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Powering Mission Critical Applications

by Michael Potts

All of the Solar Utilities Network's web pages are built and maintained on a solar-powered computer. Why? Because, just between us (don't let them hear) we don't trust our beloved local electrical monopoly to supply us with an unbroken stream of within-spec electrons …and NEITHER SHOULD YOU.

We use an industrial strength uninterruptible power supply because out here at the end of the grid, power outages are so common that we would lose work a couple of dozen times a year. In the last two years, we would have lost two weeks of work to power outages. Our power company is a little more candid than most, and they admit that the situation will get worse, because they have had to defer maintenance and cut crews to keep between the rock of rising fuel costs and the hard place of the state public utilities commission.

We are not alone. Prudent engineers almost always isolate mission critical process controllers and communications gear from the grid. In old systems where this isolation is not in effect take the air traffic control system, please down-time for the control equipment means that all those unfinished processes are just floating around in mid-air. The FAA is aware of the problem, and is installing stand-alone equipment much like that used by us here at SUN.

schematic of solar-powered uninterruptible power supply

On a good day, sun shines on our eight photovoltaic modules, and is metered in to our 400-amp-hour nickel-cadmium battery bank. On cloudy days when the grid is working properly, our solar-electron harvest is often supplemented when our inverter's built-in charger kicks in. We estimate that over the course of the year, about 25% of our electrons actually come from the grid. Direct current electricity from the batteries is converted by the inverter into the kind of electricity our computer equipment wants to see.

Days of Autonomy

When the power fails, our battery bank stores enough energy to keep our carefully designed home's energy saving equipment running for about four days without resorting to heroic measures like a fossil-fueled generator at Solarnet, we would never do that. We give up our big computer during times like those in favor of lights, running water, refrigeration, and tunes. We consider such outages to be natural holidays.

Would a system like ours make sense for you? If you want to be absolutely sure your power will not fail, you certainly need to do something. We like our solution because it will work without surprises for a couple of decades. If you are prepared to wait for the power to come back on, you only need enough uninterruptible energy to make an orderly exit from your mission critical processes. If you are as intrigued as we are by energy, and want to make it a hobby, this is a good serious project to undertake. Does it make economic sense compared to the grid? That depends on how valuable you consider your work.


In the my new book, the revised and updated The New Independent Home, we document the design and construction of a major off-the-grid homestead being built by one of my students in the wilds of cloudy Washington. To bring conventional power to the site, a spectacular mountainside far from the nearest power pole, would have cost at least $22,000, which would buy the privilege of paying a monthly utility bill. The solar-powered system which will cost much less initially, never have a monthly bill, and will still provide clean, adequate power to the residents.

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Michael Potts, webster
updated 1 January 2005 : 16:37 Caspar (Pacific) time
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