Tuesday, February 21, 2012

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...


  1. SG,

    I have read Homepower Magazine since 1987, when it started out as newsprint (newspaper) version of a magazine, long before they went "slick". I have seen homes totally off the grid with all of their needs met for about a quarter of what you quote. I have a friend in Hamilton, MT who just went grid-tied for his house _and_ his shop, where he builds acoustic guitars for sale (planers, huge sanding machines, band and table saws, etc.) and he SELLS power back to the utility company. He spent $26K on his hybrid (grid-tie plus batteries for those days without sufficient sun) system. IIRC, he uses 22 or 24 of some brand of 220 watt panels that are mounted flat on his shop roof, facing SE, and tilted the same angle as his roof. Again, he powers both his 1400 sq ft house and his guitar making shop, and still manages to sell a small amount of power to the utility company each month (net).


    Ten of these would give you 2300 watts, with footprint of 190 sq ft. MPPT charge controllers (http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/c474/) and tracking racks can further increase efficiency.

    I think your numbers perhaps might not be reflecting current state of the art. At least from the systems I have seen and the systems I have read about over the recent years via Home Power. I can email you some articles if you'd like.

  2. SG,

    Damn. Hit return too quickly. I meant to add that, given the possibility that if TSHTF, gasoline, propane, diesel, etc may no longer be available, a solar system would allow you to continue running lights, refrigeration, communication (ham radios, etc.) and such when your generator sits there rusting.

    For me, solar provides two benefits - telling the power company to "stick it" (I personally would go off-grid rather than grid-tied), plus giving me power when no one without solar has power anymore. Yes, it is certainly costlier (at present, but the prices have been steadily dropping) than grid power, but if you are into "prepping" it sure can provide some peace of mind.

  3. Patience, my friend. I'm getting there. Didn't want to put everything into a long, long post, and get 250 people saying "too long, didn't read".

    SiGraybeard @ work

  4. My power outages in the Midwest have been increasingly more frequent and longer duration. 1-4 days at a time in the middle of winter in single digit temps is no longer unusual. Solar panels are useless in February in the middle of the day under cloudy skies. For those who don't have a standby natural gas gen, woodstoves (or kerosene heaters) and gasoline gens run sparingly on and off during he day for water pumps become the norm.

  5. Greybeard,

    I'm working on a project, and I need some help getting the word out. I've started a project for a open-source encryption tool for resistance orgs. The project is here: https://launchpad.net/kg1984 Would you be able to provide some guidance and support/publicity?

    Thanks in advance

    1. Anon 1127 - I'm not sure what I can do. You're certainly way beyond me in software abilities. And I don't really understand that launchpad source page. ?