Panel Efficiency  

How much power do you really get from a solar panel?

Solar panels come in many different sizes and from many different companies. They all have the same basic characteristics. They are comprised of photo voltaic cells that convert the light from the sun to electric energy.

A solar system is the entire system that goes from the solar panel through electrical cable to the DC power inverter, and out the other side as AC power into your home electrical panel. Each of these separate parts will lose a certain amount of power, as it travels from the panels to your breaker panel. This is a normal process and can be calculated for each solar system based on the components of the system.

Each panel, each set of cables and each inverter have different levels of power loss. It is required that the manufacturer provide the level of loss as part of the data sheet for each component of a solar system.

As well, outside factors affect the overall system performance. These include: shading, clouds, temperature and how clean the panels are as well as, how old the panels are.

Below is an example breakdown of a Kaco string inverter vs Enphase micro-inverters. This also, includes the various outside factors that affect panel production. The results are similar for any string inverter vs micro-inverter.

We will use a 300-watt panel as an example.

The 300-watt panel is rated to produce 300-watts per hour at peak performance, in ideal lab conditions. These conditions include an operating temperature of 25 degrees Celsius. Each panel will have a rating for how much power will be lost due to increases in exterior temperature. For this example, we will use 0.5% of power loss for each degree of temperature rise.

Here on the coast, we generally have an average temperature on the roof, of about 30 degrees when the panels are in operation. It can get much hotter but, this is an average for the year and the hours of panel productivity.

This means that the 300-watt panel will lose 2.5% of productivity just because of the surrounding temperature. Effectively the 300-watt panel will now only produce 292.5-watts.

The next level of power loss, is through the cables from the panel to the inverter. The longer the cable run, the more power loss. This is especially true for DC cables. There can be as much as 2% loss on normal length cable runs and more on longer cable runs. We will use 2%.

This means the 300-watt panel connected to a wall mount string inverter will lose another 2% of power getting to the inverter. We are at a 292-watt level for the panel due to heat + another 2% loss from cables = 286.65-watts of power production per hour.

The Enphase micro inverters do not suffer from this loss as they are connected directly to the panels. They remain at 292-watts per hour.

Next, we look at what it takes to start the system. How much power needs to be generated by the panels for the system to actually start working. The Kaco string inverter, requires a minimum of approx. 125 watts to begin operation. Depending on the number of panels in the string, this can delay the system going into operation by as much as 30 minutes compared to the Enphase micro-inverter system. Enphase micro-inverters require far less power from the sun to go into operation. This can be as low as 1 watt. As well, the Enphase micro-inverter continues to work with less light than the Kaco string inverter. This means that the Enphase system will start operation sooner and run longer than the string inverter. While this may not be an extended period of time, it does add up.

The string inverter can easily produce 1% less per day vs the micro-inverter.

String inverter: 286.65 – 1% = 283.75 -watts per hour

Enphase micro-inverter = 292.00-watts per hour

Now we get to the inverter. The inverter converts the DC power from the solar panel to AC power that can be used in your home. The Kaco string inverter has one of the best conversion rates available for a string inverter. It still losses 4% in conversion. 284 – 4% = 259-watts per hour.

The Enphase inverter is 97% effective in power conversion: 292 – 3% = 283.24

At this point, we have a comparison of power production of:

String inverter – 300-watt rated panel actual production of 259-watts per hour

Enphase micro-inverter – 300-watt rated panel actual production of 283.24-watts per hour.

The next comparison, is the time it takes to get to maximum power output from the panel. Because micro-inverters start sooner and run longer, they get to maximum performance more quickly than string inverters. This translates to an increase in power production of at least 2.5% compared to the string inverter.  We show this as a loss of power per hour by the string inverter: 259 – 2.5% = 252.5-watts per hour.

The next comparison is shading. A big difference between string inverters and micro-inverters is that string inverters read all the power from each panel as a group. If one panel is in shade, or it is covered in bird poop, it will reduce the production of all the panels to this low production level. On the other hand, micro-inverters are connected to each panel and process the power of each panel individually. This can have a huge effect on power production. The micro-inverter system separates the panels and if one is in shade or covered in bird poop, it will not reduce the production of the other panels. In fact, this will, on average, make about a 2.5 -5% difference in total power production. Again, we reduce the power of the string inverter panel to demonstrate. 252.5-watt – 2.5% = 246-watts per hour.

Finally, we look at another common problem here, heat. A by-product of the conversion process, taking sunlight and converting it to electrical energy, is heat. This heat, in combination with the ambient heat, will lead to a problem called clipping. When a solar panel reaches its maximum temperature, it has a special safety feature that automatically shuts the panel down for a brief period. The panel then automatically goes back into operation. Not all panels will shut down at the same time, nor will they return to operation at the same time. The problem is the same as shading. When a string inverter panel shuts down, due to heat, it effectively reduces the power output of all the panels in the string. This is very brief, but it reduces the power output. The panels need to return to full power and this takes time. With micro-inverters, one panel clipping does not affect the entire system, only the individual panel. This can have a huge difference in the overall performance of the system. We calculate this as a 5% difference for our example, once again reducing the power output of the panels connected to the string inverter. 246-watts per hour – 5% = 234-watts per hour.

So, you can clearly see that the Enphase micro inverters will consistently outperform string inverters. In fact, with our example, you can see that the level of power production is substantially higher with micro-inverters compared to string inverters:

Micro-inverter average production with a 300-watt panel = 283-watts per hour

String inverter average production with a 300-watt panel = 234-watts per hour

So, the average per day production between these two types of inverters (based on 5 hours usable sunlight per day) is:

String inverter – with a 300-watt panel = 1.170 kilowatt.

Micro-inverter – with a 300-watt panel = 1.415 kilowatts.

Conclusions:

Our example shows over a 17% increase in power production with the micro-inverter. We have been able to demonstrate as much as a 14% increase with our demo systems that we currently have in operation. Given that we do keep our panels very clean and there is little shading effect, there may not be the additional loss due to shading etc. As such, we are very close to the forecast in the above example.

Cost comparison:

Until recently, it was going to cost more for a medium sized solar system using micro-inverters than it would for a wall mount conventional string inverter. Not anymore! With the introduction of the new IQ – 6 and IQ-6+ micro-inverters from Enphase, micro-inverters are now the most cost-efficient option for all home installations. Our example above used the numbers and output of the M250 micro-inverter. The new IQ-6 and 6+ will allow for up to 15% more power throughput than the M250. This means smaller systems for the same amount of power production and therefore less cost to you.

You can download this document by clicking on the PDF file here

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