Optimum Orientation of Solar Panels for Time-of-Use Rates


In California, Pacific Gas and Electric is required to buy electricity from small generators at the same rate at which they sell it. Some of their rate schedules have higher rates for times of peak usage. If you have a solar electric system connected to their grid, then rather than maximizing the amount of energy you produce, you may be interested in maximizing the amount of money you can be paid for generating electricity (or equivalently, the amount of money you can save by not buying their energy). 

Here is a typical rate. From noon to 6 pm between May 1st and November 1st, PG&E will buy or sell electricity at 35 cents per kwh. During all other times, they will buy or sell electricity at 5 cents per kwh. 

Clearly, in this situation the solar panels should be facing somewhat to the west, to capture more of the valuable afternoon energy. But how much? 

In one example, I calculated that at latitude 38 degrees north, the most economical orientation is to face 37 degrees west of south, and tilt 31 degrees from horizontal.

Your situation

Time-of-use rates differ from place to place, so it’s not possible to give specific recommendations for every situation. If you want an analysis of your situation, inquire about my consulting rates.

The fine print:

The peak period observes daylight time. My calculation ignores the fact that the last few days of October are not daylight time, and makes no adjustment for rate changes on weekends or holidays. I also assumed that solar noon (when the sun is due south) occurs at 1 pm daylight time; this will vary depending on your longitude and time zone. 

I assume the panels are fixed throughout the year. I also assume you are not storing energy in batteries. (If you were, you might be able to make money by buying power at 11 am and selling it back after noon.)

This calculation is based on an idealized situation. It assumes that you have an unobstructed view of the sky, with no trees, hills, clouds, or haze ever blocking the sun. The calculation also assumes that you are near sea level.


Written by Charles R. Landau. Copyright (c) 2002, 2003, 2010. All rights reserved.

clandau@macslab.com