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Crunching the numbers for solar energy

By Alice LaPierre
Tuesday December 04, 2001

You’ve read up on how solar panels work, and now you’re considering installing them on your building to reduce your monthly electrical bill, or perhaps to have power during an emergency. But what do you do first? 

Without a doubt, conservation and efficiency is the very first step in calculating your energy use for sizing a solar system. Without doing this step first, you will be paying for a system that is much larger than your actual needs. Conservation measures can save you thousands of dollars on an oversized solar electric system. 

Since solar systems only work when the sun is out, and are still rather expensive for the average homeowner to have installed, it makes sense to stop wasting energy first, and reduce the amount of energy you use. Compact fluorescent bulbs, EnergyStar appliances (especially refrigerators), using daylight instead of electric light, turning off lights and appliances when not in use, and using power strips with “kill” switches to turn off appliances completely will all help reduce your energy consumption. If you haven’t done these things yet, you are wasting energy and more money than any solar system could ever save you. 

Once your energy bill has been reduced through conservation measures, you are nearly ready for a solar electric installation. There are still several things left to do: 1) make certain that your rooftop or installation location has sufficient access to sunlight at all times of the year; 2) calculate the size of the system that you require; 3) contact your local utility to begin the paperwork for a Net Metering agreement, and submit an application for a rebate (more about that at the end of this column); 4) contact a licensed solar installer or qualified electrician and get at least three quotes; 5) secure electrical and other related permits. 

How do you calculate the size of your system? Begin by collecting the past year’s worth of your electric bills. Each bill indicates the number of kilowatt hours (kWh) used per month, and per day. A watt is an instantaneous measurement of electrical power, and must be measured over time to gauge usage; generally we use hours to measure use. Measured over time, 1,000 watts per hour is a kilowatt hour. An average household might use 240 kilowatt hours (kWh) in a month, with an average of 8 kWh a day.  

A 100-watt panel will generate 100 watts of electricity per hour in full sunlight. Berkeley is calculated to have an average of 5.5 hours of usable sunlight per day on average throughout the year (about 10 hours in summer, less than three in winter). A single 100-watt panel will generate 550 watts of power in a 5.5 hour period of peak sunlight (Berkeley’s yearly average). If the home uses 8 kilowatt hours of energy per day, it will take 14 or 15 panels to supply enough power on average year round: more than enough to generate excess power in the summer to retrieve for later use in winter under a Net Metering agreement. Calculate: 14 panels multiplied times 100 watts, multiplied times 5.5 hours equals 7,700 watts per hour, or 7.7 kilowatt hours.  

Here is where energy conservation will pay off. Panels cost from $300 to $700 each depending on efficiency and watts, so the fewer panels you need, the less expensive your entire system will be. You will still need an inverter and controller. Batteries are optional, since they are only needed for back-up power during the rare times that your utility power is unavailable. 

What do you do if you need more panels than you have room for? If your roof will not accommodate the 15 or 20 panels your building needs, or your site is shaded by adjacent buildings or trees, you may not be able to “zero out” at the end of a year under your Net Metering agreement. You should consider installing only those panels that will receive the maximum amount of sunlight year round; or, you may want to invest in a tracking system so that your panels receive the maximum amount of sunlight daily. If the extra costs make your payback period more than the expected life of your panels, or the extra weight means that you have to invest in reinforcing your roof, a tracking system may not be your best option. 

One other type of solar panel is available for limited applications. These are called “plug-in” panels, and have built-in inverters and controllers. They do not require licensed installers, although you will still need to have a Net Metering agreement with your utility. The advantage of these panels is that they do not require lots of complicated equipment, and can save much of the cost of installation. The disadvantage is the cost of the panels: for a 150-watt system, about $3,000, which generates less than a single kilowatt hour per day on average. 

Plug-in panels can be used with batteries, and can provide limited power in the event of a power loss, but in general their cost is too great for most homeowner applications. They are great for RV’s and boats, or for renters who would want to take the panel with them when they move.  

Once you have calculated roughly the amount of watts and number of panels you will need, verified that you have enough suitable (south-facing and unshaded) roof space, and have contacted your utility for a Net Metering agreement, you should contact three licensed, experienced installers to give you an estimate of cost for parts and labor. A qualified contractor can design the most efficient system for you. 

Rebates are available from the California Energy Commission. As of this writing, rebates are $4.50 per watt for all sizes of systems or 50 percent of the cost of equipment and installation by a licensed contractor, whichever is less, up to a maximum of $50,000. The amount of the rebate will depend on the costs of the inverter and controller equipment. To reserve your rebate, you must first fill out and send in the application forms. The system must be installed within nine months or receipt of the application. Contact the CEC at 1-800-555-7794, or go to for complete details.  

The California Energy Commission also provides a free online guide to solar energy at This is a basic guide to PV system design and installation, and provides detailed information on equipment wiring, voltage drop calculations and much more. Installing PV systems is complicated and should only be done by a qualified, licensed installer. Other rebate information can be obtained at 

After your system has been designed and drawn up, you are ready to apply for permits from the city’s Permit Service Center. For information on permitting processes, contact the Permit Service Center, 2120 Milvia Street, Berkeley.  

For other information on solar systems, net metering, and energy conservation, contact Berkeley’s Energy Office website at, or 981-5435.