Finding the right solar panel size sounds like a simple enough task.

After all, you just need to know how much power you need and buy that size of solar panel, right?

Although in theory that is how things should work, in practice we need to consider a few more things before we can be confident in our solar panel sizing.

Of course you might be interested in sizing more than just a solar panel. You might want to know how to size an entire solar array (a series of solar panels linked together).

You will want to consider solar array sizing if you are planning to install solar panels on the rooftop of your home or business. This is different than sizing a single solar panel for charging your cell phone or taking on a camping trip, but it deals with a lot of the same issues.

Think of sizing a solar installation as an extra large version of sizing a solar panel.

Now let’s take a look at what we need to consider when we are deciding which solar panel size is most suitable for our needs.

For starters, when we talk about solar panel sizes we are referring to the nameplate DC power rating of the solar module. This tells you how much power the panel can produce under standard test conditions (STC). For example, a 100 Watt solar panel will produce a maximum of 100 Watts of DC power under very specific factory conditions. This is your first clue as to why sizing a solar panel is not as straightforward as it sounds.

We all know that factory conditions can be very different than real world conditions. The real world’s “less ideal” conditions require us to scale back a solar panel’s nameplate power rating to provide a more accurate representation of how much power the panel will actually produce.

The simplest way to do this is to find the rating for the solar panel under PVUSA Test Conditions (PTC) rather than under Standard Test Conditions (STC). A list of the PTC ratings for various solar panels can be found on the Go Solar California website.

PTC ratings were developed in the 1990s by the *Photovoltaics for Utility Scale Applications* (PVUSA) project. It involves using real world test conditions to get a more accurate idea of how much power a solar panel will actually produce.

How does the PTC rating compare with the rating under standard test conditions? According to information on the website for the National Renewable Energy Laboratory (NREL), “the PV module PTC power rating is about 88% of the PV module nameplate rating”.

Now that the PTC ratings have provided you with a more accurate real world idea of how much power your panel will produce, you need to consider how much sunshine you get at your location.

Here’s a map that will help you out if you live on Planet Earth:

Let’s say you’re one of the folks that get 5 hours of sunshine per day. If you have a 15 Watt solar panel you would multiply that 15 Watt nameplate DC power rating by those five hours to get 75 (15 x 5). This means that the solar panel should be producing 75 Watt hours per day.

Remember, however, that this is under standard test conditions. When we consider the lower PVUSA rating and some additional inefficiencies we should only count on 80% of that nameplate power output. This would bring us down to 60 Watt hours per day (75 x 80% = 60). This could power a 20 Watt compact fluorescent bulb for three hours (20 x 3 = 60).

If you’re looking for a little more power, maybe you should consider a 30 Watt solar panel or a 45 Watt solar panel.

This information should give you a good start in sizing a solar panel to do some work around your home or garden. Just remember to be conservative with your estimates so you don’t get less power than you bargained for.