[Greenbuilding] A watt saved.....

[Lawrence Lile]

I don't know their rule of thumb, but let's think it through.

Let's pretend that you can install a PV system for $6 a peak watt.  This will vary more or less, more for a smaller system maybe.

Maybe there are 6 full sun hours on a winter day.  Let's say that your PV system produces 6 watt-hours each day for each peak watt installed (also a wild guess).    That's about a dollar of capital for a watt-hour per day.

Let's say you are considering two appliances that accomplish the same job, one of which uses 1000 watt-hours per day and the other that uses 500 watt-hours per day.  It will take you =  $500 worth of extra solar cells to run the less efficient appliance.  Is the more efficient appliance less than $500 more than the cheap one?  Then it is a good buy.  This is how you justify a  $1000 refrigerator on a PV system.

A tract home might use 750 KHWRs per month, or 25 KWHrs per day.  This is about $25000 worth of solar cells and inverters, if no energy efficient appliances are installed.  So you could buy a lot of really efficient refrigerators and so forth for the same $25000.  If you could invest a quarter of that money and bring your electric use down by half, you'd be way ahead.


This all becomes moot if your PV system is grid tied.  You can just borrow a little juice from the grid if your PV system is short.  Unless you are determined to have a zero energy house (a noble goal) the grid tied system probably doesn't need to follow your rule of thumb.



--Lawrence Lile,


> -----Original Message-----
> From: greenbuilding-bounces at listserv.repp.org [mailto:greenbuilding-
> bounces at listserv.repp.org] On Behalf Of John E. Beeson
> Sent: Thursday, February 19, 2009 9:05 AM
> To: greenbuilding at listserv.repp.org
> Subject: [Greenbuilding] A watt saved.....
>
> Anyone remember the rough rule of thumb that Steven Strong (and others,
> I'm sure) uses about a Watt saved is something returned (in reference to
> sizing a PV system)?  Perhaps something to do with tuning the envelope
> first will equate to some amount of kW saved in sizing a system...
> Something.....
>
> Yeah, not to specific there, but that's all I can remember on short
> notice.  Sorry.
>
>
> QUINN EVANS | ARCHITECTS
> John E. Beeson, LEED AP
> d 734 926 0425
>
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