Make a complete list of all of the appliances (everything that requires electric power) you really need. Go through room by room and think of everything you have plugged into the wall, and if it is a power hog, ask yourself if you can do without it or replace it with something more efficient. If you plan to use natural gas or propane, your job will be a lot easier, because things like a stove, a clothes dryer, and furnace, which take an enormous amount of power, can run on gas. In our case, we decided to try to do without all fossil fuels, so we had a harder time. Our stove became a wood cookstove and a solar oven, our heating system is also based on an efficient woodstove, and our clothes dryer became a clothesline. We live in a wooded area, and have an ample and renewable supply of oak firewood for our woodstoves.
For each of the electric appliances on your list, write down the rated wattage (this can usually be found on the appliance somewhere), and write down the estimated number of hours per day you usually use the appliance. Multiply these last two together to get the W-Hrs/day for that appliance. Large items that contribute 500 W-Hrs/day or more need to be considered very carefully, and probably new, efficient models need to be considered. Our list came to about 4 kW-Hrs/day for the winter and 4.5 kW-Hrs/day for the summer (see Table below), not including air conditioning. This is more than a factor of ten less than we currently use in our conventional, all electric home. As you will see in the next section, by sizing our PV system for the winter, we will have enough excess power in the summer (because of the additional solar radiation) to power our small air conditioner (up to 3 kW-Hrs/day).
|Appliance||Rated Watts||Hrs/Day (S)||W-Hrs/Day (S)||Hrs/Day (W)||W-Hrs/Day (W)|
|Refrig./Freezer, Sun Frost RF-19, 24VDC||-||-||984||-||744|
|Bread Machine||500 (mixing)||0.30||150||0.30||150|
|Washing Machine, Staber Industries||-||1/2 load||83||1/2 load||83|
|Water Pump, Shurflo 9300||100||1||100||1||100|
|Kitchen/Bath Lights (11 AC)||18||6||108||16||288|
|Toilet (DC fan)||10||24||240||24||240|
|Porch and Breezeway Fans (3)||40||10||400||0||0|
|Livingroom/Bedroom Lights (11 AC)||18||6||108||16||288|
In the table above, (S) stands for summer and (W) stands for winter. The Sun Frost Refrigerator/Freezer has a 19 Cu ft capacity, and is one of the most efficient refrigerators on the market (Sun Frost, Box 1101, Arcata, CA 95521 (707) 822-9095). Also, the Staber Industries, Inc. clothes washer is a very efficient (165 W-Hrs/load) tumbler, rather than agitator, and saves water and detergent as well. (Staber Industries. Inc. 4411 Marketing place, Groveport, Ohio 43125, (800) 848-6200). All of our household water will be collected from the roof into a 3000 gallon cistern, then pumped into a pressure tank for use in the house. The pump will be a Shurflo DC submersible, 9300 series, diaphragm pump. (Shurflo, 12650 Westminster Ave. Santa Ana, CA 92706-2100 (800) 854-3218). The lights are all compact fluorescent and the fans will probably be 24 VDC paddle fans.
You can easily move the thing around to find the optimum spot, because sometimes it isn't obvious from just looking. We used it not only to analyze the site for our solar arrays, but also to find the best spot for our garden! It can also tell you whether an array mount that tracks the sun is worth your money. This next picture is a closeup view of the pathfinder.
The Solar Pathfinder comes with all of the information you need to measure and calculate how much power you can generate at your site, and figure out how many solar modules will be required. The easiest way to show how it's done is to go through our calculation step by step.
|Month||(A) Percent of possible solar radiation available at site||(B) array tilt angle||(C) Gainesville, Florida BTU's/sq.ft. per day||(D) Tracking factor||(E) Thermal derating factor||(F) Equivalent hours at 1 kW/sq.met.||(G) W-hrs per day output per 53 Watt module||Excess W-hrs per day for 24 modules|
|Jan.||0.88||50 deg.||1530||1.15||1||4.88 hrs.||259||936|
In the table above, column A is the data obtained from the Solar Pathfinder. We found a site that was fairly clear of obstructions almost all year. The numbers are smaller in the winter months because the sun is lower in the sky, and distant trees shade the site during the early morning and late afternoon. Column B is the expected tilt angle of the solar array. We assume that we will manually adjust the array tilt angle four times a year between 50 degrees in the winter and 10 degrees in the summer. Our site in North Central Florida is at a latitude of about 30 degrees. Column C is taken from tables of climatic data, commonly called f-chart data, that are available for 250 cities in the United States. These data give values of incoming solar radiation averaged over many years, and include the attenuation of radiation due to clouds, rain, dust, etc. The town nearest our site for which data are available is Gainesville, Florida.
Column D gives a correction factor if a solar tracking mount is to be used. For our site, we have a very wide unobstructed angle to the sun, and in that case, a tracking mount will pay for itself. The correction factors listed are guesses based on data given by different manufacturers of solar trackers. The thermal derating factor (column E) takes into account the reduced efficiency of the PV modules when they are hot. Again, this is a guess based on data from the manufacturers. Column F is the product of columns A, C, D and E, multiplied by 0.00315, which is a unit conversion factor. It gives the equivalent number of hours of solar radiation at the standard level of 1 kW/sq. met., which is the radiation level assumed by most manufacturers of solar modules when they quote the output of their modules. For example, one module we are considering is the Siemens M55, rated at 53 Watts. The energy output of this module for our conditions is listed in column G.
Finally, we can estimate the number of modules we will need to supply our required power, and how much excess power we may expect during the summer. As a safety factor, we up our estimated power needs by 10%. Note that our leanest month is December, with 224 W-Hrs/day generated per 53 Watt module (column G). In December, we estimated we needed 4kW per day, and with the 10% increase, this becomes 4400 W-Hrs/day. Next, we multiply this minimum power required by 1.2 to account for losses in batteries and wiring. This comes to 5280 W-Hrs/day required in December. Divide this number by 224 W-Hrs/day per module in December to get the required 24 modules to get by in December. The last column in the table gives the excess power we should have for 24 modules, based on a use of 5280 W-Hrs/day in winter and 6000 W-Hrs/day in summer. Thus, we should be able to use about 2.5 kW-Hrs per day for our air conditioner in the hottest months of summer.
This picture shows our power situation as of Fall, 2003. We have three Zomeworks trackers, which hold the equivalent of 8 Siemens SM-55 PV's, 53 Watts each.
The next picture shows our Power distribution and monitoring setup.