What am I doing wrong here.

Pooh BearFebruary 25, 2004

Our electric bill last month was $82 for 1145 KWH.

So we pay $13.96 per KWH.

So, If I got a system that would produce one KW (1000 Watts),

how long would it take to recoup the investment.

At an average cost of $4 per watt, the system would cost $4000.

The system can only generate full power about 6 hours per day average.

That means it generates 6 KWH per day.

So, 6 KWH x $13.96 per KWH = $83.76

$4000 divided by $83.76 = 47.76

So it would take 47.76 days to pay for itself.

Everything after that is free electricity.

This can't possible be right.

Can somebody please correct it.

Pooh Bear

Thank you for reporting this comment. Undo

You need to add your fuel cost into the calculations.

If you have free fuel, let me know! ;=)

    Bookmark   February 25, 2004 at 6:07PM
Thank you for reporting this comment. Undo
Pooh Bear

No Fuel, this is PV array.
It just sits there on the roof and collects free sunlight.

Sorry I didn't specify that before.


Here is another aspect of the problem.
I used 1145 KWH last month.
Divide that by 6 you get 190.83

So to get a system that would meet my current electric usage,
The system would cost $763,300 and
it would take 190.83 x 47.76 days to pay it off.
Or 9114.2 days. or 24.97 years to pay it off.

Now lets look at something else.
Let's compare the money over a 25 year period.
After 25 years I am getting free electricity.
So for the next 25 years I am earning $82 per month.
At the end of next 25 years I have earned $24,600
Even less if electric rates go up in that time period.
But $763,300 @ 3% interest over 25 years would earn

compounded monthly
$9.03023734 x 10 ^ 34

compounded yearly
Almost 860 quintillion dollars
A quintillion is a billion times a billion.

So, with an initial outlay of $763,300
over 25 years I could earn $24,600
Or I could earn almost 860 quintillion dollars.

Hardly seems worth it now, does it.

Pooh Bear


Here is the formula I used for compound interest.

Interest Compounded During the Year

A = P(1+i/m)^mn
A = Amount in the account
P = The original deposit (the principal)
i = Interest rate per year, expressed in decimal format
m = Number of times interest is compounded within the year (4 for four times a year, 2 for twice a year)
n = Number of years compounded.

    Bookmark   February 25, 2004 at 8:47PM
Thank you for reporting this comment. Undo

You need to redo the math, you are only paying $.072 per kilowatt hour.

    Bookmark   February 25, 2004 at 9:09PM
Thank you for reporting this comment. Undo

82 divided by 1145 = .0716

If your electric bill is on a 30-day cycle you use 38.16 kWh per day.

This is the method for figuring cost per kWh for solar.

($Cost of system) / (# kWh/mo it produces)(12 mo/yr.)(Life expectancy of system yr.) = $?.?? Per kilowatt-hour.

It would only cost $4.00 per watt if the array only produced 1000 watts for one year. Generally it will cost about $0.10-0.40 per kWh to produce using solar. Your costs will depend on where you live, how much sun you get and if there are any obstructions that would effect your array. In a place like New Mexico you can expect your array to produce about 6-7 wh per day for each peak watt it is rated in a typical mid Atlantic state you are looking at about 2-3 wh per peak watt per day. So it will depend on how sunny you are.

One of the best things you may be able to do is to try to reduce your energy use. You are above the national family average of 24kwh per day. Change your light bulbs to compact florescent, they will cost more upfront but they last a long time and use about 75% less energy. Turn lights off when you do not use them, move your thermostat down just one degree in the winter and up one in the summer and so on little things will add up fast.

I hope some of this helps. It is very hard to figure this stuff out there are just so many variables that change depending on what block you live on.

You mentioned on another thread that you knew of hybrid solar panels capable of doing both domestic hot water and PV conversion. Could you provide me with more information? The only hybrid solar systems I am familiar with are amorphous and monocrystalline silicone panels combined to increase conversion efficiency.

