[as the] Radar Sweeps

A science project attacked me yesterday.  I haven’t started doing the “lab” portion yet, but I did do some initial research…

Dennis, my installer for the solar panels, mentioned during installation that keeping the panels clean would make a noticeable difference in their performance.  Makes sense;  dust and dirt will decrease the light getting to the cells.  Yesterday I took a look up from the driveway and they had gotten visibly dirty – it has been about 6 weeks since we have and any kind of precipitation here.  So I got on a ladder and hosed them down.

When I got down from the ladder I took a look at the inverter; we were generating 3.604 kW.  That is higher than I have ever seen it [now this is our first season with the panels and just days before the summer solstice, early afternoon, so it should be higher than I have ever seen it].  I stood and watched for a couple of minutes and the number was steadily going down.  It was early afternoon on a bright cloudless day; very strange for it to be going dramatically down…unless the decline was somehow related to the hosing down I just gave the array.  What was going on?  They were cleaner now but something was causing them to generate less and less, by the second.  The rate finally stabilized at around 3.250 kW.

I asked my father to come out and help with a small experiment.  He watched the inverter while I hosed them down again.  Sure enough:  as the array got wet the output went up.  By the time I whole array was covered with water we were back up around 3.6 kW.  Once it dried off, output went down again to around 3.3 kW.

What is causing this?  A couple of possibilities come to mind.  I know that the efficiency of the panels decreases with heat, but I haven’t been able to find a voltage vs. temperature chart, and in any event I don’t know how much cooler the panels get [via evaporative cooling] when I’m hosing them down.

Another possibility is that the panels become less reflective when wet.  Perhaps a thin coating of water allows more of the light to pass thru the glass, to be converted to electricity?  It could be a combination of these effects or something else.

Whatever it is, a 10% increase in efficiency is HUGE.  Time for a science project…

Any ideas?

On February 9^th, 2007 - 4 months ago to the day - we installed photovoltaic panels on the roof of our garage. Sure, we could have put them in the back, but then nobody would know we have them! So, there they sit, high above our two hybrids: my 7-year-old Honda Insight, and Ellen’s new Ford Escape Hybrid:

The goal was to supply half of our electrical needs, reducing our monthly bill by somewhat more because of the tiered billing system in which the more you use, the higher the rate you pay.

How My Utility Charges Me for Electricity

It starts with a baseline - this is the allotment of electricity they will provide me at the cheapest rate. It is determined by a number of factors, including where I live [coastal, inland, etc], my past usage, and average usage across the customer base. Staying within my baseline is smart economically, difficult from a practical standpoint - without solar panels. My utility, San Diego Gas and Electric, charges me 12.4 cents for each baseline kWH I buy from them. But the rates go up quickly: let’s say I use 150% of my a baseline one month. Those last kWH would cost me double: 23.9 cent/kWH.

And this is one of the reasons that going solar makes sense today - it shaves the most expensive kWH off your bill. Even a small installation can frequently get you out of the upper tiers. [The same argument, obviously, works for conservation - fluorescent bulbs, turning things off when not in use, more efficient appliances can all make a disproportionate contribution to reducing your bill due to the tiered rate structure.]

Early Results

So, how has it been working out? Here in Southern California we do get a fair amount of sun; since install we have generated about 19 kWH daily, on average. How much is that? Well, with 5 adults in the house and an electric range and oven for cooking we consume about 32 kWH daily, so it looks like we have accomplished the goal of supplying half of our needs - as of right now we are a little over 60%. Here is what it looks like so far:

The maroon line is a lifetime percentage of our consumption supplied by our panels; as spring edges toward summer it is steadily increasing. The choppier blue line is a 2-week moving average of the same data. You can see that starting in late April the numbers started getting quite a bit better: longer days, higher sun, lower usage of electric space heaters [and we have several]. Notice the dip in the 2-week moving average around May 9? It was hot, hot, hot and we turned on the air conditioning. Killer on the consumption. That day was sufficient motivation to finally get a solar-powered attic fan installed. This should make a big difference in the temps of the upstairs bedrooms on the coming hot, summer days, enabling us to run the A/C a little less.

A few days later - May 12, 13 - we had our first days on which we were net-positive - we generated more than we used. Yippee!

What About the Bottomline?

How about our bill? In January, the last month with no solar generated electricity at all, we paid $177. In February, with about 3 weeks of solar , we were down to about $70 - even though our consumption was almost identical - saving over $90 because of our panels. While we are generating only a little over 50% of our demand, our bills are now close to 30% of what they would be without the panels.  Our bill for May , even though our usage has been been pretty constant, was $27.40.  Here are the numbers so far:

The Technical Details

Our installation is a 3.6kW system designed and installed by Heritage Solar in Aliso Viejo. We are very pleased with their work, their attitude and their price. The system consists of 24 SolarWorld SW-175 panels tied to the grid via a Xantrex GT3.8 inverter.

Are We Glad We Did This?

No question. Clearly, this is a major investment. We paid about $21,700 upfront for our system; state and federal rebates paid the other $10,000 the system cost. I don’t think we lost any money, starting from day one: the value of our home has increased because of the solar installation by about the our out-of-pocket cost. But our goal was not just to reduce monthly bills, as important as that is. We also wanted to reduce our dependence on fossil fuels.

According to the EPA, if we generate an average of 15.5 kWH daily [and today are are at 19.8], we are preventing the equivalent of 1 pound of CO₂ from going into the atmosphere every hour, 24 hours a day, 365 days a year, every year from now on. Now, as far as global warming is concerned, that’s cool.