A peak sun-hour is roughly the amount of solar energy striking a 1-square-meter area perpendicular to the sun's location over a 1-hour period straddling solar noon in the summertime. So we can compare apples to apples, the amount of power is standardized at 1,000 watts (1 kilowatt) hitting that 1-square meter surface. By adding up the various amounts of solar irradiation over the course of a day, and counting them as units equivalent to 1 solar-noon midsummer hour (1,000 watts per square meter for 1 hour), we get a useful comparison number—the peak sun-hour.
An analogy might help complete the picture. Imagine that you have to pour sunshine into buckets that are 1 meter square, and each holds 1,000 watt-hours of solar energy. The fastest rate of filling that bucket will occur at solar noon in the summer, when the sunlight is really streaming down. At that time, you could fill a 1,000-watt-hour bucket in 1 hour (1 KWH per hour). At any other time of the day, however, it will take longer than 1 hour to get an equivalent "bucket" of 1 peak sun-hour.
On average, summertime Seattle conditions will net you 4.8 peak sun-hour-equivalents from sunup to sundown. Wintertime sees an average of about 2.5 sun-hours per day. Over the course of a year, the daily average works out to about 3.76 peak sun-hours. For month-by-month solar irradiation information for a variety of cities in the United States, visit http://rredc.nrel.gov/solar/pubs/redbook.
Larry Owens • Shoreline Solar Project
Batteryless or Backup?
I want to install a grid-tied solar-electric system, and I'm having a hard time deciding between a battery-based system and a batteryless system. Can you give me the pros and cons in plain English? Is there any way to have the best of both worlds—the efficiency and economy of a batteryless system paired with the reassurance of always having a reliable source of backup energy?
Joan Beaudet • Milton, Massachusetts
Batteryless systems are simpler, more efficient, and less expensive to install and maintain, but during a utility failure, these systems will not provide any electrical backup, even if the sun is shining. A grid-tied, battery-based system is designed to do just that, but uninterruptible power comes at a price. With the same size solar array, a grid-tied, battery-based system will yield about 7% to 10% less energy than its batteryless counterpart. This is primarily due to the inefficiencies involved with battery charging (even when the grid is functioning). And keep in mind that the batteries will need replacement roughly every seven to ten years, which can be a major expense. If you don't experience frequent or long utility failures, you will likely be happier with a batteryless system.
If your grid electricity is unreliable (perhaps you depend on a long rural line in an area that's prone to lightning or ice storms), consider a battery-based system. In battery-based, grid-tied systems, you have to install a separate AC subpanel to separate critical circuits from luxury loads. This ensures that when the system switches to battery backup, the energy stored in the batteries will not be depleted by loads that you can easily live without.
An experienced photovoltaic installer can help you determine which of your electrical appliances can realistically be backed up, and how much battery storage will be required. In almost all cases, it's unrealistic to rely on backup electricity for space or water heating, or for major cooking loads like an electric range, since the energy consumption would be far beyond the capacity of an affordable battery-based photovoltaic system. If your location experiences long utility outages, think about investing in solar heating systems or gas appliances for your heating and cooking needs.
During a utility outage, consider supplying emergency needs with no electricity. Store water in a tank. Keep a stack of ice packs in your freezer to increase its holdover period. Keep LED headlamps or flashlights or fluorescent (or gas) lanterns handy. Be ready to ignite your gas stove-top using a spark lighter or matches. Use wood heat, or gas heaters that don't require electricity. If you want battery backup for your computer, Internet connection, radio, or TV, consider purchasing an off-the-shelf uninterruptible power supply (UPS) unit just for that purpose. These preparations will keep you from being overly dependent on electricity when the grid goes down.
Windy Dankoff, founder (retired) • Dankoff Solar Products
Reliable partners for 30 years
SolarWorld California's factory began making sturdy, efficient modules and systems for off-grid homes in 1977, many of which are still in use today.
While our industry has grown and our products have improved over the last three decades, we still make the best quality PV modules around, and we still make them right here in Camarillo, California. As we celebrate our 30th anniversary, we extend ourthanks to the true pioneers of solar energy who have depended on our products for the last three decades.
Call 1-800-94-SClLAR for assistance.
4650 Adohr Lane Camarillo, CA 93012 [email protected]
SolarWorld. And EveiyDay is a SunDay. www.solarworld-usa.com
THE SUNPOWERED COMPANY*
I am writing to you from Vermont where I would like to purchase an off-grid home. I have spoken to a few local banks and have received a lukewarm response to the possibility of taking out a mortgage for a property that is off the grid. How can I find a receptive lender?
Mickel Zuidhoek • Pawlet, Vermont
Financing an off-grid home or property is not entirely different than financing a home in a typical subdivision. There are three major categories that apply to residential real estate financing— income, credit, and collateral.
Collateral is the most important factor in financing an off-grid home, and it is up to an appraiser to address the typical issues and evaluate the property's features for potential underwriters. You'll need to find an appraiser in your area who specializes in out-of-the-ordinary properties, with experience appraising off-grid properties.
Many off-grid homes are near other off-grid homes, which can be used for appraisal comparisons. Have the appraiser prepare an addendum to the property's appraisal that details other nearby off-grid properties and their sales histories. This will help show underwriters that your property is not an anomaly for the area.
Your appraiser will not necessarily be bound by the normal rule of having to use sales comparables within five miles. The lending company Fannie Mae will allow greater distances as long as the appraiser is able to support the necessity for using a sales comparable outside normal guidelines. The appraiser may also
search for older sales comparables of off-grid homes to support the value of the home. If you know of any off-grid homes in the area, let the appraiser know—sometimes sales of off-grid homes are private sales and do not show on the multiple listing system, which is how many appraisers find comparables.
Once an underwriter is able to see how the value of the property is supported with reasonable sales comparables, you will soon be enjoying your off-grid property or home.
Terry Phenicie • First Priority Financial
I've heard that the way you drive an electric vehicle (EV) can affect range dramatically. Does the same apply to fuel economy for engine-driven vehicles? Can you give me some basic pointers on how to drive so I use less energy and create less pollution?
James Fallow • Big Pine, California
Many factors affect driving range, but air drag and weight are certainly two of the most important. For an EV moving at less than 30 mph, it's the weight of the vehicle that kills driving range; as speeds increase beyond 35 mph, air drag takes over as the biggest culprit of dragging down fuel economy.
Some idea of air drag's insidious nature can be gained from data for the RAV4 EV—one of the most-studied EVs ever built. At 45 mph, the car can travel almost 150 miles on a single charge; at 60 mph, driving range plummets to about 100 miles (just imagine what happens at 80 mph).
In the case of a conventional internal-combustion-engine (ICE) vehicle, gains in fuel economy are there for the taking—if you're willing to drive at a more leisurely speed. My 1993 Dodge minivan delivers its highest fuel economy—29 mpg—at a constant speed of 45 mph. (For safety reasons, I suggest not driving at this speed on the open highway.)
When I dare to keep up with traffic on the Michigan interstate (80+ mph), my minivan's fuel economy drops to about 17 mpg.
Stop-and-go city driving also reduces fuel economy for ICE-based vehicles. This is a consequence of the operating characteristics of typical engines that are designed to operate at higher loads (and, hence, higher driving speeds), and the need for constant acceleration and deceleration. Most hybrid-electric vehicles have circumvented these problems and actually do as well, if not better, in the city as on the highway.
You can improve your city mileage with an ICE-based vehicle if you drive more intelligently. Learn how to coast, rather than braking, into a stop, and time traffic lights so you keep moving at a relatively
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