Solar-electric system not generating rated power

Question

I bought a solar PV system a couple of months ago. It has 11 410-watt panels for a total of 4.5 kW.

The first month it was cloudy and never generated over 3.2 kW of power. Now that it's been bright and sunny, I am still only getting 3.2 kW.

After doing some checking, it looks like the micro inverters are only rated for 300 watts each (0.2998 kW). I got the 400-watt panels to get more power but it looks like I would be generating the same power with the 330-watt panels they had.

Does this sound correct? Is a 25% overload on the inverter normal?

I think the inverters should be closer to 400 watts and the system should be generating close to the 4.51 kW.

There doesn't seem to be a reason for the inverters to be undersized so much. I live in California where we have plenty of sunny days. My system has been peaking out at 3.2 kW at 9:30 in the morning.

My house is fairly new with a 220A main. My system is from a local franchise (?) of a large national company. The system does not have batteries.

Answer 1

Solar panel rating is based on absolutely ideal conditions.

Absolutely ideal. In order to get those conditions you need to have a perfectly clear day, freshly cleaned panels, your panels mounted on a heliostat, or 1-axis tracker with elevation tuned for today, so the panels are dead square on to the sun's rays. It helps to be near the equator so you get less angled travel through the air.

If your panels are oblique to the sun's rays, which will be true all the time on all fixed installations except possibly a 5-minute period 3 days a year... then you will get less than rated power. And you can do the trigonometry to figure out to what degree the panel is degraded by not being square-on.

So you're going "well hold on! Those ideal conditions will NEVER happen for me!" Yes, and the solar panel installer should have set that expectation with you i.e. educated you as to this reality. Unfortunately when you're in a 30 minute consultation, thousands of words get said, most highly technical, and laymen just can't remember all of it.

Given that max is never gonna happen...

how do we size the inverter(s)? Is there any disadvantage to just going ahead and using a 410 watt inverter so that your best day, at least, really is your best day?

Yes, there is a disadvantage, actually. Electrical panels are limited on how much solar they can support. For instance a 100A service with 100A-bussed panel can support 3840 watts of solar inverter. Using the max possible inverter capacity limits your array size - if you had used 410-425W inverters, you'd be limited to nine of them. By dropping the inverter to 300W, you can have twelve. How much to downsize is really a system-design decision.

Panels are cheap

I know you just got done paying a 5-digit sum for all this, but the cost of it wasn't in the solar panels. Solar panels are 50 cents a watt. The lion's share of the cost was in the

• entitlements (permission to build)
• system engineering
• roof mounting hardware
• Labor (perilous work)
• wiring
• insurance
• financing
• customer acquisition / sales funnel

The low cost of the panels themselves, lends itself to solving difficult problems by throwing cheap panel at them.

So what you actually bought was a 3300 watt system, in which they enhanced your system's performance by oversizing the solar panels, because doing so was very cheap to do.

Answer 2

It is possible, even likely, that you may get more times of 3.2KW output (and thus, greater total output for a month) on suboptimal days with 410W panels than you would with 330W panels, but it's difficult to know to what extent that would apply without having the two systems set up side by side and comparing them. As panel output gradually decreases with age, full power output will also be maintained longer.

There may also be other factors in play such as the capacity of your electrical system for solar power, on both technical and regulatory levels that might preclude higher output. Technical being "it would not be safe due to the capacity and loading of various components" and regulatory being "you are not allowed to have more than X output as a homeowner."

Answer 3

NREL has a solar calculator on their website here

Using an arbitrary point in California I ran some simulations (using the default value for everything but the DC size and DC/AC ratio)

DC rating KW	DC/AC ratio	yearly AC kWh	yearly DC kWh
4.5	1.366	7610	8015
3.63	1.1	6168	6466
4.5	1.1	7647	8015

The difference in effective annual AC production is 1442 kWhs for the panel upgrade. Had you also upgraded the inverters, you would have only realized an additional 37 kWhs.

That's just in the first year. Over time the panels will degrade such that your effective DC/AC ratio will get lower. Your current setup has cushion in the DC/AC ratio so effective degradation will be less pronounced than with a smaller DC/AC ratio (bigger inverters).

Additionally, inverter efficiency is maximized at high output. If you download the hourly csv file you can compare the dc and ac production. You'll see that the ratio of produced dc/ac is lowest when output is the highest:

The output_ratio is the DC power over the AC power for each hour of the aforementioned simulations color coded by the row of the above table. You can see how the red and blue dots follow the same curve because they have the same inverter size. The bigger inverter's curve is pushed out to the right so you can't get as much AC power when DC power is low.

Here's a histogram

You can see that with a high DC/AC ratio that row 1 (your actual setup) has a huge count at its inverter capacity. This is what you're seeing day to day. Intuition is telling you that if you had big inverters that you'd simply push that blue bar to the right and it'd be equal height. However, that's not the case. If you increase the inverter capacity, what this shows is that you don't move that tall bar to the right. You just kind of smoosh it down into marginally higher production that peters off pretty quickly to the right of where you are. This just reinforces the idea that having bigger inverters would only have gained about 37 kWhs/year.

As an aside, in wholesale solar farm sizing, having a big DC/AC ratio on the order of 1.5, sometimes called the ILR (inverter load ratio) is normal. That's despite the fact that in a commercial solar farm the panels are on trackers so that they don't even have the same angular losses that you experience with rooftop solar.