As an owner of a solar system or when considering going solar, it’s helpful to understand what your system will output over the course of the four seasons. Here in the northeastern United States we do see significant variation in daily energy output from our systems over the course of a calendar year.
Based on real data from the Lightgauge monitoring systems we install for our customers we can closely track each system’s energy output variation during the year. If we split the year into two equal parts at the Vernal and Autumnal Equinoxes (March 21st and September 21st) we can get a quantitative handle on this variation. It turns out that, on average, 65% of our local solar system’s annual energy output is generated between March 21st and September 21st of each year. The other half of the year, between September 21st and March 21st, accounts for the other 35% of annual output.
Furthermore, if we take a look at the two month windows surrounding both the Summer and Winter Solstices (June 21st and December 21st) by comparing system outputs for June and July vs December and January we can further accentuate the seasonal variation. On average our residential solar customers see a total energy output decrease of 40-60% during the months of December and January as compared to July and August.
The factors involved in this variation are threefold. Firstly, we know that in our area we have shorter days in the winter than we do in the summer. This means that the solar system will be running for less time each day and therefore produce less average energy per day. Compounding the effect of the shorter days is the fact that the sun angle changes dramatically in the winter as well. The sun, even at it’s peak around midday, is much lower in the sky during the winter months. For most residential rooftops this means that the sun’s rays will be hitting the solar panels less directly than during the summer months. This will cause the system’s power output to be lower which also has a direct impact on energy production.
Lastly, atmospheric conditions need to be considered. Not only do the winter months provide plenty of stormy weather and cloud cover, but the effect of snow cover on the panels after a storm is significant as well. With a thin covering of snow the system will often still be able to turn on and output a small amount of energy. Larger snow accumulations on the panels, however, can keep the system from converting energy for up to a few days until the panels clear.
So how does this work with your utility billing? Won’t this cause system owners to get high electricity bills all winter long when their systems are under-producing and their usage is increased due to more time in the house, higher lighting loads, etc.? Not necessarily, and this is where net metering comes into play. When we design solar systems for customers we always look at the total annual electricity usage when sizing the system. For customer’s with adequate roof space (or area for a ground mount) this allows us to design a system which overproduces enough during the spring, summer, and early fall to build up a bank of kilowatt hours with the utility which will carry the homeowner through the winter months. Thereby the effects of reduced energy production during our northeastern winters can in fact be mitigated through correct system design, sizing, and net metering (read more about net metering here).
This is also why, for our customers who get their systems interconnected in months other than March and April, we advise them to utilize their utility’s “Anniversary Date Change” process to make sure that they are optimizing the use of their net metered energy credits over the course of the year.
If you’d like to learn more about optimizing your anniversary date please call our office and speak with one of our Technical Sales Engineers.
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