Maximum panels to add to 10kw system

new2solar · January 2012

Hello
I've got a 10kw fixed ground mount system in Ontario. Under the microfit program, the system cannot use over 10kw of inverter capacity. Currently I've got 11.7kw of panels (4 strings of (13) 225w panels feeding 2 solectria 5kw inverters) Since the array is 400ft from the inverters and meter, and the system is a fixed mount, I'd like to add more panels to minimize system losses.I'm looking for input on how many panels to add without wasting money or frying inverters. Since the price of the panels have fallen so much, I thought about adding two more strings of 13, but I think there would be alot of wasted energy. Looking at the inverter efficiency, I think dropping the strings to 11 panels each, and adding two more strings may be the way to go. The inverter efficiency looks to be better when the input voltage is a little closer to the output voltage, rather than being too high. I'm not sure the Ontario Microfit program allows adding any panels, but I've sent in a request to find out. Any thoughts on this would be appriciated. Thanks

mike95490 · January 2012

Re: Maximum panels to add to 10kw system

To minimize losses, you used suitable grade of wire.

To maximize harvest, you need to install arrays on trackers, to get close to the full power, for more hours of the day, since you are limited on inverter capacity.

BB. · January 2012

Re: Maximum panels to add to 10kw system

Adding to what Mike says... Installations can lose as much as 3% of their output in their wiring... If you spend 3x as much money for thicker wire, you could reduce it to about 1% losses. May not make sense to change an existing install.

If your system 10kW maximum is based on inverter ratings, I like to use a derating factor of ~0.77 which allows for losses, hot panels, dirty panels, etc. For a 10kW system:

10 kW / 0.77 = 12.99 kW of solar panels for maximum "cost effective" installation (rule of thumb)

However, with GT inverters, you can add more solar panels and get more power in the mornings/evenings, and the inverters will (by design) limit themselves to their maximum rating.

If you wish to really get into the details, you can use PV Watts and output the hourly data. Pull that into a spread sheet and setup a formula that limits the output to 10kW max, and adds up the rest of the hourly data.

You can then try various data points and see what the marginal increase in total power output will be for various array sizes.

Some sample data for a 13kW system (all rest defaults) for Toronto (based on 20 years, typical date data used):

1969, 1, 15, 05:00, 0
1969, 1, 15, 06:00, 0
1969, 1, 15, 07:00, 0
1969, 1, 15, 08:00, 0
1969, 1, 15, 09:00, 2685
1969, 1, 15, 10:00, 6319
1969, 1, 15, 11:00, 8567
1969, 1, 15, 12:00, 9887
1969, 1, 15, 13:00, 10088
1969, 1, 15, 14:00, 9508
1969, 1, 15, 15:00, 7783
1969, 1, 15, 16:00, 5970
1969, 1, 15, 17:00, 2243
1969, 1, 15, 18:00, 0
1969, 1, 15, 19:00, 0
1969, 1, 15, 20:00, 0...
...
1981, 7, 22, 04:00, 0
1981, 7, 22, 05:00, 0
1981, 7, 22, 06:00, 0
1981, 7, 22, 07:00, 19
1981, 7, 22, 08:00, 2177
1981, 7, 22, 09:00, 4819
1981, 7, 22, 10:00, 6247
1981, 7, 22, 11:00, 7830
1981, 7, 22, 12:00, 9315
1981, 7, 22, 13:00, 9488
1981, 7, 22, 14:00, 8792
1981, 7, 22, 15:00, 7794
1981, 7, 22, 16:00, 6474
1981, 7, 22, 17:00, 4361
1981, 7, 22, 18:00, 1742
1981, 7, 22, 19:00, 21
1981, 7, 22, 20:00, 2
1981, 7, 22, 21:00, 0
1981, 7, 22, 22:00, 0

-Bill

new2solar · January 2012

Re: Maximum panels to add to 10kw system

I wanted to go with extra panels rather than a tracker to have no maintenance issues, or storm worries. I didn't realize PV watts had that type of info...I should be able to run a few scenarios to calculate the cost effectiveness of any futher system expansion. Thanks for the info!

Ralph Day · January 2012

Re: Maximum panels to add to 10kw system

Any changes would require ESA approval to the work...more permitting and cost. How much over the 5kw can each of those inverters accept? Look on the spec sheet for them, they may only be able to take 5.??kw. What would be the financial gain if you fry the inverters? My advice (as a microFIT owner too) would be to look at and enjoy the statements from the utility and not worry/wonder how much more it could be8).

Why were the inverters not at the array? it's much cheaper in wire to send 240vac 400 feet than dc the same distance. Maybe change inverter location?

Ralph

new2solar · January 2012

Re: Maximum panels to add to 10kw system

Hi Ralph.
First I'd like to say you've got a great system. I've taken a look on enphase at your output, it rivals some trackers in my area! As far as mounting the inverters at the house, the way I understand electricity, you'd have far less line loss running 400 volts dc ( two circuits, four 1/0 cables total feeding two inverters) 400ft than running 240v ac that distance with two cables. I also wanted to have the inverters in the house for ease of maintenance, climate controls etc. I used the Tigo system (I'll send you the link if your interested) to have similar monitoring to the enphase. The price of the solar panels have dropped like a stone, so I could add some at a reasonable cost. I talked to the inverter manufacturer who said as long as I stay within the inverter voltage and amperage specs, they'd be able to operate in derate mode all day. It was something I was pondering, but now that I've played around with pvwatts, its probably not really worth the investment.

Maximum panels to add to 10kw system

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