If Ed Begley is around maybe he can help also! 
My home is directly southwest facing and needs a completely new roof (composite shingles)
I would like to place solar panels across the entire front of the roof. approximately 900 square feet in the front of the house.
1. Do I reroof underneath the panels first or can I leave the old shingles on underneat.
2. Will this be adequate to take me off the power grid in my 1800 square foot home and outside lighting in garden aea?
I have a washer, dryer, fridge, dishwasher and electric stove, and air conditioner to run.
thank you for all your help. I need to make this decision before having the new roof put on.
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{ 5 comments… read them below or add one }
First, I am not sure you would be better off being off the grid. If your state has net metering, you can “bank” power when making more than you are using. You can get it back when you use more than you are making. Batteries are an unnecessary, costly accessory. Unless you have critical need for power when the utility has an outage, you can do without them. In any event, buying and maintaining a small generator would be less costly.
Using one manufacturer’s panels (they call them modules), you could get a 7.5 Kw system on your 900 sq ft. This means something around 5 kW effective.
Look up INSOLATION TABLESon the Internet to find the average annual sun energy where you live.In my area it is 5 hours. 5 x 5 is 25, so you would get an average of 25 killowatt hours per day. Look at your utility bill, and see how much you now use. This will put you in the ballpark. It is likely, though, you won’t get a full 900 sq ft of modules. And, the angle from north-south your latitude and the pitch of the roof will all contribute to the efficiency of the system.
My 6 kW system gives me 20kWh per day. Living alone, this is much more than I use for the items you mentioned. I keep track of daily electric meter readings. From a four-year record, I know roughly how much surplus I would have, and use that to augment heat from the gas furnace. Actually I used no gas last winter, except for heating water.
Read all you can on the Internet, and in the green search box at the top of the page.
Certainly you want to reroof prior to mounting the modules. Solar contractors like to say the modules protect the roof, but really it is only partial protection.
Just rough guessing here. a 115 watt solar panel I have on my home is right at 3 foot wide and 5 foot long. That is 15 sq. feet. So you will get about 7.6 watts per sq foot. Times 900 = about 6,900 watts in solar power on your roof. If you get 5 hours of peak sun a day you will produce about 41.4 kWh a day.
That is just over twice what I use in an 1700 sq foot home.
Yes you will need to replace your roof because it could leak and you would have to remove all the solar from your roof to fix it in a few years.
So being you are replaceing your roof why not save a little money and install Solar roofing shingles. Should be about the same or less being you are going to reroof and add solar. And no one would know you had solar unless you told them.
http://store.oynot.com/sh17warosh.html
Certainly pv electric is way cool albeit very expensive, your roof will need to be replaced , you can get pv shingles If you can get grid connected and sell the (Juice ) to the elect co that’s cool .Watching your elect meter go backwards really warms my heart. Completely off grid is out there, but to thumb your nose at the utilities has its own special rewards.Also not being fuellish is good for the head(let em eat their oil)I don’t believe 7-8kw is enough juice however, a great site for info is Bp, they have calculators costs etc . Also energy star.gov can help. South face is great for Pv.Bp is British petroleum
Replace roof with enameled steel roofing first. The electrical out put of the panels has to match the max load of your house use. Along with the right size of battery storage for night time use . and have the correct electrical controllers .
Buy some books on solar design.
I expect you will need to remain hooked to the grid if you have an air conditioner. While you may be able to produce the amount of you consume each day you run the risk of having a spike when your AC turns on that the panels can’t supply.
You need to consult a professional installer of solar systems before you spend any money.
Most people can reap large benefits from just spending a couple of grand on a very good sealing/insulation job on their home.