Choosing correct size PV panel for inverter

[stevehull]

Am looking at a lot of solar PV packages. Have decided to go with the Enphase microinverters for a variety of reasons.

But here is the quandry. A lot of selling sites couple the M250 inverters with panels that seem too small in wattage.

Let's look at the smaller M215 model for a bit. The guaranteed specs say that it can handle up to 250 watts in and will put out 215 watts. But performance data, from lots of people, shows that the M215 will put out 240 watts and some claim 250 watts out without clipping. Let's focus on this performance (real world) data.

We also know that the nameplate PV output and real world output are different. There are immediate losses of 10% and then degradation with time (10-20%).

So, let' take a 280 watt rated panel. Fresh out of the box it will put out (at best) 252 watts (0.9*280). The wiring drop is insignificant as the inverter is mounted behind the panel, but the inverter has a 96% efficiency. Thus, the output of the M215 would be 242 watts (252*.96). This is below the clipping value that many see in real world applications.

Furthermore, with time, the panel will degrade. At one year most panels have a further 10% loss so that panel that put out 252 watts, fresh out of the box, will now only put out 227 watts (252 * .9). Again, well within the M215s performance range.

The difference in cost between a M215 and a M250 is about $20. I will likely be putting up 35-40 panels, so this is not an insignificant amount of $. Additionally, the cost per watt of 300 watt, or higher wattage panels, is a lot higher than the 280 watt panels.

So why are sellers bundling 260 watt panels with M250s? (I suspect their bottom line, not mine is the factor).

Seems like I should couple M215 microinveters with a 280 watt panel . . .

Comments?


Steve