Here are two charts I made using data from the output of our grid-tied solar array and data from the Midcontinent Independent System Operator (MISO), the people who keep the regional grid (including Minnesota’s) running.
This shows the total load the grid is using in a given hour and the total production in that same hour from our solar array. Our array faces due south. The shape of MISO’s load curve is fairly consistent from day to day, and so is our production curve (on sunny days). The peaks are not coincident, and solar production drops off dramatically in the peak load time. Data from our home use is even uglier (on weekdays).
The second chart compares the same solar production to price. For this one, I averaged all the locational marginal prices at the Minnesota Hub for all the days in July (average prices can vary somewhat by day and hour). The blue line basically shows how expensive wholesale electricity is in Minnesota at a given hour based on supply and demand (side note: check out this great real-time map of marginal electricity prices in MISO at different locations). Solar production seems to match this curve better in the morning, but still misses the opportunity to offset some of the more costly electricity in the afternoon. In July, west-facing panels could frequently be producing until 8 pm and beyond, when the price peak seems to start dropping off.
Rooftop solar usually doesn’t point west, probably because the incentives and utility rates (at least in Minnesota) are designed to maximize production rather than meeting peak load or reducing electricity prices. The New York Times has covered this dilemma. At current levels of solar generation in Minnesota, the issue of west- vs. south-facing panels doesn’t matter much. As generation grows however, this issue is something for utilities and regulators to consider.