14kW Tesla solar plan; DC/AC ratio of 0.8?

[TesslaBull]

I just got the plans for a Tesla Solar 14kW system. I'm concerned about the inverter size, but not because they are too small which is the concern I often see.

AFAIK, it is common/optimal to somewhat undersize the inverter to arrive at a DC/AC ratio of ~1.25 although I understand there is some leeway here and can actually go higher and actually improve overall power generation.

My concern is that they have a 10kW inverter connect to 7600 watts of panels, and a 6kW inverter connected to 6400 watts of panels. Tesla has them listed as 9600 watt and 5760 watt. Using these numbers, I get DC/AC ratios of 0.79 for one array, and 1.11 for the other. This seems highly unusual to me. My concern, especially with the .79 array is that overall production will be drastically lower than an optimally configured system with a smaller inverter of perhaps 6kW for a 1.25 DC/AC ratio.

I'm in PA so I don't have extraordinarily high production or optimally oriented roof. My 14kW system is only being guaranteed to produce 14,800kW/year. So I can't see how clipping would be a big concern.

 

[wwhitney]

My concern, especially with the .79 array is that overall production will be drastically lower than an optimally configured system with a smaller inverter of perhaps 6kW for a 1.25 DC/AC ratio.

I don't believe that's correct. My guess would be that the 0.79 ratio array might perform epsilon better. But for any practical purposes, they will perform the same.

Cheers, Wayne

 

[TesslaBull]

I went onto PVwatts to estimate production using the system design, and then using smaller inverters to increase the DC/AC ratio. I only increased production by 89kW/year. The tradeoff with undersized inverters, to my understanding, is that cost is lower(possible plus), production can actually be higher(another plus), but since they run closer to peak capacity, they generate more heat which could affect longevity. So maybe a small loss in possible production is worth cooler, potentially longer lasting inverters. And since larger inverters cost more but were included in my design, it may be a good thing. So the added cost is not a downside for me.

I sent a message to Tesla an waiting to see how they respond.

 

[yblaser]

I have a 0.73 ratio and haven't seen any adverse impacts. Inverter efficiency is usually pretty flat until you get down to less than 10% of rated power. At that point you aren't generating much power anyway so any impact because of inverter efficiency is small.

 

[abp5070]

I just got the plans for a Tesla Solar 14kW system. I'm concerned about the inverter size, but not because they are too small which is the concern I often see.

AFAIK, it is common/optimal to somewhat undersize the inverter to arrive at a DC/AC ratio of ~1.25 although I understand there is some leeway here and can actually go higher and actually improve overall power generation.

My concern is that they have a 10kW inverter connect to 7600 watts of panels, and a 6kW inverter connected to 6400 watts of panels. Tesla has them listed as 9600 watt and 5760 watt. Using these numbers, I get DC/AC ratios of 0.79 for one array, and 1.11 for the other. This seems highly unusual to me. My concern, especially with the .79 array is that overall production will be drastically lower than an optimally configured system with a smaller inverter of perhaps 6kW for a 1.25 DC/AC ratio.

I'm in PA so I don't have extraordinarily high production or optimally oriented roof. My 14kW system is only being guaranteed to produce 14,800kW/year. So I can't see how clipping would be a big concern.

Depends on which direction your panels will be facing. I have 26 panels (10.4kw) all facing SW. Production is estimated 13,300. I am also in PA near philadelphia.

 

[TesslaBull]

I reached out to Tesla, and they got a response from engineering which they passed along. I was hoping for technical details as to why they chose a 9600 kW inverter for 7600kW of panels, but all I got was basically that the system is designed to be as optimal and efficient as possible.

One thing I did notice on my own was that the Delta inverters seem to have a very low startup voltage of 35 volts for all their inverters regardless of size. Unless I understand things incorrectly, this is what typically allows smaller inverters to produce power earlier in the day and later in the afternoon. If Delta's large inverters turn on early and stay on late just as well(as an undersized inverter) during off peak conditions, the larger size should be a great thing as they will potentially run cooler, never clip, etc. Larger inverters also cost more afaik so I'm getting it for no extra charge.

Also, cheapest quote I have gotten other than Tesla has been $2.80/watt:
11.7kW with 12,909kW production for $32,765, I keep SREC's
Tesla: 14 kW with 14,800kW production for $27,500, I get no SREC's.

Tesla is $5265 cheaper upfront, and produces $378 more electricity/year. SREC's estimated at $505/year and subject to go down or go away over time. BEST case scenario, SREC's net me +$127/year after paying over $5,000 more upfront.

Gonna submit plans to HOA. Tesla is applying for permits.

 

[dareed1]

I reached out to Tesla, and they got a response from engineering which they passed along. I was hoping for technical details as to why they chose a 9600 kW inverter for 7600kW of panels, but all I got was basically that the system is designed to be as optimal and efficient as possible.

One thing I did notice on my own was that the Delta inverters seem to have a very low startup voltage of 35 volts for all their inverters regardless of size. Unless I understand things incorrectly, this is what typically allows smaller inverters to produce power earlier in the day and later in the afternoon. If Delta's large inverters turn on early and stay on late just as well(as an undersized inverter) during off peak conditions, the larger size should be a great thing as they will potentially run cooler, never clip, etc. Larger inverters also cost more afaik so I'm getting it for no extra charge.

.....

As yblaser pointed out earlier, the idea that smaller inverters provide better efficiency at low irradiance is technically probably true, but the effect for modern inverters is negligible. I'm not sure if the start up voltage being 35 volts is meaningful, but Delta claims MPPT operates from 50 to 480V for all the inverters in the Mx-TL-US series. If PA is anything like CA, your installation will also include MCI or RSS devices, which themselves can limit production early/late in the day.

I suppose that Tesla is offering a 10kW inverter because they already have some in inventory. (They use their own inverter in most of the US.) The 10kW M10-TL-US inverter has 2 MPPT channels, while the recently introduced M10-4-TL-US has 4. The 6 and 8kW inverters have 3. This only matters if you have more than 2 panel orientations going into the 10kW inverter. I guess that Tesla proposes to install the 2 MPPT channel version.
Delta Inverters