I am having a hard time figuring out the math for DC to AC size ratio. Maybe your covering it further on in the course or not. I used a online calculator and came up with a number, that seems right but who knows. Hoping someone can help me out. Lets just say I am pairing 285 Watt Modules with Enphase M250 Invertors. Typically we see the inverters topping out at about 265 watts. How would I figure out the DC to AC Ratio for this combination?
There are many factors in determining the dc to ac ratio.
You can relate the same line of thinking for microinverters as you can for string inverters and MW inverters in many cases.
1. As PV goes down in price relative to the rest of the price of the system, then the dc:ac ratio can go up.
2. If you are limited by the ac size of the system, such as how many inverters you can fit on a breaker for microinverters or what is the largest interconnection you can make on the load side (120% rule) or for a MW inverter, what is the max inverter size that the utility will let you connect.
3. Are you orientated to get full sun or are you east or west facing. Are you in an area where the air is thick (prevents full power) or are you in an area where the temperatures are extra how (prevents full power).
Remember that PV is rated at STC and conditions of STC or better are very rare. Depending on the location, STC or better conditions are far from normal. STC conditions will only happen when you have 1000W per square meter hitting the PV in the plane of the array (not at an angle) 25C cell temperature (not ambient, cell temperature is often much hotter than ambient) and 1.5 Air Mass (thickness of the air due to angle of the sun, humidity, pollution and elevation). These conditions if they do happen, do not happen long enough to make a significant dent in production in most places. You might find a rare spot on a windy mountain top above the clouds, but installer pay on Everest usually is out of reach of most companies.
Besides the STC conditions not happening much, another thing is that there are your normal losses, such as inverter inefficiency, soiling, wire loss, module mismatch and module degradation.
To give you a microinverter example that is a bit extreme. I got a good deal on 190W Enphase microinverters a few years back and installed them with 240W modules. That gives you a dc:ac ratio of 240/190 = 1.26:1. I was not concerned and did not look back. I suppose if I looked at the monitoring every day I probably could find an hour per year when the inverter is making it's maximum power, when it was cold, windy, bright and the sun was aiming directly at the PV. Maybe I could have made 5W more that day. If that module made 5W hours more per year and the inverter lasted 25 years, then that comes out to be 5Wh x 25y = 125Wh = 0.125kWh and 0.125kWh x $0.30 per kWh = 4 cents. In this example I owuld make 4 cents more electricity if I had a larger inverter over 25 years. Would you do 10x or 100x better than 4 cents? 100x 4 cents is $4. Big deal.
In your case, you said that you are using 285W modules with 250W microinverters. That is a dc:ac ratio of 258/250 = 1.14:1. I do not think you would ever be able to make 250W out of that microinverter any place on this planet below 15,000 feet elevation. I personally would be comfortable with a 1.3:1 ratio in most situations. I think that 1.2:1 is a conservative ratio and that there is no reason to ever worry about anything less than 1.2:1.
Another thing you can do is look at the PV output at NOCT, which is 800W per square meter, 20C ambient temperature and 1.5 air mass. This is going to happen a lot more often than STC and the power is a lot less. NOCT conditions performance is often about 74% of STC.
Some MW scale utility PV projects use a 1.5:1 PV to inverter ratio. This is because they have a limited interconnection for the project. For example, lets say the utility does not want more than 100MW coming to the grid at any one time. Your project is going to spend $10 million making a substation and transmission lines for an interconnection. When buying PV in bulk, the price is good and the land in the desert is cheap. I did some work for a company using this ratio for these reasons. They will have times in the middle of the day where they will be putting out 100MW and the utility likes to know that they will have no spikes above 100MW coming from that power plant.
Here are a few solarpro articles on the subject: