# How many watts for solar



## haypoint (Oct 4, 2006)

I have room on my roof for 60 300 watt solar panels. My electric useage is around 200 Kw a month, about 6 1/2 Kw a day.
I have a lot of clouds in October and April, but lots of clear skies the rest of the time.
What do I really need for solar panels, as far as total wattage capacity?
What I'm looking at is around $1. a watt.


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## Gray Wolf (Jan 25, 2013)

Going to charge a big battery bank and run the house off-grid or be grid- tied to reduce monthly bill? 

If grid-tied, cost per kwh to buy, how much per kwh you will get selling power back, and how much you want to reduce bill will determine how many panels. System payback will be long. Check incentives.

Don't forget snow. Snow will evantually slide off but may also eventually (depending on mounting configuration and roof slope) build up and block the bottom of the panels and kill output. Shade kills!!!

We have a row of wall mounted panels on the house that shed snow with no problem. I also have some ground mounted ones by the house that I held up about 2'-3' from the ground to let the snow build up some before I have to clear it away. Roof ones would never clear on our house as we have snow on roof for a month or more at a stretch.

Lots of Snake Oil salesmen out there. Check and confirm. And then check again.

Welcome to the sunshine!


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## haypoint (Oct 4, 2006)

I had planned a battery array, but would still be able to sell to the utility. Will check to see what they pay. 
Snow would be a problem for me. I do have verticle surfaces, but figured I'd lose too much energy by not being at right angles to the sun. True?

If I need 200 Kwh a month, let's say I'd need to produce a monthly average of 400 Kwh. How much capicity do I need to get an average of 13 Kwh a day?


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## wy_white_wolf (Oct 14, 2004)

I'd need a closer location than N Michigan and check your states net metering law, but you should beable to get full offset with @ 2000w grid tied system.

If you want off grid I'd guess it would have to be closer to 6500 watts for 200KWH hours monthly in the winter.

WWW


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## Gray Wolf (Jan 25, 2013)

For us up north, the winter tilt kind of hugs the walls but you'll still need angle mounts. We have 3' eves so the snow coming off the roof doesn't whack the panels. Also, the placement and summer tilt has them serve as awnings over the windows.


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## SolarGary (Sep 8, 2005)

haypoint said:


> I had planned a battery array, but would still be able to sell to the utility. Will check to see what they pay.
> Snow would be a problem for me. I do have verticle surfaces, but figured I'd lose too much energy by not being at right angles to the sun. True?
> 
> If I need 200 Kwh a month, let's say I'd need to produce a monthly average of 400 Kwh. How much capicity do I need to get an average of 13 Kwh a day?


Hi,

You can check on how much of a hit you will take for vertical panels and also get an idea what your production by month is by running PVWatts Version 1

http://rredc.nrel.gov/solar/calculators/pvwatts/version1/

Just pick your location off the map, change the tilt angle to what you want and fill in how large an array you want to look at.
Do this for a tilt of vertical and for a tilt equal to your latitude.

If you want to be grid connected but have battery backup, then there is no need to have an array that produces more on a yearly basis than you use on a yearly basis. Net metering in most places won't pay for any excess you produce on a yearly basis.
The DSIRE site will probably have your net metering rules: 
http://www.dsireusa.org/index.cfm

200 KWH a month is really good -- you must have worked on consumption to get there.

Gary


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## moeh1 (Jan 6, 2012)

I'm starting to look harder at this too, found this website tracking various panel prices.
http://www.ecobusinesslinks.com/surveys/free-solar-panel-price-survey/


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## ralph perrello (Mar 8, 2013)

I am a full time RV'er looking for a place of my own.
I have 400 watts of solar on the roof and 4 - 12 volt golf cart batteries. I charge my batteries to 14.8 volts @ 80 F deg. I have a temp sensor and that voltage varies with the battery temp. I am single. With a minimum amount of conservation I have power to spare.
I use propane for refrig, hot water and heating when not plugged into the grid. I power my 1000 watt microwave, stereo, vacuum cleaner, drill, blender, mixer etc. Not all at the same time of course.

The first thing you need to do is some serious conservation practises.
The next thing to do is to analyze your power usage so you know exactly how much you use each day. Then size your solar system to these numbers.
When getting power from the sun it is not practical to use any appliance that has a heating element in it. A coffee maker is OK if you shut it off when the beverage is brewed. A/C is not practical either. These all use more than a practical solar system can produce.

A good quality controler is a must and the wiring must be LARGE!
My system was installed by the mfg. as directed by a solar "expert" who did not know his --- from a hole in the ground. I had a junk controler and wiring way too small. I replaced the 30' of #8 wire with #2 wire and installed a top of the line controler. I fused the positive wire into and out of the controler as that was not done either by the "expert". It works like a charm now as long as I do not park in the shade. When I found out why my system did not work as per the books said it should I did my own research and found out why.


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## wy_white_wolf (Oct 14, 2004)

ralph perrello said:


> .... I fused the positive wire into and out of the controler as that was not done either by the "expert". It works like a charm now as long as I do not park in the shade. When I found out why my system did not work as per the books said it should I did my own research and found out why.


If you get a short between the battery and your fuse at the charge controller what protects your wiring?

The fuses should be at the battery and panel end of the wires to do any good. Also if you have 3 or more panels making up the array wired in parallel each panel should be fused individually at the junction box.

WWW


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