Wattage controller

Rainbowbeach · July 2012

Basically I want to increase the output of my system in the morning and afternoon, whilst limiting the peak production to suit the inverter. My plan is to increase the number of panels and thus increase production in the lean times and by placing a wattage regulator between the panels and the inverter I can limit the amount of watts at peak times to suit the inverter. I have 5400watts on the roof with a SMA 5000 inverter, mostly the system produces around 3-4 kwh but for half the time (morning and afternoon) it is around 3kwh.
Anyone have an answer for me on how to achieve my goal.

waynefromnscanada · July 2012

Re: Wattage controller

Even if such a device existed, I believe you'd end up with a lower overall efficiency per $ spent. Yes, you could achieve a steady output for longer hours, but to get there, you'd be throwing away more energy during peak times than you'd gain in the off peak hours, and using a greater investment to get that lower overall efficiency.

vtmaps · July 2012

Re: Wattage controller

Welcome to the forum

There are two usual ways to do what you want.
1) a tracker, not feasible for a roof mount.
2) two arrays of panels, one array facing south east and one array facing south west. Each array will need its own inverter.
--vtMaps

ggunn · July 2012

Re: Wattage controller

Rainbowbeach wrote: »

Basically I want to increase the output of my system in the morning and afternoon, whilst limiting the peak production to suit the inverter. My plan is to increase the number of panels and thus increase production in the lean times and by placing a wattage regulator between the panels and the inverter I can limit the amount of watts at peak times to suit the inverter. I have 5400watts on the roof with a SMA 5000 inverter, mostly the system produces around 3-4 kwh but for half the time (morning and afternoon) it is around 3kwh.
Anyone have an answer for me on how to achieve my goal.

You don't need a "wattage regulator"; inverters already essentially have them. If you overload an inverter with current, it will just clip the excess during high production times. DO NOT overload it with voltage, though. You can add another string but do not make the string(s) longer than where string Voc on the record coldest day exceeds the max DC voltage of the inverter.

EDIT: I just saw in another sub-forum where you were asking about the max wattage for a charge controller. In your question in here you did not specify that it was a battery based system, and I assumed it was grid tied. What I wrote is true for GT inverters, and I would think it would also be true for an MPPT charge controller, but I don't know for sure. That is a bit outside my bailiwick.

BB. · July 2012

Re: Wattage controller

Why do you want to limit the output power? Is this because you have a limited main panel/branch circuit so that you cannot go over 20 amps or something similar?

As "ggunn" says, any well designed GT Inverter (SMA makes very good product) is capable of working to much larger solar arrays and limiting it output current to "rated" specifications without damage/problems.

In theory, you would need to put a DC breaker on the solar PV cabling/input to the inverter to limit maximum array current to rated wiring/input current limit to GT inverter (don't want a "large" solar array to fry the wiring or inverter PV input and cause a fire).

-Bill

Rainbowbeach · July 2012

Re: Wattage controller

Thanks for the replies , my system is a grid connect, the SMA inverter has a capacity of 550volts, each panel is 45.2volts, I was think inking of 3 strings of 11 panels, currently it has 3 strings of 9 panels, with 11 panels this could send the watts produced at midday up to around 5500watts, but only for a short period, the volts would be around 500 volts, max is 550 volts.
So the excess watts would just be clipped, this won't damage my inverter? as long as the volts don't exceed the 550 volts which is maximum. If this information is correct then each string would have 11 panels each 200watts and 45.2volts. Anyone care to calculate if my thinking is valid. At peak times now the system will produce 5100watts but only briefly, my estimate is that with the increase in panels around 5600watts at peak and increase in other times due to more panels. Just want to make sure that the inverter won't get damaged by the increase in watts. Clipped, meaning not allowing the watts to pass through.?

