Help for newbe?

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Posted by Bob Coco on August 18, 2009, 01:47:45 PM

Hi Folks, I just discovered AltE and what a great company and this board is excellent info. I'm planning a small off-grid system and need some help. Basically want to use the system as back-up power and to reduce some power costs as well. I want to start small and have room to expand. This is what I have in mind. I already have a 2000w inverter and two large batteries in my motorhome which sits unused for most of the year, so I want to take advantage of them for now. What I want to do is put 4 200w panels on the garage roof and use them to charge the batteries in the motorhome. I plan on adding 2 more batteries in the bank later. The distance from the roof to the motorhome is about 25'. Should I put the MPPT controller/combiner in the garage and just run wire to the batteries, or should I put the controller in the motorhome and run wire to that from the combiner located in the garage? Does it make a difference? Also what amp controller would be minimum for 1000-1400w of panels? Eventually I want to expand to this capacity but not want to have to purchase another controller when I do. Thanks.

Posted by Ken Hall on August 19, 2009, 02:37:28 PM

Re: Help for newbe? (Reply #1)

I think if you supplied more info, it would be easier to respond to your questions.
How many watt-hours do you propose taking out of the batteries “to reduce some power costs? On a foggy day with less solar, will you reduce that figure, or do you need a reserve ? What Is the figure (watt-hours) for backup power on an occasional basis ?

What make and model is your inverter and how are you going to supply the power to the house ? Are you attempting to connect it into the house wiring or just running extension cords ?

Just how big are your two large batteries and are they true deep cycle batteries, or are they “RV deep cycle” ? The make and model of them will give us the answer if you don’t know.

What is the general location of this system ? We need to know the hours of full sun.

With answers, I think we can give you better recommendations. Right now there is enough gray area that it is hard to say anything. The batteries in the RV indicate that you are probably thinking 12Vdc, but you might be better off with 24Vdc.

Posted by Bob Coco on August 20, 2009, 06:18:34 PM

Re: Help for newbe? (Reply #2)

I don't know what the exact watt-hrs will be as that is hard to predict. If I lose power in the summer, I just need to run some lights and misc small items. If winter, perhaps some more. It also depends how long power is out and time of day. When I use the inverter in the RV with existing batteries, I can get 4 or more hrs of running TV, lights, water pump and even the heater blower thru the night. Something similar for the home would be fine. I have a generator and can use that to charge the batteries if/when needed. I lost power for 7 days last winter and really didn't like filling the fuel every 2 hrs or so, especially during the night and at $4 a gallon. Not to mention that when I finally got to shut the gen off after running 7 days, it seized up, prob from being overworked.

The inverter is a TRACE 2000w, I'm not sure what the batteries are, they are deep cycles I got at Sears. I run them and inverter at 12v in the motorhome. The motorhome is parked about 20' from the house, I am going to run conduit underground from house to RV. If power is out in summer, we'll just live in RV till power comes back on and only run a small circulation water pump (28w) on our aquarium in the house. So, not much load with that and a few lights.

I have a south facing roof and should get sun from approx 10:30 am to 3mp or later. I'm having 3 trees cut down in the spring and that should give them more exposure in the AM.

Greg at altE has helped with planning and I've got 4 Evergreen 205w panels coming, a 80W MTTP controller, and varius other things to get me started. In the future I'll be adding 2 more 205w panels and at that time a separate battery bank and invertor for the home.

Thanks,Bob

Posted by Ken Hall on August 23, 2009, 12:13:16 PM

Re: Help for newbe? (Reply #3)

You don’t know how many watt hours you intend to use and you do not know what size batteries you have. You feel you are OK because the batteries have served you well in the past.

Step one in the proper design of a solar system is determining how many watt hours you intend to draw from it on a daily basis. (The AltEe university has a calculator that can help)
Step two is sizing the battery bank to support that load, including any reserve that may be factored in.
Step Three is sizing the panel array.
You are apparently doing it by “got my batteries, ordered my panels, will worry about watthours later”. (A lot have people have done that, and end up paying for it later. They “fry” or “smoke” the batteries.)

Batteries will accept certain loads under occasional use (such as RVing) that are abusive when done 7 days a week, 365 days a year. RV/Marine batteries are not true deep cycle batteries, and do not stand up well in 7/365 service.

Unless Sears has some battery I am not aware of, your 2 battery bank is about 200 amp hours (plus or minus about 10%). That is a terrible mismatch with your 800 watts of Panels.

While I wish you the best of luck with your new system, I fear that you will be buying batteries much sooner than you expect to.

Ken

Posted by Thomas Allen Schmidt on August 23, 2009, 08:22:29 PM

Re: Help for newbe? (Reply #4)

I would tell you to forget about the RV.
Are you planning to turn your existing on grid home to off grid? If so, I say more power to you!. I think its a great idea. Its not going to be easy and its not going to be cheap. Look at your light bill. The national average for grid electricity is about $0.12 per kWh or kilowatthour so if your electric bill is $60.00 a month then your using about 500 kWh's per month or 16.6 kWh's per 24 hours. That 800 watt PV array you want might offset about 3.2 kWh's of that 16.6 it all depends on a lot factors. Namely, the number of hours of equivalent full rated charge you can expect from a PV module. Go here http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/ and enter; Average, Annual, Flat Plate Tilted South at Latitude then find your place on the map. If where you are shows a number 4 then that 800 watt PV array you want, might make 3.2 kWh's.
If you are converting your on grid home to off grid, the smart money is on reducing your dependence on electricity first. To give you an idea, I've lived of grid for about 30 years now. Only within the past 15 have I had the luxury of PV. We use about 72 kWh's per month which if I had to pay $0.12 for it would be about 8 dollars and some change. But I do have an LP bill that averages out to about $25 a month. There is four of us in the household. Sometimes five.

Is it your plan to turn your existing on grid home to off grid? Are you willing to give up air conditioning and outdoor lights that burn all night long? How will you refrigerate and pump water? These are all concerns when making a move to off grid living. Just how much electricity can you live without? Think about it. Oh! Get yourself a subscription to home power magazine. Learn about electricity in particular Ohm's Law and the National Electrical Code. Most importantly, scower the web looking for the best prices. Just when you think you have found a good price, lok again.

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