    Bookmark   February 25, 2004 at 10:09PM
Thank you for reporting this comment. Undo
Pooh Bear

I can't seem to find any links to anything about the hybrid solar panels.
I read about them a few years ago.
How they work, is they take a standard PV solar panel.
They put a curved glass panel over it that acts as a magnifying glass.
This concentrates the suns energy on the PV panel.
This causes it to generate more electricity.
But it also causes more heat in the solar cells.
Heat in silicon solar cells in bad for them.
So, the back them with a water channel, and circulate
water to carry off the excess heat. This water can be
added to the houses hot water system.

I had some catalogs with these type of panels in them.
But that was 7-8 years ago when I was looking at solar options.
Maybe somebody can have better luck searching
for this than I did.


If I called a typical company that deals with PV systems,
and told them to come out and install a system,
and I just got the bill when they were done,
It would cost about $4 per watt for the system.
If they installed a 10KW system it would cost $40,000.

Pooh Bear

    Bookmark   February 25, 2004 at 10:49PM
Thank you for reporting this comment. Undo

Change your light bulbs to compact florescent, they will cost more upfront but they last a long time and use about 75% less energy. Turn lights off when you do not use them, move your thermostat down just one degree in the winter and up one in the summer and so on little things will add up fast.

Not good advice.

The 4' tube fluorescents are economical and durable, the compacts are not, I have tried them, they do not last and do not emit the necessary light.

A problem exists, the "rich" are asking the "poor" to reduce their standard of living by 10 to 20%, but this is not the solution, at least in the long term.

The solution is too eliminate waste through far better engineering and design.

A house for the 21st century would indeed be nice..Most houses are designed for the 19th century (mine, my neighbors,etc)..

As an example, my antique of a house is good in some aspects, and only cost me $50,000, the "energy efficient" 21st century home would cost $100,000, so we have quite a quandary here.

The utility bills for the new home would be about $100 monthly, for the old home they are now $300 monthly..

I would have to pay $50,000 more in order to save $2,400 annually..

In the future, a strong possibility exists that electricity(from the utilities) will cost more than 8 pennys per watt and that the solar array will have a true cost of less than 8 cents per watt. As the array system could cost $10,000 to purchase, the interest rates will be a very, very strong factor.

The competition will be intense as well can be imagined.

    Bookmark   February 26, 2004 at 9:54AM
Thank you for reporting this comment. Undo

We have been using the compact flourescents for many years, and found most to hold up very well, at least to the 10,000 rated hours and usually longer. It is true that they fudge the numbers on how much light the give compared to an incandescent, but they are still much cheaper to run. We have found it takes a 32 watt compact to be equal to a 150 watt incandescent in our end table lamps. An 8 watt does just as well as a 40 in ceiling fixtures. So I would say that they run on average about 20% the cost to run.

    Bookmark   February 26, 2004 at 2:05PM
Thank you for reporting this comment. Undo

I agree with booster about the CF bulbs. Used properly they are quite a good solution. They do not last as long as the advertised time, if you use them in applications where they are turned on and off frequently. I've put them in most of the lights that are left on for extended periods. If a 15W doesn't provide enough light as an 60W incandescent, then use a 19W -- it's still a considerable saving.

    Bookmark   February 26, 2004 at 11:03PM
Thank you for reporting this comment. Undo
Pooh Bear

We use CF lights. Love them. Less than $4 each.
We use less than 100watts of light bulbs thru-out the house.
We have our thermostat set on 62 degrees.
I am the only one home most of the time.
And I stay in one room. This room has a lot of electronics
in it so it stays warmer than the rest of the house.
And I can use a small space heater to supplement the heat.
Most of our electrical use goes into all this electronic equipment.
So we are definitely not wasting any electricity.
I just use a lot of electronic equipment all day.

Pooh Bear

    Bookmark   February 27, 2004 at 12:55AM
Thank you for reporting this comment. Undo


It sounds like you are doing a lot to reduce your energy consumption. Do you have electric heat and/or stove? I just wonder based on your monthly use it seems high considering you are conserving in other areas.

As for the $4 per watt. It could only be $4.00 per watt if you 10kw system were only going to produce 10kw over the entire life span.

LetÂs say you get that 10kw system for $40,000.

LetÂs say you live in an area of moderate sunlight and you can expect to produce 3 wh per day for each watt of your system.
3wh x 10,000 w system = 30,000 wh per day

30 kWh x 365 days = 10,950 kWh per year

LetÂs say you get 30-year panels, they could last longer but I think this is the best warranty given now.

