How Much Would My PV Array Produce on a Day Like Yoday?

As easy a question as that sounds (and as often as I get that question), there is no easy answer.

I would need one or more irradiance sensors, the temperature of the modules, their wiring configuration in the array and down to the inverter, the equipment specs and on and on.  The easiest way for me to tell you would be to call a current customer with a specific system size and say “Hey, go look at your inverter or your monitoring software and tell me what’s going on.”

Since doing the above isn’t always easy or convenient, we approach the problem in a slightly different way.  Your electrical bill isn’t (usually) billed on an instantaneous measure of power, so why look at your solar installation that way?

Why telling me a straight answer so difficult?

Well, even though a solar array only contains a few components, the way they interact can be complex.  Let me see if I can simplify.  Instead of telling you how much your system might produce at this instant, how about letting you know how your system will perform over time.

Estimating System Performance

Generally speaking we will provide you an estimate of your solar array’s annual performance.  Using one year as our guide is useful for a variety of reasons:

  1. It eliminates seasonal variations;
  2. We can use data from historical years to average out sunnier and rainier years;
  3. Most of our incentives and benefits are communicated to us by the state and utility on an annual basis.

There are tools that will allow us to model your system output on an hourly basis, but those are usually more appropriate for engineering studies of larger systems and not residential applications.

Factors Affecting Performance

Your performance can be affected by:

  • Geographical location;
  • Shading;
  • Tilt of your roof;
  • Orientation (do you face south or east or west or some mix of the three?);
  • Equipment installed;
  • Voltage drop along wires;
  • Age of your panels;
  • Dirt;
  • Ambient Temperature;
  • And on…

When we model your system performance we take into account and plan for as many of these items as possible.

So how much will my system produce?

One of the most common used tools in the solar industry for general numbers is PVWatts.  Other modeling tools include RETScreen,  PolySun, Solmetric, T*Sol, PVSyst, System Advisor Model, and a host of others.  RETScreen, PVWatts, and System Advisor Model are all free tools.

The best thing you can do to find out your estimated production is to talk to your solar contractor.  I should warn you though; even between contractors you may get different answers.  To ease your mind A&R Solar provides a Highest Performance Guarantee so you have the confidence that our systems will perform how we say they will.