Links To Sections Of This Page:
My First System
Energy Produced By The First Small System
Enhancing The Current System
I bought a small system in mid-July 2000 to learn more about making and using measurable amounts of solar power. I have succeeded in producing enough power that I am definitely going to build a larger system.
I have a single solar panel producing electricity, which is stored in a portable 12 volt battery unit (which contains a sealed battery and a charger). A inverter is connected to the battery unit to convert the 12 volt DC power to 110 volt AC power, which is what normal household appliances use. A power strip is plugged into the inverter, and I plug things into the power strip when I want to run them via solar-generated power.
If you are thinking about building your own system, keep in mind that the battery I selected was too small to store as much energy as my solar panel could produce (in the summer months at least). I knew that I would expand my system in the future, so I intentionally over-sized the solar panel and inverter so I could reuse those components.
You could choose to either buy a larger battery and charge controller (for about the same money as my portable battery) or save money by buying a smaller solar panel (32 watts, maybe even 21) and inverter (140 watts, maybe even 75) as I did. I decided that I wanted to skip straight to a utility intertie system like I did in my Phase II.
Equipment List
I use the laptop every day and the lawn mower once every week and a half during high-growth periods and once every two or three weeks in the middle of the summer. I have had enough extra energy in August to run one or two table lamps (compact fluorescent bulb, 15 watts) in the family room for a few hours on many nights. In December, the smaller amount of energy is better suited to just running lighting and recharging batteries (i.e. not the laptop).
This way of generating power is not going to save you any money over buying electricity from Baltimore Gas & Electric. We pay about 8 cents per kilowatt-hour in Maryland. So the power produced by this solar panel system each week would cost only about 4 cents! Prices for solar panels have fallen a lot since the 1970's, but not enough to compete with coal-fired power plants. However, I expect power prices to rise in the next ten years. And I know that large utilities will continue to generate almost all of their electricity with fossil fuels which pollute.
The rate at which you drain power from a battery has a big impact on the total power you can produce. The Multi-Power Source unit can last a long time (eight or more hours) if you draw very little power from it (10 watts). Push your consumption up to 60 watts, and it will not last more than two hours.
This makes sense when you think about it -- the MPS energy capacity is rated at 17 amp-hours total. That's measured when the energy is removed at a constant rate over a twenty hour test period -- less than one amp at a time over all of those hours. That's a very light load. Watts is a unit of power defined to be the product of amps and volts. So 17 amp-hours times 12 volts equals 204 watt-hours. For long life, batteries should not be discharged too deeply (i.e. more than some proportion of their total capacity). You should leave 20 to 50% of the capacity unused before recharging. So one shouldn't expect to get more than 100 to 150 watt-hours out of the MPS at a time.
And of course the inverter is not 100% efficient. My particular model is rated at 90% efficiency. So you use about 10% of the battery power just to drive the conversion process.
In practice, 65 to 75 watt-hours a day is a common "yield". On sunny days when I can connect and disconnect loads to the MPS during the day, I have been able to get more total power through the system. (I'm discharging and recharging the battery multiple times during the day.) The one day record is 142 watt-hours. That kind of hand-holding takes more time than anyone will want to put in as a regular matter.
I wrote the following section before deciding to buy a utility intertie system. See this page for what I actually implemented. But I have left this section in place so you can follow the thought process about an alternative strategy.
I decided that the battery capacity of the MPS is just too small. I think that the 64 watt solar panel can produce more energy per day than can be stored in the battery. Of course this may not be true in the winter months! But I figured the next step to take was to buy larger batteries. Since winter is not yet upon us, I will be able to see how well I can maintain the charge with a moderate amount of sunshine. The fact that Real Goods advertises a 10 watt solar panel to use with the MPS supports my conclusion.
The MPS is a handy little portable power source. It has a built-in battery charger, built-in flashlight, safety fuse, molded carrying handle. It comes with two (short) jumper cables to use in jump-starting a car. And two charging cables: one for a car cigarette lighter and one for a home power outlet. One of its charging input jacks has a voltage stabilizing circuit. This is what makes it possible to connect a solar panel to it safely. The voltage produced by the panel varies all over the place as lighting conditions change. But batteries need to be charged at steady, regulated voltages. That voltage stabilizing feature does the trick. The two batteries within the MPS are sealed, low-maintenance batteries. One cannot add more to the unit (it has a molded plastic case).
For the same amount of money ($150) as the MPS, one can buy two six-volt "deep-cycle golf-cart" batteries with a much larger capacity -- 220 amp hours EACH. I could have also bought a sealed battery with "gel" electrolyte, but I gathered that they did not last as long as they should when charged by solar power (too much current too fast). And they are more expensive than golf-cart batteries, so for $150 I would have been able to get an 86 amp-hour gel battery. Remember, you should only assume that you will be able to use half of the battery capacity, so that gel battery would only yield 43 amp-hours. I decided that if my new golf-cart batteries do not get enough power input from my one panel, then I would rather buy a second solar panel and have a lot of capacity (220 usable amp-hours versus 43). BUT, it could have gone either way -- the lack of maintenance on the gel battery is very attractive.
Since the MPS has a built-in charger, I didn't have to worry about that component in my current system. So I have ordered a new charger to use with the two golf cart batteries. (September 7 2000) Again, I bought something with much more capacity than I strictly needed. I bought a Morningstar ProStar 20M for $160. I could have used the $30 SunGuard-4 (also from MorningStar). But the ProStar is big enough that I could expand the number of solar panels and batteries without having to buy a new controller. It also has a LED panel which displays the amount of current produced by the solar array, the battery voltage, and the amount of current being taken out of the battery. There are a bunch of other features of interest to me (this link is to the manual in PDF format).