There are many factors that go into choosing a panel wattage for your project; power density, inverter compatibility, physical size, availability, cost. It can be a difficult decision that comes down to a lot of the specifics of your project. We’ll take a look today purely at panel wattage and some of the implications it can have on a project.
When looking at panel wattage, one of the biggest factors to take into consideration is the physical footprint of the module. Generally speaking, the higher wattage a panel is the larger the footprint for that module will be. This obviously has a large bearing on how many modules you can fit into a given space and is very important when you’re working with a limited roof space. Modules come in 2 general physical layouts; 60 Cell/120 Half-Cut Cell and 72 Cell/144 Half-Cut Cell. The 60 Cell/120 Half-Cut Cell used to generally fall into a footprint of 65” x 40” and the 72 Cell/144 Half-Cut Cell in a footprint of 80” x 40”. With the advent of changing cell technology and individual cell sizing inside of panels the physical footprint of modules has started to become non-standard and should be taken into consideration when looking at a penl and not taken for granted based on the cell quantity designation.
Another hugely important factor when deciding between a higher wattage panel and a lower wattage is the inverter system with which you will be pairing your modules. For most microinverter and optimizer systems the manufacturers will have a maximum DC power input for each individual unit that cannot be exceeded. So in the case of something like the P400 SolarEdge optimizer, the rated panel output power to be paired with this optimizer cannot exceed 400W. So it’s important to confirm these when making your selection. Most inverter manufacturers will have a calculator on their websites to indicate whether or not a specific panel will be compatible with a specific inverter or optimizer.
In a similar vein, you will want to check your DC to AC ratio between the PV panel and the inverter. An ideal DC to AC ratio is 125%, for example pairing a 375W PV panel with a 300W AC output microinverter will give you this ideal ratio. The idea with this is that each inverter system has a minimum start up voltage that needs to be reached before it can kick on and start converting from DC to AC power. By oversizing your panels slightly you meet that start up voltage requirement earlier in the morning and maintain the voltage needed to operate the inverter later into the afternoon giving you several extra hours of production per day. While you may get some clipping at peak production hours during the middle of the day, you make up for this significantly over the course of a year by having those extra hours per day that your inverter will be operating.
So in conclusion, it often isn’t the most appropriate to choose the highest wattage panel in a given situation but is highly dependent on your specific use case. There are many other factors to consider when choosing a panel, but hopefully this gives you a few to consider when making your decision.