On Being Confused Therese E. Peffer Do you get lost in the discussion of watts, Amp-hours, inverters, controllers, PV, UL, vac, and VDC? How does someone new to renewable energy put it all together? It helps to think small and simple. To get myself started (and explain to my folks what the heck this solar stuff is all about), I decided to take the first step at designing my own teensy system. Defining energy wants and needs The first step is finding how much energy is needed. Both electrical and thermal energy should be considered. Some appliances, such as refrigerators, run on either propane or electricity. Energy needs will make a long list for a house, and a smaller list for recreational vehicles. My space is pretty simple: a place where I will hang out and sleep. I will limit my discussion to electrical needs, and not worry about cooking, space heat, and hot water. But I'll think about it Ð let's see, solar thermal, propane, wood heat, hydrogenÉ Figuring out the total electrical load requires making a list of appliances Ð anyone can do this. I will need a few lights, and I'd like to play my cassette player/AM/FM radio. I have a clock radio, but could use my wind-up (mechanical) clock. If this were a house or a recreational vehicle, the needs and wants would be different. For a house, one could add more lights, refrigerator/freezer, radio/stereo, computer, microwave, television, blender, toaster oven, etc. An RV might have a few of these items, too. To invert or not invert? For appliances that use 110 or 120 volts ac (alternating current), I will have to use an inverter, a device that converts the electricity from 12 Volts DC (direct current) from the batteries or the source to 120 vac. Here's another decision to make Ð I could use DC lights. My cassette player / radio will run on either, and I don't need my clock radio. I must compare the cost of an inverter with the expense, availability, and quality of DC lights. I need to consider other DC versus ac appliances in case I expand my system in the future. Choosing an inverter depends on how much energy is used, and the sensitivity of appliances. Some appliances (radio, computer, television, lights) will buzz or hum on certain inverters, or not work properly at all. How much? How much energy will I consume? How much storage will I need? It depends upon my use. Now I get to check the back and undersides of my favorite appliances to read how many watts of electricity they draw. Anyone can do this Ð this is a good exercise for those who want to conserve as well as those planning a system. I have a Panasonic compact fluorescent light; the box says it uses 18 Watts. Since Watts = Voltage x Current, I can calculate how much current the light is drawing. 120 volts x current (in amps) = 18 w Current (in amps) = 18 w / 120 v Current = 0.15 amps. I'll use natural light as much as I can, but I will need light at night Ð 2 to 6 hours per day (consider long winter nights, and dark stormy days). To figure amp-hours of usage, multiply 0.15 amps x 6 hours = 0.90 amp-hours per day. It turns out this figure isn't exactly right; I learned a few lessons here. The box lists the current drawn at 0.26 amps. Why the discrepancy? I wondered, too. Richard said the ballast on the light uses some electricity; the wattage rating is for the light bulb. Another mistake is that the inverter as well as the utility (in the U.S.) delivers electricity at 117 volts on the average. This changes my figures to 0.26 amps x 117 volts = 30.4 watts Ð a far cry from the 18 watts listed! After a consultation with the Wiz, I realized that my calculations reflected the current drawn from the utility or inverter, and not from the battery to the inverter. If the light or boom box was plugged into the utility grid, I could measure ac amperes and volts to find wattage. In a stand-alone system, you are your own utility. I must figure the Amps and Watts drawn from the battery. For those on the grid who want to calculate the amount of energy used, use the wattage listed on the appliance. Since I'm figuring for my off-grid system, I need to add about 10% to compensate for the amount the inverter consumes. It would be more accurate to measure the volts and amps directly. If I have my light on for 6 hours a day, using power from the utility, I will use 0.26 amps x 6 hours = 1.56 amp-hours. Figuring in watts would be: 30.4 watts x 6 hours = 182.4 watt-hours a day. For my stand-alone system, I add 10% of 30.4 to 30.4 watts to get 33.4 Watts DC from the battery. Multiply by 6 hours to get 200 Watt-hours per day. This exercise piqued my curiosity. When I lived in Berkeley, (and paid $0.11 per kilowatt-hour), I used mostly incandescent lights. I took a look at Home Power issue #20 Ð the article on efficient lighting Ð and decided to run my own test and actually measure amperes of current consumed. Would it pay to purchase a more efficient light? For those wanting to conserve: compare regular incandescent lighting with my compact fluorescent and another compact fluorescent brand. INSERT TABLE The lights were run on an Exeltech 250 watt sine wave inverter. I assumed a use of 5 hours a day, 365 days a year. Cost was based on $0.11 per kilowatt hour from the utility grid near the San Francisco area. (The cost for those off the grid would be much higher). After 10 months, my "18W" light paid for itself in savings (figuring about $0.50 for an incandescent that lasts 6.5 months versus my light at $8.00 that will last 4.9 