($40,000)/ (10,950kwh per year)(30 year life of panels) = 0.12 cents per kWh

It is still more than you are currently paying for electric but not as bad as you thought.

    Bookmark   February 27, 2004 at 11:41AM
Thank you for reporting this comment. Undo

Hey there, Pooh,

are you sure that much of the warmth in your occupied room doesn't come from the hot air generated by the resident?

Jes askin'


    Bookmark   February 27, 2004 at 7:01PM
Thank you for reporting this comment. Undo
Pooh Bear

LOL @ Ed.
Don't think so. Since I'm home most of the time by myself,
that would mean I would be talking to myself.
My alter-ego says I'm not heavy on the conversation.

Actually, our house uses gas. My little space heater is electric.
It was also very cold last month.
Our normal electric bill is about $60 per month.

This is a new house. Only about 6 months old.
It is sealed very well. Insulated well too.

All this electronic equipment must use a lot of energy.
I have a bunch of audio/visual equipment that runs 24/7.
And I have my own internet server that runs 24/7.
Along with two computers that are always working.
If I am not at the computer then I give it tasks to perform.
It works while I am asleep, or gone for the day.
I do a lot of uploading and downloading.
I have bot programs to help me with this.

Last time I checked into PV systems, it ranged from $3 to $5 per watt.
This was for a complete system. And a large system.
Smaller systems cost more. This was 7 to 8 years ago.

    Bookmark   February 27, 2004 at 11:25PM
Thank you for reporting this comment. Undo

To some degree, cutting electrical usage during times when you are heating your house is not extremely critical, especially since your electric rate is not too bad and natural gas has gotten expensive. All you're doing by keeping all that stuff running is changing your home's energy balance a little away from gas heat and more toward electric heat, in the form of the heat that your equipment is generating and is contributing to your home. So, as long as you are running heat in your home, saving 1 kwh of electrical usage does not save you 7.1 cents. It only saves you about 3 cents, which is roughly the price difference per kwh between gas heat and electric heat (at your electric rate).

However, in the summer when you are running a/c, your net savings that come from cutting 1 kwh of electrical consumption is far higher. This is because then not only are you spending the 7.1 cents for the kwh of power, but you are spending roughly another 2 or 3 cents to have your air conditioner get rid of that kilowatt-hour's worth of heat. So, in the summer, saving 1 kwh of power consumption saves you around three times as much as it does in the winter -- around 10 cents.

    Bookmark   March 4, 2004 at 8:36AM
Thank you for reporting this comment. Undo


I use just about as much electricity as you do. Unfortunately, I have an electric water heater. I recently installed a 220V timer to the water heater that only provides electricity to the water heater from 5am-9am and 5pm-9pm. It has saved me about 10-15% in electricity costs. All the materials cost me less than $60.

If you have an electric water heater I would recommend a timer.

(Amarillo, TX)

    Bookmark   June 12, 2004 at 10:33AM
Thank you for reporting this comment. Undo

That can be an excellent thing to do, but in order for it to work, your utility has to offer an on-peak/off-peak rate option, which means that they come out and install a special meter which has a clock on it and enables them to charge you a different rate at different times of the day.

Also, some utilities do not use a timer for off-peak water heaters but instead use a remote control system where they can shut off your water heater remotely for a certain number of hours per day if they are experiencing high electricity demand.

So the first step is to call the utility and see which (if any) of these plans they offer. They can then advise you on the best way to take advantage of that rate plan.

    Bookmark   July 17, 2004 at 2:20AM
Thank you for reporting this comment. Undo

Well, not really. I think what Lee was getting at, is that if you only need hot water at the times that he has his timers set to 5-9 both am and pm, then that is when you need to keep water warm. If you know that there will be no demand for hot water and you use a standard hot water tank that keeps the water warm all the time, this set up will definately reduce the power usage, no matter what rate you are paying.

Could be interpreting incorrectly here but I think that is what he was getting at, if not sorry Lee.


