DG size much lower than designed AC KW. Is that ok? (IQ8+ inverter with 395 panel)

[Amyyuma]

Help! Anyone knows if the DG size (distributed generator) on application to energy company can be significantly lower than the designed AC KW?

We are in the process to install photovoltaic panel on roof through a company. They quoted and signed contract with us for a 9.48 KW DC system peak power for 1st year annual production of 9,127kwh using 24 REC395AA panel and 24 inverter from Enphase Energy, Model: IQ8PLUS-72-2-US. They recently submitted application to energy company for a DG size: 7.2kw. It's such a big difference between 9.48 kw to 7.2kw, and they put in the contract a DC to AC ratio of 1.36. when I asked about this big difference, they said 7.2kw is based on inverter peak rating of 0.3*24=7.2 KW. Is this normal to have such large DC to AC ratio? Does that mean my peak output will be somewhat capped at 7.2kw as per application? If so, does that mean I unnecessarily paid for extra DC KW? They say this is industrial practice. Is that so?

[Ampster]

Help! ..... Is this normal to have such large DC to AC ratio? Does that mean my peak output will be somewhat capped at 7.2kw as per application? If so, does that mean I unnecessarily paid for extra DC KW? They say this is industrial practice. Is that so?

AC - DC ratios of About 1.25 are fairly common. I once had a system that was 1.5 to 1 and surprisingly it performed up to estimates despite clipping on sunny days. Some will argue that the early ramp up and latter ramp down makes up for any clipping. Are you getting a performance guaranty? Many factors go into system design and it would be hard to conclude anything without running your orientation, tilt and location into PVWatts to compare it to a similar system with more expensive higher capacity micros and a lower DC - AC ratio.

[SolTex]

We have a full Enphase system with IQ7+ inverters and LG 380W (nominal) panels. Your confusion is normal and common to new Enphase system owners. We shared your concern until I finally dug around the internet for explanations that cleared up the matter in my mind.

Your installer is giving you the correct information and valid answers, but they could do a much better job of explaining the logic behind the numbers. According to Enphase technical papers, a properly designed system using their micro inverters will have a DC-to-AC ratio of around 1.25 although this is not a hard and fast rule. Slightly above or below that number is fine, and can be advantageous in some situations. It's just a guideline.

Your installer says your ratio is 1.36 which is correct (mathematically). Your panel DC rating (STC) is 395W divided by your inverter max continuous AC output of 290W equals 1.36.

Your system cannot produce any more power than the combined max output rating of your inverters, regardless of the panel DC rating. Your IQ8+ inverters are rated at 300W peak AC output power times 24 inverters equals max peak SYSTEM output of 7,200 watts, or 7.2 kW. No, you were not cheated, and you did not pay for bigger than necessary panels. Your system is well designed according to prevailing industry practice, and should perform very well under a variety of weather conditions. You can sleep peacefully tonight.

The two links below may help. You can also do a Google search for "DC-to-AC ratio" or "ILR ratio" (Inverter Loading Ratio) to find more info.

https://aurorasolar.com/blog/choosin...rter-clipping/

https://tandem-solar-systems.com/wp-...ries-DS-US.pdf

[bcroe]

My opinion, the more clouds your area experiences, the higher the
DC to AC ratio is justified. Taking that to an extreme here, I orient
many panels E and W to avoid clipping and get a longer solar day,
under clouds any orientation works with the dispersed light. My
DC/AC is over 2.

And do not forget, some of the DC generated is actually lost in
transmission and reduced by inverter efficiency. Bruce Roe