In the first two threads we looked at putting together some deep discharge batteries, an inverter, some wires, a sub-panel, and a fused disconnect to yield an energy storage system that can provide a little power to a home. We found that we could probably power some lights (especially fluorescent), a microwave, a furnace, and even a shallow well pump. We learned how to size things so that we bought a large enough inverter to handle our peak loads, and enough batteries to handle the sustained loads.

In this thread well look at keeping those batteries charged.

There are several chargers we can use. The most common are a) a generator, b) solar panels, or c) wind power. And, of course, if utility power comes on for 2 or 3 hours, the batteries get charged during that time. Hydro-turbines are rare and wont be discussed here.

Its important to realize that the same central setup........inverter and battery pack.......is used, regardless of whats selected to recharge the batteries. Once we provide for it, everything else is add-on. (Actually, we can also add-on to the battery/inverter package we have in order to beef it up as well.) The idea is to start with that core, get it in place, and then add the chargers.

A generator is the last thing we should be looking to buy, even though we will probably need one before were finished. Unfortunately, it is often the first thing we buy....we understand them -- put gas in, pull the starter, and plug the extension in. Sort of a second cousin to our lawnmower.

Solar

If you dont have big loads, solar isnt too bad. For example, lets look at using the solar panels just to keep our simple battery pack charged. We calculated that we would need about 4500 W-Hr per day from the batteries. How much solar does this take. Well solar panels range from very small ones suitable for trickle charging the battery in your car up to 120W units. Most common home power units are between 50 and 120 watts each. Lets look at a very common and good unit, the 64 watt module made by UniSolar. These are an excellent unit, and cost roughly $300 each.

First, how many hours per day does the sun shine where you live? For our calculations we are going to assume that we get 4.5 hours of useable sun during the winter months. For your calculations I would recommend looking up National Solar Radiation Data Base User's Manual (1961-1990) and finding how much sun youll get during those short winter days.

Now, for my 4.5 hours, I can get a total of 4.5 x 64 W = 288 W-Hr from each panel. Since I need 4500 W-Hr per day, I need 16 of these solar panels. This is optimistic, since it assumes the panels are facing the sun......which means I have to install a solar tracker or I have to manually shuffle the panels around a bit to get this much power.

During the summer, things are much better. We receive twice as much sunlight, meaning that we need only half as many solar panels to generate the same amount of energy. Anticipate using 8 panels. The 8 panels cost $2400. However, the 8 panels will only supply about 2250 W-Hr. in the winter.

Even 16 panels (at a cost of $4800) will only charge our battery pack on those winter days when the sun actually shines. So, if youre going to go this route, it might be wise to consider adding some wind power.

Wind Generators

See Wind Energy Resource Atlas of the United States

Using data from the Wind Energy Atlas, I find that where I live the wind in the winter is Class 7 -- that is it averages from 15 to 21 miles per hour. This is enough to drive an Air 403 wind generator to put out from 70 to 150 W. The generator will contine to output this much power 24 hours a day, resulting in from 1680 to 3600 W-Hr. So, this single $595 wind generator can provide much of the power needed during the winter. The summer is another matter -- the generator only puts out about 20 W in the lazy, warm, humid air. Thats still enough to provide 480 W-Hr.

Combining Wind and Solar

To generate 4500 W-Hrs in a day requires:

Solar only, winter------------16 panels, or

Solar plus wind, winter-------8 panels plus 1 wind generator

Solar only, summer------------8 panels

Wind only, summer-----------10 wind generators

Solar plus wind, summer----- 8 panels plus 1 wind generator.

The combination of 8 panels and 1 wind generator works well for me. Your mileage may vary, especially since I live in the Apalachian Mountains where it gets windy on the ridge crests. As noted in the Wind Energy Atlas, In contrast to valley and plain locations, the daily maximum wind speed for mountain summits and ridge crests generally occurs at night; this situation occurs because the frictional boundary layer is more shallow as a result of the absence of solar heating and associated vertical mixing. So, even in the summer, the highest wind speeds are encountered at night -- when solar power is unusable.

Wind and solar power go well together (for me).........one works when the other doesnt.

Modern wind generators begin to put out power when the wind is blowing only about 11 mph. In a 30 mph wind theyll put out full power, perhaps 500 W. A popular model the Air 403 will cost about the same as 2 solar panels. Where I live I can expect about 2300 watts per day from one Air 403.

And, a wind generator will put out power at night when solar is dead. It usually blows during a rain storm. On the other hand, on a calm still day the sun is usually shining brightly, so that some power is available.

Either, by itself, is prohibitively expensive if you want to run the system to the max. A combination is less expensive and more reliable (a better chance of providing power). As I said, your mileage may vary, especially if you live where the wind isnt a factor.

Generators

Ive read posts that say that the writer wont use a generator, that theyll simply use solar panels or wind generators. Thats nice if the sun always shines or the wind always blows. Seldom happens, so most people have to resort to a generator sometime. But, if solar and/or wind power are selected, generator use is minimized. That should be our goal since it means fuel use is minimized.

In the Y2K equation, fuel is a big unknown........except that we can predict that fuel costs are going to rise substantially.

This is my own opinion only, but I believe that power costs will go through the ceiling as a result of Y2K. I see two factors. The first is that the power industry has spent billions in remediating the problem. The CEOs have the choice of passing this cost on to the public or the stockholders. Care to guess what their choice will be? The second factor, of course is rising fuel prices. If oil prices go up it cost more to run oil fired generation plants, diesel train costs (for hauling coal) rise, and the industry has a rising energy cost environment in which to raise prices. So, Id look to offset some of these costs with a solar/wind combination. This is in addition to having the source of power when its needed and having the ability to sustain operations for a long period of time if need be.

How big a generator do we need?

How big is the inverter? Select a generator that is large enough to drive the inverter at full capacity and have some left over. You might want the leftovers for such things as washing clothes, pumping water from a well, sewing, additional lights, or for some emergency, such as powering spotlights.

In my own setup, I have a deep well (3/4 HP pump). I can draw water using my batteries and inverter, but I prefer to use those for other things, so I hook the 240 volt pump to the generator and draw water when the generator is running. By the way, you can get a lot done if the generator is run for about 3-4 hours a day, and everyone knows when water will be available. The lines form outside the bathrooms just before power up is scheduled.

Also, the generator must be capable of charging the batteries. We can expect that this can take up to 70 amps peak. Leave 1000 watts for battery charging to be on the safe side. If you have a 2400 watt inverter, add 1000 watts for charging and another 2500 watts for the pump, for a total of 6000 watts. You can probably get away with a smaller generator, say 5000 watts, but dont go for a really small unit.

Utillity Power

The system weve described switches automatically from utility power to inverter power: the inverter does this automatically.

This has the advantage that -- when power is restored briefly-- we have the ability to charge the batteries from the utility input. This is automatic, with no generators. It isnt automatic if we add a generator, because then we have to add a transfer switch to select between AC power from the generator and the utility. Thats not a hard thing to do..........and is definitely a requirement unless you want roast lineman for dinner.

Sources, References

4Winds

WindSun

Jade Mountain

Mr. Solar

Hope these help

-- de (delewis@inetone.net), July 09, 1999

Answers

de, this is a fantastic series that you have put together. Sure enough, in my case I first got a 15KW diesel generator, because I am completely comfortable with the concept. (Also got a pre-packaged battery/inverter system from The Juice Page.)

But I'm on a mountain top, where the wind often roars and the sun usually shines. Thanks for all the info!

-- Jack (jsprat@eld.net), July 09, 1999.