    Bookmark   July 19, 2004 at 1:12AM
Thank you for reporting this comment. Undo

That's an interesting idea, but putting a water heater on a timer without having an off-peak rate is not any different from just lowering the setpoint temperature of the water heater -- something you can do with no timer.

If you let the water heater operate just between 5 a.m. and 9 a.m., and 5 p.m. and 9 p.m., that's 8 hours a day. Yet, according to the Energy Guide sticker on my electric water heater, they are assuming that an average water heater only runs about 3 hours a day. So, if you just leave the water heater alone with no timer, it won't run anywhere close to 8 hours a day -- unless you use a LOT of hot water. (And if you use that much hot water, you couldn't do this, because you'd be running out of hot water between times the water heater is timed to run.)

So, the only savings from a timer comes in the fact that the water is being held at a lower average temperature than it would be if it were just allowed to run as needed. This means that you could receive the same savings simply by lowering the water heater's temperature setting.

    Bookmark   July 19, 2004 at 4:29PM
Thank you for reporting this comment. Undo

1145 kw month = 38kw/day or an average of 1.59kw per hr.
If we could insure that the load were to remain constant for the entire 30 day period you could rely upon a 1.59 kw generator. 746watts of electrical energy equals 1 horsepower of mechanical energy, so in a perfect world, it the generator could produce electrical energy exactly equal to mechanical output you would need a 2 hp gas motor. A 2hp gas motor typically consumes 2 gal/hr and fuel is $1.95 a gallon so you need $3.90/hr for fuel x (24 hours x 30 days) or 720 hrs @ $3.90 hr = $2808. But even that will not do the job, because your electrical demand is not constant. An electric range requires 50amps at 220volt (P=E x I where P is watts, E is electomotive force(volts) and I is inductive force (amps) therefore the range requires 50 x 220 or 11,000 watts. To produce 11,000 watts you would need a 14.74 horsepower generator (11,000 watts divided by 746 watts electrical per horsepower) you would need another 10 or 15 horse power when the AC comes on, etc etc etc. Over the years i have heard many people say they were going to buy a gernerator and produce their own power, but in truth, if everything worked perfect, you still could not produce enought electric to satisfy your home for a thousand times what commercial power costs.

    Bookmark   August 3, 2004 at 3:47PM
Thank you for reporting this comment. Undo

Yes, the problem with a small generator lies in the inherent inefficiency of an internal combustion engine. When you're only seeing a small fraction of the energy value of the fuel going in actually coming out as electric power, and the rest being wasted as heat, you see why you can't save any money with a generator.

I'm not sure a 2 hp gas motor uses quite 2 gallons of fuel an hour - I think it might be somewhat less than that. But even if it uses 1 gallon in three hours, a gallon of gas contains about 150,000 BTU of energy, and if you're producing a solid 2 hp for three hours and turning all of it into electric power, you're yielding about 4.5 kwh (about 15,000 BTU) of energy for your 150,000 BTU input -- a paltry 10 percent efficiency. At $2 a gallon for gas, that's about 45 cents per kwh, which is roughly 5 times the average U.S. electric rate, and that doesn't count upkeep and repair on your generator.

The only way you can economically generate your own power with any type of engine is if you have some use for the waste heat, such as to heat your home or for a business to use in some industrial process.

    Bookmark   August 4, 2004 at 10:52PM
Thank you for reporting this comment. Undo

you should check to make sure you are not including a customer charge in your rate calculuation. In fact, there should be a $/kWh rate listed on your bill.

If you provide the name of your power company, someone here can look up their tariffs to check.

    Bookmark   August 23, 2004 at 11:38AM
Thank you for reporting this comment. Undo

try the online calc. at this site: http://rredc.nrel.gov/solar/calculators/PVWATTS/

Here is a link that might be useful: pvwatts

    Bookmark   August 23, 2004 at 1:23PM
Sign Up to comment
More Discussions
Radon system
Can somebody tell me why there's condensation outside...
Anyone get SRECs for their solar panels
We've had solar panels installed and working for 3...
Annie Deighnaugh
solar pods?? is it worth it?
first that I remember saw mentioned in "Farm Show"...
PV installation ROI review
My wife and I are researching solar panel systems for...
Geothermal in south bay area
I am wondering if anybody has feedback on geothermal...
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™