Correct my math please, my wife doesnt believe me.

[merlyn2000]

Correct my math please, my wife doesnt believe me.

i would very much like to 'solar-ize' my home, and have done **some** searching for information. let me describe what i've come up with.

my electric bill states that i used 39kWh average per day, from mid april to mid may [i think thats a bit high and we need to adjust our usage but for the sake of discussion, let's go with it]. using the formula:

100W X 30h = 3000Wh = 3kWh	([url]http://en.wikipedia.org/wiki/Watt-hour[/url])

i calculate that we use 1625W average (39000Wh / 24h = 1625W). for the sake of simplicity, and not knowing how to calculate actual needs for myself, i used information from this post:

http://www.solarpaneltalk.com/showthread.php?t=484

specifically this portion quoting Jasun:

To get about 2000 watts of electricity, you'll need about 400watts of solar panels.

A good average price of solar panels is $5 a watt. So you are looking at roughly $2000 for a system to supplement your needs.

**Assuming** a perfect installation with no losses due to wiring or less than perfect solar cells,
i then used information about Ohm's law from this site:

http://www.the12volt.com/ohm/page2.asp

to calculate the wattage from some cells sold on ebay, "108 3x6 .5v3.6a Evergreen solar cells +wires flux sldr ", indicating that each cell provides 1.8W. the math looks like this:

P = E x I

-or-

3.6a * .5v = 1.8W

at 1.8W i would need approximately 225 cells. again, this assumes no loss due to imperfect cells, or installation issues, or wire/heat loss, as well as proper location and orientation of the panels. But at the prices i found on eBay, i would be in spending in the range of $450-500(US) for the cells. my electricity usage was $450 for a peak use winter month last december. considering that i have some electrical and electronic experience, i could do *some* of the simpler wiring, as well as building my own panels. i also have a friend who is a licensed electrician who would be willing to assist where able.


my wife says i've been listening to too much late night radio [coast to coast with george noory], and that i'm crazy to think that i could solar-ize our home for anywhere south of $10,000. i know that i haven't included controllers, batteries etc, but the panels alone. so let me know ... where is my math wrong?

thanx
merlyn2000

[MarineLiner]

(39000Wh / 24h = 1625W) for the sake of simplicity

...
my electric bill states that i used 39kWh average per day, ...
i calculate that we use 1625W average (39000Wh / 24h = 1625W). for the sake of simplicity, ...
$10,000 ...

[indalecio.feng]

First welcome you here.
Second, 39KWh is a lot for the daily electricity.

Can you please list all the equipment you using normally. And the better please tell how long you use the equipments?

[merlyn2000]

as far as the extreme usage, i think its a problem with the HVAC system. i know the house isnt insulated well, but i think there is another problem stemming from the HVAC and i havent figured it out yet. my heating bill for one month last winter was $450.

so, with that in mind, the first thing i plan to do is to insulate better, and then replace the heating with a wood pellet system. once that is done, ill begin to save for the solar system.

BTW, usage for last year was 29kWh/day, this year is 39kWh/day for the same period.

i work this evening, but will begin to start tallying the usage for each item this week.

[Mike90250]

Mike here, Just a quick stab at this.

1) I'll assume you are considering Grid Tie system, unless you want the extra expense of the battery backup. (be aware no batteries = no usable power, even if the sun is out)

2) $5/watt installed is a good start

3: 39KWH / day is a lot, conserve, conserve, conserve. Insulation is a LOT cheaper than PV panels.

4) Solar hot water is the First place to spend some $$, install a couple rooftop water heaters, unless you have really inexpensive heating fuel.

5) Ignoring your location and seasons for now, Typical equinox day has 12/12 house of day/night. Of those 12 hours, the 1st 3 and last 3 (6hours) carry very little energy. That leaves you 6 hours of mostly usable sunlight, for a well oriented southern array. 39KWh / 6 = 6.5 KW needing to be harvested, per hour.
A WILD guess would be a 9 KW array installed will meet most of this demand. Add a pair of 5KW grid tie inverters, and the permit from your city and electric company, and you are good to go. May want to re-shingle your roof unless it is pretty new, PV should be good for 20+ years, hate to have the labor to yank it to replace bad shingles in 10 years.

You will harvest more in summer, and less in winter.

Good luck

[MarineLiner]

Harvest time about 6hours

5) ...., Typical equinox day has 12/12 house of day/night. Of those 12 hours, the 1st 3 and last 3 (6hours) carry very little energy. That leaves you 6 hours of mostly usable sunlight, for a well oriented southern array. 39KWh / 6 = 6.5 KW needing to be harvested, per hour.
A WILD guess would be a 9 KW array installed will meet most of this demand. Add a pair of 5KW grid tie inverters, .....

[iamian]

If you want to.. here are some additional sources for additional calculating data.... just don't get information paralysis. ...

The bellow is a bit simplified ... and assumes DC power transmission ... it will give you a few ideas and methods of additional detail ... and there are yet further levels you can go to ... to continue to get better and better detail ... but each level gets more and more complex.... I find that generally beyond this level to start getting into the skin effects , magnetic fields , etc... doesn't tend to be worth the extra effort.

--------------------

Solar Radiation Data Manual published by the NREL lists more details about the solar energy for 239 locations throughout the U.S. ... just pick the closest location to you... I find it is easier to do the rest of the calculations for the rest of the system if you are in if you convert the given BTU/ft^2 in Kwh/M^2 ... just multiply the BTU/Ft^2 by 0.003152.

