Dec 28

Solar Powered Generator Specifics: 25% for the 99%

Posted on Dec 28, 2011 under Uncategorized | Comments are off

Image: Salvatore Vuono /

Today, we will look at the basic requirements for a solar powered generator capable of meeting 25% of your normal household electrical needs. As you may recall from the last post, my solar backup goal for the masses, “the 99%,” was to provide a reasonable comfort level throughout a long power outage, lasting a week or more. My recommendation was to install a solar backup system capable of delivering 25% of normal power usage on a continual basis, “24/7.”
As mentioned previously, it is best to start with power usage figures from your own power bills. To account for seasonal differences, it’s best to look-up (or add up) your annual power usage in kilowatt-hours (kWh), and divide by 12 to get your average monthly usage. Next, calculate your daily use by by dividing 30 into your average monthly usage. Divide that number by 5 (U.S. average “peak” sun hours), and multiply by 1.5 to account for system losses. To this point, you’ve calculated the system size (in kilowatts) required to cover 100% of your electrical needs. Now just divide by 4 to get 25% of your electrical requirement, the size I have recommended for your solar backup system.
To get a sense of  what size system would be required for the average U.S. home, we will use national averages to illustrate here. As stated in the last post, the average U.S. household uses 11,496 kWh of power annually, or 958 kWh per month. Using these figures in the calculation described above, we arrive at a hypothetical system size of 2.395 kW, or about 2400 watts (W).
So a solar backup system with ten 250 watt solar panels would meet this 2400 watt requirement, and then some (since 10 x 250 W = 2500 W). Monocrystalline solar panels are recommended. This technology is mature, and these solar panels are the most efficient panels commercially available today.
A bank of sixteen 100 amp-hour deep cycle batteries will provide adequate energy storage. Sealed lead-acid batteries are recommended to reduce maintenance and avoid spillage. The batteries are 12 volts each, but they will be wired in “series-parallel” to create a 48 volt system. For this 19,200 watt-hour system, the 48 volt configuration will improve system efficiency (as compared to a 12 or 24 volt system).
To maintain proper system voltage, and prevent battery overcharging from the solar panels, an 80 amp controller will be required. A remote digital display for the solar control module will allow for system monitoring, and will store a year’s data on a removable Secure Digital memory card (SD card).
For a system of this size, a 4000 watt inverter is in order. This inverter would be large enough to handle short-duration 240 volt applications, such as a well pump, and selected 120 volt household circuits. Since the solar backup system will only be providing one quarter of normal power usage, careful consideration should be given to which circuits and outlets your system should supply, once in “backup” mode. Give special attention to those electrical needs which are not portable. Examples include: well pumps, furnaces, attic fans and refrigerators. Of course, a portable appliance can always be moved to a powered outlet, like a radio or laptop computer, for instance.

So where are the electrical priorities in your home? What are your “core” power needs? I hope my solar backup solution as presented here has provided you some “food for thought.”

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