years!) You could read by the light of one Osram for 5.5 years ($21.50 each, plus lifetime cost of $29.35), or use 10 incandescent light bulbs ($5.00 total and $83.49 for the lifetime cost)! The savings ($88.49 - $50.85 = 37.64) is almost enough to purchase two more Osrams. Next appliance Ð the Boom Box Next, I checked my tape player / radio. I have to admit, I had fun doing this. I don't remember ever reading the back of my boom box so carefully. In the city, hooked up to the utility grid, I didn't care how many watts or amps it drew. The back of the box says "ac 120v 22w, DC 12V, 8 D cells, UL listed." I will run the box on DC, not through the inverter. Calculating with these figures, I get: DC, 22 W / 12 V = 1.8 Amps. If I were on the grid, I would calculate: 22 w / 117 v = 0.19 amps. (On the inverter: assuming that the inverter is about 90% efficient (it consumes 10% of the DC input Ð only 90% of the electricity feeds appliances), 10%(1.8 amps) + 1.8 amps = 2.0 amps). If I have it on for 6 hours, I will use 11 Amp-hours (on the inverter, 12 amp-hours). I was able to run some actual measurements. I don't have a plug for DC power yet, so the measurements were on inverter power. On the Heart inverter, there was a distinct hum. Some results were fairly obvious: the tape player uses more power than the radio, and high volume takes more juice than low volume. I also took readings with the boom box plugged into the Exeltech inverter. No hum! Another discovery was that my boom box is a phantom load! That is, when the power is turned off, but it is still plugged in, my box is still drawing power! I recorded 11.5 watts drawn from the battery, and 2.6 watts from the inverter. What does this mean? If I left my boom box plugged in but turned off back in Berkeley, then I would pay about $0.21 a month. Not substantial until you think of all the other phantom loads (television, microwave with running clocks, stereos etc) and add them all up Ð quite a price to pay for appliances waiting to be used! For those living off the grid, phantom loads can drain your battery if you're not careful! In order to save 276 watt-hours a day (11.5 watts x 24 hours), I will unplug my boom box when not using it, or put it on a plug strip that I can turn off when I'm done listening. My measurements were 13.8 Watts (15.15 Volts x 0.91 Amps) with the radio playing at a reasonable level, and 16 Watts (15.24 Volts x 1.05 Amps) to play a tape. The "22 watts" printed on the boom box assumes playing a tape at a screaming high volume, but is a fair figure to use if you don't have a volt or amp meter. To figure amp- hours, I'll use 1.05 Amps x 6 hours = 6.3 Amp-hours per day. All Together Now! For 1 light and 1 boom box, I will need about 1.8 Amps for the light and 1 Amp for the music. Multiply by 6 hours gives 16.8 Amp-hours at 15 Volts. This is equivalent to (27 W + 16 W) x 6 hours = 43 W x 6 hours = 258 Watt-hours per day from the battery, using my measurements. (If I hadn't had a meter and just used calculations: 30.4 w for the light and 22 w for the music = 52.4 watts. Multiply by 6 to get 314 watt-hours from the grid. Add 10% to get 57.6 Watts times 6 hours = 345.8 Watt-hours from the battery.) Making calculations gives an estimate. The next step Now I have an idea of energy use for one day. For those who want to conserve, figuring energy loads shows where your energy bill goes. For those planning a renewable energy system, it is only the first step. The next step is to plan the source of energy. I'll probably use electricity from the sun. One photovoltaic module will give me about 45 - 50 watts per hour of full sun, stored in a battery. I'll need 6 hours of sun and one panel for 6 hours of light and music a day! I haven't discussed priorities Ð what if I want to use my 1500 W toaster oven (for three minutes, about 75 watt-hours). Do I want to expand my system? I haven't talked about batteries. Battery Basics on p. 30 explains about amp-hour capacity, but how many rainy days can I last without sunshine? Nor have I discussed controls, regulation, instruments to measure, wiring, location of batteries, or what happens when I leave for a week or two. I haven't priced anything. What I have just done was a small step. Richard has a computer program to list, prioritize, and calculate the energy load, and compare the energy source and storage! However, it helps to list and calculate just a few appliances if only to be able to ask the right questions. To think a little more clearly. And to appreciate all the nuances of setting up a system. Conclusion First, my apologies to installers for trivializing this procedure. I have taken a very minor step. It was a good exercise. Sometimes in reading this stuff, I get lost in the amps, watts, etc. It helped to realize just what a single light bulb uses. For those who are interested in living on renewable energy, this exercise is necessary to plan and execute your system. For those who wish to conserve, it helps to spell out the lights, stereo, and other appliances in terms of energy usage. You can put a price tag on it by calling your utility or looking at your bill to see how much you are paying per watt. See how much your television draws when you are not using it, and see how much you'd save every month by putting it on a plugstrip. Replace an incandescent with a compact fluorescent light Ð when does it pay off? Access Author :Therese E. Peffer, c/o Home Power, POB 130, Hornbrook, CA 96044 ¥ 916-475-3179