine the cost per day to operate

cost per day = power usage x length of time operating, in my example, this would be (250 watts / 1000) kw * 10 hours/day * .12 dollars/kw-hr = 1.74 dollars per day

 

Try that math again.

[Larry Grenier]

Dam! You are a geek Thanks

 

I did calculate that the bulbs wold last about 13 years @ 10hrs/day

Edited October 21, 2010 by Larry Grenier

[Origami]

Just as a rough rule of thumb, I often pass this on:

 

1 Watt, if consumed 24 hours per day, 365 days per year, will cost you roughly $1 per year. If you do the math, that's 8760 hours per year or around 11.4 cents per kWhr. Now the price of a kWhr varies from jurisdiction to jurisdiction, but 11.4 cents is probably a little on the low side here for you Marylanders.

 

Here's a table from the Department of Energy on the subject covering cost per kWhr by state:

http://www.eia.doe.gov/electricity/epm/table5_6_a.html

 

So, if your skimmer runs continuously and consumes 30 Watts, it's costing you roughly $30 per year to operate (electricity only). If you have a return pump that's burning 130 Watts, that's another $130 per year.

 

If you're running 400 Watts of lights for 8 hours a day (1/3 of a day), that's costing you around $400/3 = $133 per year.

 

And so on.

 

It's admittedly a rough calculation, but it gets you in the ballpark and provides a sense of additional understanding.

[Chad]

Try that math again.

 

Hah, go fumble fingers!! Another lesson in "never trust the calculator the first time!!"

[bluce]

We can even make this more complicated for you due to the time value of money. The present value of the money spent for the LED lighting fixture is worth more than the future cost savings.

Example say your new fixture costs $2000 and you figured that your going to save $300 / year in electricty + an additional $100 for bulb replacement. Initially you would think that you would recoup your investment in 2000 / 400 = 5 years.

The problem is that your $2000 (due to time value) isn't worth as much in 5 years as it is today.

 

So the Future value (FV) of the initial $2000 invested 5 years from now is P * (1+i) nth Where P is the present value, i = interest rate (lets say 5%) and n = the number of years.

2000 * (1+.05) 5th power = $2552. Actually it would be less because your not paying out everything on the 5th year. So in this example it would take more time to actually recoup the true value of your present day investment

[hypertech]

Dam! You are a geek Thanks

 

I did calculate that the bulbs wold last about 13 years @ 10hrs/day

 

Don't be so quick to say that. Lots of people have bulb failures after a year or two. Saltwater is unfriendly to electronics. I'd be surprised to get more than 5 years out of an led fixture before having to rebuild something.

[thewire]

We can even make this more complicated for you due to the time value of money. The present value of the money spent for the LED lighting fixture is worth more than the future cost savings.

Example say your new fixture costs $2000 and you figured that your going to save $300 / year in electricty + an additional $100 for bulb replacement. Initially you would think that you would recoup your investment in 2000 / 400 = 5 years.

The problem is that your $2000 (due to time value) isn't worth as much in 5 years as it is today.

 

So the Future value (FV) of the initial $2000 invested 5 years from now is P * (1+i) nth Where P is the present value, i = interest rate (lets say 5%) and n = the number of years.

2000 * (1+.05) 5th power = $2552. Actually it would be less because your not paying out everything on the 5th year. So in this example it would take more time to actually recoup the true value of your present day investment

 

 

Yes I agreed with you assesment but you also have to take a few factors into consideration the heat issue that required you to run fans during summer/fall/spring months minus winter. Also MH induced heats into the house which required running of A/C. Water evaporate faster, hence more RO water cost..

[Integral9]

Umm... Ya'll are forgetting that electricity is not sold at a completely flat rate and the rate varies depending on the time of year.

http://www.dom.com/d...riffs/index.jsp

 

for example: If you are a Dominion Power customer the "Basic Residential Rate" is:

http://www.dom.com/d...fs/pdf/vab1.pdf

 

(a basic customer charge)

$7.00

+

a distribution charge which amounts to:

2.233 cents per kWh for the first 800kWh and 1.260 cents per kWh for anything over 800kWh

+

an Electricity Supply charge which amounts to:

(from June - September)

3.795 cents per kWh for the first 800kWh and 5.773 cents per KWh for anything over 800kWh

(or from October - May)

3.795 cents per kWh for the first 800kWh and 2.927 cents per kWh for anything over 800kWh

= your Bill

[Integral9]

I made a spreadsheet for you all: PowerBill_v1.xls

[Origami]

Umm... Ya'll are forgetting that electricity is not sold at a completely flat rate and the rate varies depending on the time of year.

 

 

Not forgetting. Just simplifying where we can to allow for a reasonable comparison. In the end, we're gonna pay the bill no matter how thoroughly we plan. After all, these formulas change every time the utility applies for a restructuring of their rates. This happens far too often for most of us.

[beatle]

I made a spreadsheet for you all: PowerBill_v1.xls

 

That's a really useful spreadsheet, thanks. I could compute the kwh of all my gear based on the number of hours used, but it was challenging to wrap my head around the rate schedules. I modifed the spreadsheet's columns to read "After Tank" and "Before Tank" instead of "Power Consumer" and "New Power Consumer" since it was originally designed to show you how much you can save by reducing your power consumption, not how much a reef tank costs you. "Savings" becomes the important column which should actually read "extra cost" with the new columns.

 

In the end, with 350w of lights run 9h/day, 24w of fuge lighting in a reverse photoperiod (15h), 30w of circulation, 80w return, and a 33w skimmer all run 24h, the total comes to $92.13/year which isn't bad. This assumes you have a base of 1000kwh per month and you're not factoring in any additional load on your AC during the warmer months or your water bill, which needs its own calculator!

[Integral9]

lol, yeah my column names were fairly off-the-cuff, although I did not mean to put an 'r' instead of a 'd'. hehe. The column names don't matter in the calculations though. I also didn't include taxes... I figure if we can get to 95% accuracy, that's close enough.

 

If you are like me and your whole house is electric and like me you hit that overage charge every month, then just using the overage rate is enough as your household appliances will take up the rest. Also, the distribution fees don't go up in the summer time or after the first 'X' kWh (at least for a Dominion residential consumer), so I didn't include that either.