Damilola Ajayi

I just noticed something unusual today. If I were to size a 250/100 MPPT using a solar panel of 37.2Voc and 9.07Isc. I would do this 250/37.2 to know how many I can connect in series and 100/9.07 to know how many strings I can connect in parallel.
But today while checking Blue solar MPPTs datasheet, I noticed they specified the Max. PV array current to be 70. As opposed to 100. 100 is the output current of the MPPT.
We have a site that the MPPTs were sized using this method. And they've been operating for a while. Any help understanding how MPPTs should be sized exactly?

I am designing with a Sunny Tripower 20000TL-US. It has two MPPTs. from my calculations, each can take 75 280W Canadian solar CS6k-280P panels. Making a total of 42kWp PV array. If the nominal output is 20kW. Is there not some waste here? Also the tripower would be connected to a sunny island which has a battery bank. Would the batteries be charged by the excess power from the PV. What really is overpanelling? Can't find anything online.

If I’m using STC. I give a tolerance of 30% for inefficiency. I believe that’ll take into consideration the atmospheric conditions, possible shading etc. I have attended a training before where it was said that we should use NOCT. that’ll mean a bigger panel array in Terms of area. And more cost. Thoughts?

Marcos Buenijo wrote:

Peter Mckinlay wrote:Not being smart, its called a switch. One opens it when grid goes down and closes it when the grid is up.

Opening the switch isolates the grid so house only is on draw.

Unfortunately, this will not work. First, as Peter Smith noted, a grid tie inverter requires an external 110 VAC. However, it will not work even if this feature were somehow bypassed. Imagine you are powering your home directly off the solar panels while they're getting full sun. Well, what happens when a cloud comes by and shades the panels? The system requires a voltage buffer (grid or battery back up), which is another way of saying a means to store energy is necessary to fill in the gaps during such transients.

Matt, I recommend you investigate what is called an "AC coupled grid-tie with battery back up": http://www.wholesalesolar.com/AC-coupling.html . Basically, what this configuration does is provide an external NON-grid source for the required 110 VAC using a battery system and separate inverter. This "tricks" the grid-tie inverter into functioning. There is a little more involved, but this is the basic idea. In principle, it's possible to keep a battery system charged with a grid-tie system. Now, if the grid is lost, then an inverter on the battery can generate a 110 VAC local grid for the grid-tie inverter thereby allowing the panels to function. It seems counterintuitive, but it's possible to charge the battery with the solar array (using the grid-tie inverter to supply a battery charger) while the battery is powering an inverter that provides the 110 VAC local grid.

Hi Marcos, I believe in the grid-Tie Inverter system with battery back up. The batteries replace the function of the grid when there’s a grid outage. What happens when the grid has been out for a good time and your batteries are now dead? How are the batteries charged if your PV Inverter system is totally down now.