Oversizing Inverters - Should You?

[discodanman45]

Attached is a graph that I was shown when people said that oversizing an inverter was a smart thing to do. The more I think about it, I think that the graph is outdated and doesn't give any benefits to the newer generation of more efficient inverters. If I had to recommend a system to someone after all the clipping I have experienced, I would say don't go over 1.1 DC:AC ratio. Here is my justification. The link will take you to a graph of the efficiency testing of a SolarEdge 7600 http://www.gosolarcalifornia.ca.gov/...US%20(240).pdf
As you can see from the graph it is very efficient even at 10%. The ramp up times and the energy you would gain from it would be minimal, if any. You can even argue you would lose energy because at higher loads the inverter actually loses efficiency. Can anyone give me a good reason for oversizing an inverter, besides the cost of the inverter and possible costs to modify the panel? The graph below is complete nonsense for current systems and people make it seem you are getting a better return by clipping. After seeing the efficiency graphs of new inverters I would never suggest anyone to oversize an inverter for any reason.

[JSchnee21]

If you consider just the SolarEdge lineup, the cost differentials on upsized inverters is pretty small (usually only a few hundred dollars). Assuming no heroics are needed to upgrade the electrical panel (if any upgrades are needed at all), I would tend to agree. As with many contractor and labor centric jobs, contractor and labor overhead (e.g. margin) is a big percentage of the total cost of the job. So it's much more cost effective to buy a little more up front than to risk having to upgrade again later.

Given that the Inverter is the most likely component to fail -- due to heat and electrolytic capacitor failure (for non HD models) -- I'd much rather have a larger one which was only running at 50-70% capacity than a smaller one which was getting pegged day in and day out. The one exception here might the HD series, which while the spec's look great on paper still concern me due to their lack of active ventilation and very high operating temperatures.

[JSchnee21]

One of the questions/concerns that frequently comes up on this board is the impact of the size of your load center (breaker box) and whether your are getting a Line Side or Load Side tap. In some locations, I've heard, line side taps are not permitted. So the output from the inverter must be run into the load center as a load side breaker. The size of this breaker is limited by the current carrying capacity of the busbar in the breaker panel which depends on how many amps the breaker panel is rated for. One of the reasons the 7600VA is so popular is that this is the largest size you can feed in as a Load Side tap on a standard 200 amp panel.

For me, line side taps are fine. Only real limitation (save cost) was the size of my roof, and PoCo breakpoints / restrictions on % of utilization (105%-110%) and liability (<12 kW DC as I recall)

[foo1bar]

I think the graph you attached is poorly done.

But there is a real argument to be made for having more DC nameplate wattage than the inverter nameplate.
I do not think the efficiency improvement in early AM / late PM is significant.
(I'm not sure if that's what the graph is intended to show - or if it's trying to show increased power from a larger array during those times, but getting clipped during peak hours.)

I think the real argument is the extra power/energy for not much extra cost is significant. (Or looking at it another way - the extra cost for increasing the inverter size to the DC array wattage can be signifcant and have minimal benefits)
If you have 1.2::1 DC::AC ratio compared to a 1.0::1.0 DC::AC ratio, you will see 20% more power during the day *except* when clipping.
With a 1.2::1 ratio, you will be clipping a lot less than 20% of the time. (for many cases, it'll be 0% of the time)

Why I think it was a good idea for my installation to oversize the DC significantly:

1> I've never seen the DC nameplate wattage being produced by my modules. (I'd bet that's the case for vast majority of installations - getting 300+W production out of a 300W module happens rarely if ever (maybe in snowy climates they manage to get a cold sunny day with good reflections off the snow/clouds)
1a> I don't have a perfect south-pointing roof - nor is it the perfect slope for solar.

2> I won't ever see >40% of my modules at their peak at the same time. (multiple orientations)

3> Moving to next size up inverter would have probably added ~$500, without adding any more kwh of production.


I have 8960W of DC and a 7600W inverter - and I think I could add a fair amount more DC watts without clipping.
My highest production day recently was 6/16. On that day my peak power was 6.719kW - so I could have 10% more DC wattage and still wouldn't have been clipping.
It's possible that other days had higher highs - but was cloudy at other times so wasn't highest production day, but I don't know of a good way to find that in the SE interface.
Flipping through the graphs day by day I saw the next day I hit 6.727kW - which is basically the same.
So I could have been at a 1.33:1 ratio and not have any clipping.
I could have even gone to a 1.4:1 ratio and I think it would have had such minimal clipping it would have still likely been better choice (lose up to 7% for less than 4 hours/year; gain 7% the other 99% of the year)

[bcroe]

I would say don't go over 1.1 DC:AC ratio.

As you can see from the graph it is very efficient even at 10%. The ramp up times and the energy you would gain from it would be minimal, if any. You can even argue you would lose energy because at higher loads the inverter actually loses efficiency. Can anyone give me a good reason for oversizing an inverter

I am hardly concerned about efficiency at 10% power, because one or two percent of 10% is a pretty
small number. Your situation may be entirely different than mine. But my 15KW inverter plant has
generated over 150 KWH in one day, you can't do that. It does it with carefully oriented extra panels.
The same design usually produces half that even under clouds and rain, your design can't do that.

Curves I am chasing are something like this.
Bruce Roe

PVm17Jn16.jpg

[discodanman45]

Oversizing should only be done if you plan out systems as thoughtfully as people posted. The problem is now that some companies are stressing inverters to maximize profit on their end. I don't think it is very common because many companies I get estimates from actually oversized the inverter, but I have been seeing more of it with the 155% inverters on the market. The costs of inverters has gotten to the point where going larger isn't a big deal to profit. The only two reasons to oversize in my opinion would be if you can't line side tap and can only hook up a 7600 inverter or if you live in a place like Australia that limits your inverter size.