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Old 07-23-2009, 11:14 AM   #1
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


Considering the fact that I live in a desert with ample amounts of sunlight, I've been thinking about building my own solar array for my property. The thing is, I'd like to know just how powerful of an array I'd need to either meet or exceed my current power consumption. I'm not really sure if I can gauge this from my electric bill as I think they only do occasional readings of the meter. What do you guys suggest? I'd imagine that a simple ballpark figure should be fine...

I currently have a 1600 sq.ft. home with a rather modern central air system. I run about 4 computers 24-7 and don't expect to get a large flat screen for a few more years. I have no major electric appliances beyond that -no electric water heater, stove, or dryer...

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Old 07-23-2009, 12:09 PM   #2
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


Quote:
Originally Posted by perpetualjon View Post
Considering the fact that I live in a desert with ample amounts of sunlight, I've been thinking about building my own solar array for my property. The thing is, I'd like to know just how powerful of an array I'd need to either meet or exceed my current power consumption. I'm not really sure if I can gauge this from my electric bill as I think they only do occasional readings of the meter.
Your electric meter is just like the odometer on a car, it records continuously. The utility only reads the meter occasionally, but you can read it any time. If you want to develop an accurate picture of how much power you use and when, just go out and read your meter in the morning and evening and keep a log of it. You'll be able to see how much power you use during the day and overnight, and how it varies through the week and as the weather changes.

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What do you guys suggest? I'd imagine that a simple ballpark figure should be fine...

I currently have a 1600 sq.ft. home with a rather modern central air system. I run about 4 computers 24-7 and don't expect to get a large flat screen for a few more years. I have no major electric appliances beyond that -no electric water heater, stove, or dryer...
If you are planning on going off-grid completely, you would need to do a very thorough analysis of your peak and average power requirements. If you are planning on staying connected to the grid but trying to offset your usage, then only your long-term power usage matters - the grid will make up the difference when the air conditioner is on, for example. All you need to know is how many kilowatt-hours you use per month, and size your solar system to match.

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Old 07-23-2009, 12:28 PM   #3
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


So, I'm seeing solar panels that give me a voltage and amperage rating while some will actually go ahead and just list the wattage output (which I can always calculate just with the voltage and amperage values anyways). I'm assuming that my "watts per hour" rating would be based on how many hours if good sunlight exposure I'll get within a given day. For instance, if I build a solar array that is rated at 500 watts of output, and 10 hours of good sunlight in a day, that means that I'm theoretically producing 5KW-hrs?

In my case, I was planning on staying on the grid so I didn't need to build a massive battery setup in my garage for the evening times (although I'm still thinking about it)...
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Old 07-23-2009, 12:47 PM   #4
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


Quote:
Originally Posted by perpetualjon View Post
So, I'm seeing solar panels that give me a voltage and amperage rating while some will actually go ahead and just list the wattage output (which I can always calculate just with the voltage and amperage values anyways). I'm assuming that my "watts per hour" rating would be based on how many hours if good sunlight exposure I'll get within a given day. For instance, if I build a solar array that is rated at 500 watts of output, and 10 hours of good sunlight in a day, that means that I'm theoretically producing 5KW-hrs?
That's roughly correct, but it's much more complicated than that. The angle of sunlight on the panels greatly affects the power output. Temperature also does, and the actual intensity of sunlight (solar flux) may be less or more in your location than the test conditions used to determine the panel rating. All solar panels have multiple rating values that are important when designing a system. The panel specifications include wattage (usually measured at 1kW/m^2 irradiance), voltage and current at peak efficiency, open circuit voltage, and short circuit current. All of those parameters are required when designing a system. You cannot really calculate panel wattage with just voltage and current ratings (unless the voltage and current are specified as being at the maximum power point) because PV devices are nonlinear impedance sources with some strange characteristics. The voltage and current values you see may not be achievable simultaneously.

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In my case, I was planning on staying on the grid so I didn't need to build a massive battery setup in my garage for the evening times (although I'm still thinking about it)...
That's the easiest way. You will need to use grid-tie inverters with maximum power point tracking (MPPT), and there are some special rules about disconnects for grid-tied PV systems. Also need to get the power company on-board to set up net metering.
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Old 07-23-2009, 12:49 PM   #5
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


http://www.gaisma.com/en/location/de...alifornia.html
At an insolation of 8 kwh per sq. meter per day at 10% efficiency and an average usage of 4A@240v [23 kwh per 24 hr day, but peak needs are much higher] you'd need 23/0.8 = ~30 sq. meters plus a means of energy storage to get you through peak demand periods and periods with no sun.
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Old 07-23-2009, 01:09 PM   #6
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


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http://www.gaisma.com/en/location/de...alifornia.html
At an insolation of 8 kwh per sq. meter per day at 10% efficiency and an average usage of 4A@240v [23 kwh per 24 hr day, but peak needs are much higher] you'd need 23/0.8 = ~30 sq. meters plus a means of energy storage to get you through peak demand periods and periods with no sun.
Wow! What an awesome site!! Here's my actual link for the town I'm right next to...
http://www.gaisma.com/en/location/la...alifornia.html

So that amounts to about 322 sq ft of solar panel space I need. I think I can go WELL over that amount... I've got plenty of roof space on my 25' x 25' garage as well as the main house... Thanks for the help!!
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Old 07-23-2009, 01:15 PM   #7
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


http://www.eia.doe.gov/emeu/consumpt...on_tables.html

You get about 5/8th of the energy out of batteries that you put in so I'd look to other means of storage.
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Old 07-26-2009, 08:08 AM   #8
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


A couple of years ago, I had a guy quote me a ballpark figure of about $7 per watt (watt...not kilowatt) for a solar installation. Since this calculates out to over $30,000 just to run a standard water heater (batteries not included), I never got too excited about it.

Have you had any estimates on what your project is likely to cost?
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Old 07-26-2009, 09:52 AM   #9
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I'm thinking of building a solar array. What's a good way to measure my pwr needs?


To pay back 30 kilobucks in 10 yrs @ 8000kwh/yr you'd have to be paying ~38 cents/kwh.

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