A customer recently asked me if we were orienting his solar panels to true north or magnetic north. He and I are both private pilots, so I knew what he was asking about – magnetic declination. The true north pole (based on the axis upon which the earth spins) is not at the same location as the magnetic north pole. Solar panels generally produce the most energy when oriented to the south – true south.
What makes my job easy is that true and magnetic north in Southwest Florida are pretty close. The current magnetic declination is about 5 degrees west (to further complicate things, declination changes over time, and the rate of change also varies!) That means that magnetic north is five degrees to the west of true north. To align a solar array to true south, it would need to be oriented to 185 degrees on the compass.
Five degrees may seem like quite a bit, but it doesn’t result in any significant difference in energy production. In fact, the National Renewable Energy Labs PVWATTS calculator tells us that energy production is slightly better with panels oriented to about 175 degrees, which is five degrees east of true south and ten degrees east of magnetic south (see another blog post on east vs. west solar panel orientation for some reasons why this may be the case). The difference is only 0.2%, essentially negligible in terms of energy production.
Magnetic variation in the lower 48 states can be as high as 18 degrees, so in some locations the variation may be more important. In Southwest Florida the issue of true vs. magnetic north is far less pronounced. Unfortunately, we usually do not have much choice, as the orientation of our roofs predetermine the available orientations for solar panels.