Even further accuracy can be had by figuring out how the shade of your location will effect the solar panel instillation location ... this can be done manually ... or it can be don electronically for you with a rather expensive tool.. the SunEye... if you do it yourself don't forget to account for the change in solar angle throughout the year... the SunEye does it for you automatically.

keep in mind your solar panel is not 100% efficient ... some manufactures will give you the efficiency curves for their panels... this would allow you to convert the above available solar energy into PV expected output energy.... or keep in mind that 1 solar hour day = ~1kwh /day... so if your solar panel is rated for a power output under 1 sun ... that is the amount it will give you in 1 solar hour day ... you could convert the kwh/day of available energy into solar hour days ... and then apply that to the specs of the panel you are considering buying.

--------------------

The loss in the wires can be calculated to a reasonable estimate ( without getting overly complicated ) with the help of charts like this one.... the chart shows you the distance point at which a given gauge of wire under a given amps of current will have a 2% voltage drop... this chart starts with 12VDC ... if you run 24V double the distances... if 48V quadruple the distance ...etc... to calculate the estimated power lost in the wire ... first find what the exact voltage drop is ... 2% of 12V = 0.24V ... 2% of 24V = 0.48V ... then applying Ohms law multiply by the current flow in amps by this voltage drop to get the watts used / wasted in the resistance of the wire itself.... example : 10 Amps at 12VDC has a 2% voltage drop at 7 feet in a #12 gauge wire ... 2% of 12VDC = 0.24V * 10 Amps = 2.4Watts of energy loss at 7 feet ... or ~4.8Watts of energy loss at 14 feet.

Higher Voltages allow further runs before having the same loss of watts of power ... because electrical loss is related to current flow ... not voltage ... but watts of power is Volts * Amps ... so you can get less loss by increasing the Voltage and still transmit the same amount of watts of power.... 120VDC at 1AMP transmits the same amount of power as 12V at 10 Amps ... but will generally have only 10% or 1/10 of the power loss over the same distance and wire size.

The Thicker the wire the further you can go before getting the same loss with the same voltage and current ... a #2 Gauge wire could transmit that same 12VDC 10Amp load 72.5 feet before it looses the same 2.4 Watts ( 2% Voltage Drop ) of energy.... instead of the 7 feet before the #12 gauge wire had the same ~2.4 Watt energy loss.... or a 120V 1 Amp load on a #2 gauge wire would go ~724 feet before having the same ~2.4 Watts of wire loss.

-------------

In general $1 of improved energy efficiency with better insulation and appliances ... would take ~$2 worth of solar energy generation to offset the same amount of energy.... so start with the efficiency upgrades first.

-------------

building your own panels is not an easy thing ... and it does consume massive amounts of your time ... but it also has the potential to greatly reduce your costs.

for example : you can buy raw solar cells here for about ~$1.34 / Watt for the raw 0.75Watt cell... this will not be your finished cost as you still have to pay for the other materials needed for the assembly ... and it assumes you pay nothing for the massive amounts of your time that you will invest... also ... if you plan to go this route ... start with building a single panel ... then you will see about how much work is involved in it ... and you can decide from there if you want to build the rest of the other panels yourself as well.

[Photovoltaic Supporter]

Okay first of all Mike90250 is on mark... conserve absolutely, roof condition absolutely now for sizing... A 9KW system would do the trick in almost all situations, that's a BIG system for a home...least I think so...but then again my home isn't so HUGE and most don't put in systems such as this....maybe commercial...small commercial that is. Anyway. I suggest a 4.5KW to start, should get you where you need to be for a lot less and you should do grid tie and maybe a small battery backup system. you’d stay under $ 20K with an average 6-10 year pay off / break even point... and end up after rebates, tax incentives and all...at about $ 13K or so out of pocket.. complete... What ya think Mike...that sound close to you? Anyway, Conserve is key and positioning also key...a tracker if pole mounted is a real good choice...that's the route I'm going with my ad on. I'm doing a 2KW pole mount with tracker...not sure if motorized or oil filled hydraulic system yet.. but surely pole mount for 20-40% bump in efficiency. And no roofing issues. My roof is good but I have sixty trees in my yard, yep 60...so there's a bit to consider for now...It aint rocket science but it feels like it is!

[Mike90250]

My 4.5KW system supports a house without air conditioning. I could easily see needing 2x to run and AC system. Some people use a lot of electricity. Conservation is much cheaper then throwing another 10 PV panels on the roof.

tracker mounts, seem to need maintanince/repairs often, and it's been suggested that
adding more fixed panels is more cost effective.

[EdgarNewt]

So, is the $5/DC Watt meant to be the original price, or actual cash out (after rebates/discounts). I live in Southern California and am being quited approximately $8/Watt before the rebate. After the rebate, it drops to under $5/watt (this is the amount I would be out of pocket, not the $8/watt price). After the tax benefit, the price drops to under $4/watt.

Does this seem like the market rate or should the pre-rebate price already be at approximately $5/watt?

Thanks.

[longtermsolar]

I am in california too, and the $8/watt is pretty reasonable for the pre rebated figure. There are quite a few people I know who have gotten recent quotes for $7-7.50/ watt, so you may still be able to find better on the up front number.

[solar_dave]

I am in california too, and the $8/watt is pretty reasonable for the pre rebated figure. There are quite a few people I know who have gotten recent quotes for $7-7.50/ watt, so you may still be able to find better on the up front number.

$6 a watt installed (or slightly less) is more the norm in AZ with quality componenets.

[Sunking]

$6 a watt installed (or slightly less) is more the norm in AZ with quality componenets.

AZ prices do not apply to CA.

[ionized]

Is solarbuzz.com a good resource for price?

[stephenkain21]

i hate math....

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