Do I have this right?

[49north]

My solar system at our cabin has 3 X 100 watt panels (12 volt), and I was trying to figure out how much value in electricity it produces.  The utility company at my city house charges about 6 cents per kilowatt hour.  If I have the math right, it takes those solar panels about 3.5 hours to produce a kilowatt hour.  So, in a day with 5 hours of useable sun, I'd get roughly 1.5 kilowatt hours, or 9 cents worth of electricity.  Can that be all, or have I got this wrong somewhere? Thanks.

PS.  not that it's ALL about dollars and cents.  The solar system provides power off the grid, and is far better than my gas-hungry generator.  And it's always fun to see a guest's amazement when we run power tools purely off the sun.

[Nando]

You are correct !!!

Nando

[Sue Karber]

Be thankful, we pay $.10 a Kilowatt hour here in Oklahoma with a extra cent for fuel charge.

[SamoaPower]

I envy all of you!

Grid power here in American Samoa is currently 26 cents per kWh and going up every month.

Bah, humbug on fossil fuels!

[oztules]

Ditto 30c per kwh here now (flinders island)

[DanG]

Divide total electric bill by KWH so you've added in taxes, miscelaneous fees, 'delivery' charges THEN add the price of a stamp to return their bill plus the cost of the check you write - even thus the economics pale to the buying power of 99.9% of the population subscribing to grid power...

[commanda]

There are 2 ways of looking at this.

One. It's a good thing they have a 20 year warranty, because it will take that long to recover the purchase price.

Two. What would it cost (in 10's of thousands of dollars) to get the grid connected to your cabin. Now, do you see how much you're saving?

Amanda

[wdyasq]

Your numbers are right but the panels are putting out ~18-20V and your batteries are only 12-14 volts.  This means you waste 30-40% of the "Watts". Knock that figure down to about $.06 a day.

The ways you have to increase output, and the order that makes sense, is to add a tracker.  If you get enough panels, an MPPT will pay for itself.

Ron

[oztules]

Are you sure about this? If I was sold a panel that was 100W at 18v, i would assume that it's current at that voltage must be 100/18=5.5amps (approx). I would further have expected that as I shifted the differential between the panel volts and the batt volts further apart (no regulator for simplicity), a corresponding increase to 100/13.7 = 7 amps or so... still roughly 100watts for the panel.

Now I don't have any panels, so I don't know, but I would want some approximation of this to occur, or else solar cells just became even more expensive per watt to buy....you'd be throwing away near half the panel, and so eficiency for a 12v system would come down to around 3-4%......surely not?

Have I misunderstood your contention?

..........oztules

[commanda]

surely not?

Surely yes. Solar panels are essentially constant-current devices. Google solar mppt for the solution.

Amanda

[oztules]

Well!! I'll be buttered on both sides..... I didn't realise that.....don't think I'll be lining up to buy a heap of those then.

Hornswoggled..........oztules

[wdyasq]

You will if that is the only way you can get reliable electricity.  Few realize just how expensive RE is compared to "Grid-tit".

Ron

[49north]

Yes, that's a much better way of looking at it. In fact, there is sometimes chatter about bringing the grid to our little corner of the island.  It involves stringing cables underwater and across hills, and astronomical amounts of $$.  Makes my solar system look better all the time.  

[geoffd]

We brought a house in the middle of no where.  It had a phone but no electricity.  The local electricity company wanted 60,000+ euro to install the 2km wire to the grid.  Then I would have had to pay them for the KW/HR.

We ended up with a system based on a 24volt 1300AH battery bank with an Outback 3KW (3024) Inverter plus (at the moment) 2 x 160W PV panels, soon to be 4.  The prices of around 8,000 euros and no on going electricity bills was a fair swap!

Cheers

Geoff

[Opera House]

There's more bad news.  That voltage is specified at a rather moderate temperature like 25C. Not likely for anything dark sitting out in the sun.  That doesn't have any effect if you are hooking it on a 12V battery, but it means you never get that power even with a MPPT. They are higher voltage than apparently needed because of temperature, resistance and diode losses. Now sit with a current meter and watch the panel drop to almost nothing on a bright sunny day when a tiny wisp of a cloud passes by!

[Clifford]

Ummm..

Try a controller with a digital readout, and check your Amps and watts that the panels are actually producing.  Or, at least test the power that is actually being fed into the batteries with a basic ammeter.

A panel rated at 100W...  may not produce 100 watts / hr for 5 hrs / day....  And, you may need to adjust angles and have a solar tracker if you want to maximize power production.

On the bright side...

Add your basic Power Feed Connect Fee to the "basic equation" ($10 to $20 / month even if you don't buy any power) so your savings are a bit higher.

Also, some utilities charge if you have to extend their wires.  I.E.  Not paying the fees to extend power to your house might pay a portion of the cost of the system.

An additional consideration....

You may be "leaner, meaner, and more efficient" when running by solar...  Say, you cut your power consumption by a factor of 10...   So, if you generate 6 cents of solar power, if you had a "power feed", you would likely have bought 60 cents of power.

Of course, if you generate 1500 watts, and you only use 500 watts, then you really can't say that you generated 9 cents worth of power when you "wasted" 2/3 of it.