Go Solar! Estimating PV system size.

I live in Hawaii and have been dreaming of "going solar" for a few years.  I'll admit that my main incentive for doing so was the allure of saving money.  Electricity in Hawaii is very expensive, in large part because the state depends on imported oil for about 76 percent of its electricity needs.  There have been months where I have paid 45 cents per kilowatt-hour for my electricity.  (In 2010, the national average was 11.5 cents per kilowatt-hour.)  Reading how high my electricity bill is probably makes you wonder why I have waited so long to move to alternative sources. Initially, I was waiting for the cost of some of those alternatives to come down.  Then in the last couple of years, I just couldn't find the time or energy to do the research required to make a sensible purchase.  In September 2011, I felt it was high time I got this project in gear.

My engineer friend, Steve, one of the most able and trustworthy people I know, is passionate about alternative energy and I contacted him to see if he had time to install a photovoltaic system for me before the end of the year.  Unfortunately for me, he did not have time to do the work, but he generously offered to come over to review my situation and give me some pointers.  He spent a couple of hours at my home and left me with a valuable basic understanding of PV systems.  He also did some back of the envelop calculations that gave me a ballpark idea of the size of the system I should contemplate building.  Thank you so much, Steve!

Ok.  What kind of information did we use to estimate system size?

1. How much energy do you generally use?  Consult your past electric bills to calculate this information.  In my case, the average use for 2 adults in our household over a 12 month period was 22.8 kilowatt-hours per day.
2. How much sun does the area usually get?  The average number of peak sun hours per day in my area is about 4.8.  The Hawaii State Geographic Information System (GIS) has links to Solar Radiation Maps for each of the major Hawaiian Islands and an energy unit conversion table on their web site.  Here's the link: http://hawaii.gov/dbedt/gis/miscmaps.htm
3. Where will you put your solar panels and how is the site oriented to the sun's path?  Our roof is very high and unshaded by trees, buildings or other obstructions in the sun's path.  It is also oriented with its roof-line on basically a southeast to northwest axis.  This is a terrific piece of luck.  Steve had a gizmo that looked at the sun's path in relationship to my roof.  A solar contractor has access to similar instruments.
4. What do you usually use electricity for?  The idea here is to take stock of your energy use and decide on an appliance-by-appliance basis if you could benefit by changing your approach.  In Hawaii, heating water accounts for about 30% of the typical electric bill.  Clearly, examining our approach to hot water heating should be first on our list.  A little on-line research told us that a solar hot water system was likely to reduce our hot water heating bill by about 90%, so we decided to install a solar hot water heater as our first priority.  (See a separate post on the solar hot water system.)  Going down the list of our other large appliances, we determined that our gas stove was a good thing, our old refrigerator should be replaced with a more efficient model when it is financially practical to do so, and that we would stick with our electric dryer rather than buying a propane dryer, at least for the time being.  Our dryer is in good condition, so I'd hate to replace it now.  Also, we paid $5.60 per gallon for propane purchased today.  That's pretty expensive and likely to go up as time goes on.  So, for the purposes of estimating the size of a PV system, we decided to use our current average less the probable effect of a solar water system.
5. Here's the equation Steve shared with me: Kilowatt Array Size = Kilowatts Required  /  (Efficiency of the System  X  Ave Peak Hours of Sun).  The amount of Energy we want to generate is about 16.6 kilowatt hours per day.  This is our 22.8 average use, less 90% of 30% of that same number in order to allow for the likely contribution of the solar water system.  Steve estimated the efficiency of a PV system at 70%.  Our Average Peak Hours of Sun is 4.8 hours/day.  Kilowatt Array Size = 16.6/(0.7 X 4.8) = 4.94.  So, we determined the likely size of our system should be about 5 Kilowatts.

These were Steve's back of the envelop calculations.  My next step would be to consult with some solar contractors to get their opinions on the system we should build.  To be continued in another post.