A Solar Cell Backup Power System - 1

Solar electric power is the stuff of liberal dreams.  Free power from the sun, that's non-polluting and doesn't affect the environment at all! (Not bloody likely, but ...some other time).  It's almost as good as those magic unicorn farts!

On the other hand, a lot of us look at the solar power constant - the sun delivers about 1100 Watts per square yard at the earth's surface - and say "how do I get me some of that?"  It's not that it's "green" - it's more like "free money" - how much better does it get?

The price of photovoltaics is coming down, but just isn't competitive with grid power except in the remotest places.  A modest home typically has a 20 kW electric supply distribution (the fuse panel) and wiring.  Such a solar photovoltaic system would cost well over $100,000.  If it's going to cost you even half that to get power lines run to your remote house, it's a no-brainer to go with solar.  After you pay to bring the power lines to your house, you pay the electric company every month.  After you buy the solar power system, the largest parts of the expense are over.  Yes, you'll have to do upkeep on the system, but let's ignore that for now - everything in a house needs upkeep.

Everyone on the power grid has experienced periods when the grid goes down.  Although the longest period we have ever lost power in 28 years here at Castle Graybeard was just over 30 hours, after the first of the '04 hurricanes, it sure isn't guaranteed to stay that way.  Between the two storms, most of our neighbors must have gotten "window rattler" generators, judging by the increase in noise.  Those rely on gasoline, and a continuous cycle of filling the gas tank, driving around town to find an open gas stations to fill the source of gas for the generator, and repeating.  What if things are down long enough for the local gas supply to get scarce, like after a Katrina or big quake?  Wouldn't a solar powered system that could run a modest amount of stuff be a tremendous thing to have in a long blackout? 

Your house (in the US) probably contains five or more identically rated circuits: 120V at 15 Amps.  In power, that's 1800 Watts (120V * 15A).  If you had 1800 Watts in this situation, you'd probably come close to using every watt you could get out of it.  A refrigerator would run long enough to get everything cold, and then you could unplug it for a while to run something else, if needed.  Refrigerators are pretty efficient, generally using a couple of hundred watts for a while, then idling back down to less than 50.  Maybe you'd make ice for your neighbors (you'd have enough for yourself).  You couldn't use your electric oven or range, but you could run a microwave oven which typically uses less than 1000W.  Chances are you'd be recharging batteries continually.  Let's see what it would take to design a solar electric system to deliver 1800W. 

(Typical standalone or off grid system)

How big a set of panels would be required?  Since the sun beats down at 1200 W/sq. yard, isn't the cell area just 1800 watts, divided by that number, or 1 1/2 square yards?  Not really - that's only true if turning light into electricity was 100% efficient.  The efficiency of the cells varies, but one of the best types, monocrystalline Silicon, is 16% efficient.  That means you'll only get 16% of your potential 1800 watts, 288W, from 1 1/2 sq. yards.  Divide the 1 1/2 by 16% (.16) and you see you need 9.38 square yards - over six times bigger.  Since one square yard is 9 square feet, that's an 84.38 square foot panel, just over 9 feet on a side (if square).  Unfortunately, that's still going to be too small.  A panel rated for 1800 watts is only going to produce 1800 Watts on a cloudless day, if it's perpendicular to the sun's rays at all times - which means tracking the sun with the solar panel mounts.  Add another 30% or more to the size of that panel to make up for not steering it.  Now it's gone from 9.38 square yards up to 12.2, or 109.7 square feet.  Getting pretty big, isn't it?  And that's just the solar cell area; any construction material, a frame, and so on, makes them bigger.

Hold on a minute, though.  Your panel will only provide power when the sun is up, and to get 1800W usable 24/7, you'll need to store power in banks of batteries.  Even worse, to store 1800W while you're using 1800 W means you need even more area dedicated to solar panels. If you really want to store 1800W and use 1800W every hour, it has to be twice the size.

It turns out that to create a single circuit that will run 1800 watts 24/7 is a big deal with photovoltaics, and that's one reason that the technology isn't very widely used, yet.  It's well over 220 square feet of solar cells.  Do you really have enough room to duplicate this five or six times over, to match the circuits in your AC breaker (fuse) box?  This is why, if you have access to gasoline, diesel, propane or natural gas, an internal combustion engine driving a generator is a far more practical way to power things.

Finally, did you notice that solar cells are less efficient than gasoline engines (typically around 20-25%)?

Next time, planning...