How to calculate AC from DC system stats

[jimboski93]

My utility allows a maximum 10 kW AC for residential systems. I've pointed this out as I've been fielding quotes, and I'm getting quotes for systems that are clearly keeping the DC rating under 10 kW, and other quotes for systems at 11 kW or even 12 kW.

Is there a "conversion factor" or some other method for me to understand the AC rating on these systems?

[bcroe]

My utility allows a maximum 10 kW AC for residential systems. I've pointed this out as I've been fielding quotes, and I'm getting quotes for systems that are clearly keeping the DC rating under 10 kW, and other quotes for systems at 11 kW or even 12 kW.

Is there a "conversion factor" or some other method for me to understand the AC rating on these systems?

The maximum energy to the grid will be controlled by the inverter automatically limiting. Here the
commercial systems are rated by that AC output rating, but can be designed with a larger max DC
panel equipage. The relation is called DC to AC ratio.

The main concern of the PoCo is the peak AC power pushed onto the grid, DC:AC is usually set
higher than 1:1 for best performance. SOMETIMES the PoCo will dictate max DC:AC, I think that
should not be allowed. Bruce Roe

[oregon_phil]

Is there a "conversion factor" or some other method for me to understand the AC rating on these systems?

Bruce is trying to tell you that the size of the inverter determines the AC power going into the grid. If you get a 10kW inverter, then in theory, you can get 10kW AC out to the grid. But the actual amount of power is determined by a bunch of factors. The book Solar Power for Dummies is often referred to as a place to get educated on solar power.

It seems a very popular size of inverter for the US is 7.7kW because most 200 amp electrical panels can integrate 7.7kW solar input from your inverter without modifications. Only your electrician can tell you for sure, however. A 10kW inverter will need more electrical integration thought than a typical 7.7kw inverter.

The total DC rating of your solar panels can be limited by your inverter's DC to AC ratio. For example, if SMA DC to AC ratio is 1.4, then a 7.7kW inverter can have 7.7 times 1.4 worth of DC solar paneling or 10.78kW if all panels are oriented in the same direction. There are alot of examples with high DC to AC ratios achieved by differences in orientation. Search bcroe on this forum for his large layout. I wish I had the room.

Take your quotes and start dismantling them piece by piece. Get all the manuals from the people quoting the jobs, and get the solar power for dummies book.

[Mike90250]

To know the DC - AC efficiency, one only needs to look at the inverter specs Most are >95% conversion

But 5kw of PV will NOT produce 5KV of power. Panels are calibrated in a test lab. Your panels live in the real world, where, if they had perfect aim at solar noon, might give 80% of nameplate. But the atmospheric soup, dust, misalignment and the simple fact that it's not always noon on your panels, will reduce that even more.
Your mileage may vary. All sales droids lie.

[Ampster]

.....
Is there a "conversion factor" or some other method for me to understand the AC rating on these systems?

No conversion factor needed. The AC rating should come from the inverter described in your quote. Since your utility limits the AC capacity just look for the inverter size on your quote. If the sytem has micro inverters you may need to multiply their capacity times the number of units specified, which usually is equal to the number of panels.

Systems are often described by their DC rating. That is why you are seeing lager numbers. Efficiency is a different matter and as explained earlier will usually be better than 95%.

[SunEagle]

No conversion factor needed. The AC rating should come from the inverter. Systems are often described by their DC rating. As Bruce described the ratio of the DC capacity of the panels to the AC capacity of the inverter is known as the DC to AC ratio.

I have found that most inverters have an AC wattage rating which is how you determine the circuit breaker size for the main panel connection. The DC to AC ratio will get you the panel wattage that the inverter can handle.

[PVAndy]

I have found that most inverters have an AC wattage rating which is how you determine the circuit breaker size for the main panel connection. The DC to AC ratio will get you the panel wattage that the inverter can handle.

Mass is similar in that net metering allocation is oversubscribed. The only single phase systems to get net metering are a max of 10kW AC. We typically use 10kW SolarEdge inverters with up to 15,5 kW DC STC of panels. Note that some 10kW inverters are considered to be higher than 10kW based on max power output. See the CA inverter list for data https://www.energy.ca.gov/media/2366

Andy

[Ampster]

........We typically use 10kW SolarEdge inverters with up to 15,5 kW DC STC of panels. .......

My Solaredge system is running at a DC to AC ratio of 1.5 to 1, which is close to the max that Solaredge recommends. I do see some clipping but an analysis of different scenerios showed that the loss from clipping was not much and the earlier ramp and longer period of output got me more production than a smaller ratio. I think my POCO may have put a limit on inverter size so the higher ratio was optimal for my needs.

[RottenMutt]

on enphase, you can set a grid profile to not export more then 5 or10kw, so if you are home during the and can use the excess power then install a larger system. you can set the pool pump to run during the day, etc or more solar to cover vampire loads.

[PVAndy]

on enphase, you can set a grid profile to not export more then 5 or10kw, so if you are home during the and can use the excess power then install a larger system. you can set the pool pump to run during the day, etc or more solar to cover vampire loads.

Some utilities will accept that. We were the first installer in both Mass & Conn to get export limit systems approved. It took a lot of engineering work & a complex utility witness test.

[Will792]

Some utilities will accept that. We were the first installer in both Mass & Conn to get export limit systems approved. It took a lot of engineering work & a complex utility witness test.

I didn’t have any problems with approval of my 17KW system with 72 Enphase IQs in CT EverSource territory. I used 1.35 to1 DC to AC ratio, which probably was a bit too low, since my roof has East-West 4/12 pitch planes. I see very little clipping, mostly in May.

[Will792]

My utility allows a maximum 10 kW AC for residential systems. I've pointed this out as I've been fielding quotes, and I'm getting quotes for systems that are clearly keeping the DC rating under 10 kW, and other quotes for systems at 11 kW or even 12 kW.

Is there a "conversion factor" or some other method for me to understand the AC rating on these systems?

It is not hard formula but rough estimation of number of hours in a year when panels would produce more power than inverter(s) can convert. Since different houses design, roof orientations, temperature, air gap between roof and panels, thermal coefficient of panels and so on affects this estimated amount of power “lost” due to clipping it is difficult to get precise number. Most likely you do no really need precise number and 1.2 to 1.5 range is a ballpark target.

Personally I consider 1 to 1 DC to AC designs to be wasteful, unless for some odd reason you want to convert every single drop of photons for a few minutes in days in May when it is sunny but cool but pay extra for wiring, permitting, inverters and so on that would be used well under 100% all other times.

[Ampster]

......
Personally I consider 1 to 1 DC to AC designs to be wasteful, unless for some odd reason you want to convert every single drop of photons for a few minutes in days in May when it is sunny but cool but pay extra for wiring, permitting, inverters and so on that would be used well under 100% all other times.

I agree, and I have a system that has a DC to AC ratio of greater than 1 to 1 with shade and two orientations. Part of my system is as high as 1.5 to 1. I was also constrained by my utility with a kW AC limit, but the economics suggested that i could optimize total generation in kWhrs by increasing the DC capacity.