Solar Panel Ratings
This is the third in a series of articles that we’ve written as a guide for people who are just getting started learning about solar panels and how to use them. In our last 2 newsletters, we talked about energy (measured in Wh or kWh) and power (measured in W), because these are two important things to understand before you really get started. If you want a reliable system that’s not oversized (translation: more expensive than you need), then you want to size your solar panel so that it produces enough energy each day to provide the energy that the load uses each day, with some left over. These are the basic steps in this process, and we talked about the first two in the previous articles in this series:
STEP 1: Calculate the Wh needed by the device you want to run
STEP 2: Find the Peak Sun Hours at your location
Step 1 and Step 2 are pretty easy, once you know how to get the information you need. Now let’s talk about Step 3:
STEP 3: Figure out the right panel size
In our previous articles we talked about how nice it would be if you could use the rated power Wmp of the solar panel to calculate the energy that could be produced in a day by a solar panel – just like you use the rated power of the device in Step 1 when you calculate the energy needed by the device you want to run. But there are some problems with doing that. To explain this better, I’ll talk a little more about solar panel ratings.
The first problem with using Wmp in Equation 1 (May newsletter) is that the manufacturer’s power ratings for solar panels are measured in the lab under specific test conditions, called STC (standard test conditions). The goal of these measurements is to be consistent, so that one solar panel can be accurately compared with another. Unfortunately, they are out-of-the-box lab values, not real-world values. An independent system has been developed using a different set of test conditions (PTC, or Performance Test Conditions) to create a more realistic rating. Ratings produced using the independent PTC ratings are typically 10-15% less than ratings produced. This may make the STC ratings seem like a “scam”. You could also think of it as the first stage of a work-in-progress, with the positive side being that an improved system is being developed.
The second problem with using the manufacturer’s power rating Wmp in Equation 1 is that the power output of the solar panel is not really measured directly. It’s calculated from measurements of voltage and current. You will see these ratings listed as Vmp and Imp alongside the power rating Wmp of a solar panel. Vmp (voltage at maximum power) is the voltage the panel will produce at maximum power. Imp (current at maximum power) is the current the panel will produce at maximum power.
So for example a 10W panel might have these ratings.
Wmp = 10W Vmp = 17.5V Imp = 0.571A
The rated power of the panel Wmp is calculated by multiplying Vmp by Imp. You could check for yourself that multiplying the 17.5V X 0.571A comes out to the 10W that the panel is rated to produce.
You may have heard that solar panels produce 12V, not 17.5V. The 12V is what is called a “nominal” value – it’s used to distinguish a solar panel designed for 12V devices from a solar panel designed for 24V devices, or one designed for 6V devices. Actually, a “12V” solar panel produces around 17.5V, whereas a 12V device like a lightbulb really runs at about 12V. This voltage difference between the solar panel and the load is the second problem with Wmp.
Because only 12V gets used by the device, the difference (17.5V – 12V) is not really useable voltage. So, part of the Wmp of the panel is not really usable power. If the panel is only supplying 0.571A, and the lightbulb is running at 12V, 0.571A, then the lightbulb is only running at 6.9W even though it’s hooked up to a 10W solar panel.
So here’s what to do about these problems with Wmp.
1. As a first rule of thumb, always add 10-15% to your energy needs, to make up for the real-world conditions that make the manufacturer’s ratings higher than the STC ratings.
2. The second rule is to avoid the inaccuracies that come from the voltage differences by not using the rated power at all. Instead, we’ll show you in the next newsletter how to use the rated current Imp of the solar panel. The current is what actually runs an electric device – the voltage is what makes the current flow. As you’ll see, using Imp instead of Wmp really makes the calculations easier.
All prices are in USD.