Rainbowbeach · July 2012

Re: Wattage controller

Bill,

So could I risk putting 5600watts through the 5000SMA inverter at peak times, the excess will get clipped?, so as to allow for greater production at other times. The voltage with 11 panels would be around 500v per string and the inverter is rated to 550volts. Excess watts is ok as these are clipped where as excess volts will damage the inverter? Is this thinking correct

Cariboocoot · July 2012

Re: Wattage controller

Rainbowbeach wrote: »

Thanks for the replies , my system is a grid connect, the SMA inverter has a capacity of 550volts, each panel is 45.2volts, I was think inking of 3 strings of 11 panels, currently it has 3 strings of 9 panels, with 11 panels this could send the watts produced at midday up to around 5500watts, but only for a short period, the volts would be around 500 volts, max is 550 volts.
So the excess watts would just be clipped, this won't damage my inverter? as long as the volts don't exceed the 550 volts which is maximum. If this information is correct then each string would have 11 panels each 200watts and 45.2volts. Anyone care to calculate if my thinking is valid. At peak times now the system will produce 5100watts but only briefly, my estimate is that with the increase in panels around 5600watts at peak and increase in other times due to more panels. Just want to make sure that the inverter won't get damaged by the increase in watts. Clipped, meaning not allowing the watts to pass through.?

You've got your Watts and Volts mixed.
The input of the SMA is limited to 550 Volts, but that is not the same as the Watts. What size/model inverter is it? Some are 3kW, some are 7kW, et cetera.
Eleven 200 Watt panels is 2200 Watts. Three such strings would be 6600 Watts total. If this is a 6kW Sunny Boy that is fine. Whereas the inverter will "clip" extra power as the others have said, there is a limit to how much they will clip. Generally 10% over is not a problem, and depending on the climate 25% over may be okay (panels get hotter midday, lose power).
If the 45.2 Volts is Voc you're fine with eleven or twelve panels per string unless you have cold temps. In that case it may exceed the maximum input Voltage of the inverter.

All grid-tie installs need to be done with proper permits and permissions, and more importantly engineering. Almost all inverter manufacturers have calculators for the array size you can put on their inverters. Use it. Putting more than the recommended limit on will void the warranty.

solar_dave · July 2012

Re: Wattage controller

If you want to add capacity how about pulling a permit and adding a few panels with Enphase inverters? A AC panel can be used as a combiner to get you to a single back-feed breaker. Of course if your up against the main panel limits for back-feeding then you have a different problem.

BB. · July 2012

Re: Wattage controller

Roughly, I use a 0.77 derating for solar arrays... Accounts for "hot weather", dusty and aging panels, wiring+controller losses, etc. So a 5kW inverter would be "maxed out" (cost effective wise) at:

5,000 watts * 1/0.77 = 6,494 watt max "cost effective" array.

Above that amount (or if you have lots of peak power in sub freezing/snow on ground days), you might want to look at enlarging your system past that point.

You will get some clipping of output power at 6,500 watt array--But not too much in my area (just south of San Francisco CA on the "bay side" of the coastal hills. It has not happened very often to my system (I maxed out around 3kW to 3.3 kW on a 3.5 kW array).

Many times, the maximum size solar array depends on the main panel for your home... For example a 200 Amp panel can have 20% or 40 Amp branch circuit for solar:

40 amps * 240 VAC * 0.8 NEC derating = 7,680 Watt maximum solar output.

There are games that can be played--You can have a 200 amp panel with a 175 main breaker and have a total of (40 amps + 200 amp-175 amp breaker=) 65 amp maximum solar branch circuit.

-Bill

ggunn · July 2012

Re: Wattage controller

Rainbowbeach wrote: »

Thanks for the replies , my system is a grid connect, the SMA inverter has a capacity of 550volts, each panel is 45.2volts, I was think inking of 3 strings of 11 panels, currently it has 3 strings of 9 panels, with 11 panels this could send the watts produced at midday up to around 5500watts, but only for a short period, the volts would be around 500 volts, max is 550 volts.

SMA has a great tool for figuring this stuff out. It's called Sunny Design and it's a free download at http://www.sma-america.com/en_US/products/software/sunny-design.html. You must either use this tool or hand calculate what your string Voc (open circuit voltage) would be at the lowest temperature your array will ever see (I use the record low temperature for the system location in the database at http://www.weather.com/) to find the maximum string length you can use. The maximum Voc is the most critical number in designing a PV system because the penalty for exceeding it is so harsh - damage or destruction of your inverter and voiding of your warranty by the inverter manufacturer. In fact, most inverters have a "black box" recorder which stores a reading of the highest DC voltage the inverter has ever experienced, so if you submit a warranty claim they can tell if you've exceeded their limit. It gives them license to void your warranty, even if you have filed a claim for something unrelated to overvoltage.

From the wording of your post it is not clear to me that you understand this: in the middle of the day in the summer when the sun is the strongest, the voltage produced by your array is actually at its lowest. The highest voltage it will produce will be at dawn following the coldest possible night.

Wattage controller

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