[RE-wrenches] PV watts question

Tue Jan 28 10:29:42 PST 2025

Most modern string inverters have a DC-DC boost phase to allow a wide DC voltage operation. Whatever voltage the inverter side works at is where the inverter will be most efficient, and thus generate less heat.

So it is not really accurate to say high voltage is worse than low voltage for the “heat kills” concern (which I agree with).  The same is true for DC-DC optimizers – they contain a boost/buck circuit that has an optimum efficiency point in the midrange of the DC operating voltage...

Current a bit more complicated but generally efficiency curves for inverters are pretty flat above 10-15% loading.   Obviously the same efficiency operation generates more heat when processing more power – so good thermal design matters.  And removing as much power conversion equipment from under the array should help reduce thermal load and cycling too (but that is definitely a biased opinion =)  )

Mike

Michael Mahon (he/him)
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From: RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> On Behalf Of Jerry Shafer via RE-wrenches
Sent: Monday, January 27, 2025 10:21 PM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Cc: Jerry Shafer <jerrysgarage01 at gmail.com>
Subject: Re: [RE-wrenches] PV watts question

Wrenches,
So MPPT,s working at high voltage high and high current with even with the electronic switching you will generate heat and over time heat kills
Fun times

On Mon, Jan 27, 2025, 8:04 PM Jeff Clearwater via RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>> wrote:
Ahh I see the light now on the inverter size point. Still I wonder if all MPPts are made equal in terms if they ability to clip high ratios with no ill-effects.

Sent from my iPhone

On Jan 27, 2025, at 7:55 PM, Jeff Clearwater <jeffc at villagepowerdesign.com<mailto:jeffc at villagepowerdesign.com>> wrote:
Yes I’ve wondered about this too and there is much debate on forums etc about exactly how this works.

Large PV plants have long used 1.4 order of magnitude ratios to maximize economics (but much of that was under “per watt” incentives rather than performance based incentives).

However I’ve long wondered what the true effect of red-lining MPPTs is. I’ve seen it as more of a lifetime issue but I wonder that if you consider overheating of the circuit if we might actually be clipping more than just squaring the top off the curve.   Perhaps PVWatts modeled this?  They are pretty thorough there at NREL.   Or perhaps they have a bug or sloppy programming. It would be worth asking them. Anyone got a buddy at NREL we can ask?

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On Jan 27, 2025, at 12:43 PM, jay via RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>> wrote:
HI David,

My point exactly.

On Jan 27, 2025, at 1:35 PM, david quattro via RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>> wrote:

Jay you are right - that doesn’t make sense because with more PV, the system will reach “max AC out“ power sooner every morning and hold it longer into the evening

On Mon, Jan 27, 2025 at 12:20 PM George McClellan via RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>> wrote:
Probably due to clipping.  The panels are generating more energy than the power electronics can process, so that excess energy gets cut off (or clipped).  It looks like a 1.2 ratio is optimal.

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-----Original Message-----
From: RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org<mailto:re-wrenches-bounces at lists.re-wrenches.org>> On Behalf Of jay via RE-wrenches
Sent: Monday, January 27, 2025 11:56 AM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>>
Cc: jay <jay.peltz at gmail.com<mailto:jay.peltz at gmail.com>>
Subject: [RE-wrenches] PV watts question

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HI All

I’m working with a customer who’s pretty savvy and is using PV watts for his design.

What I can’t figure out is the kwh/year goes down if he uses dc/ac ration of 1.5 instead of 1.2 which is the default.

Hes in Santa Fe NM.

.8   ratio=  1663 kwh/yr
1.0 ratio = 1668 kwh/yr
1.2 ratio =  1671 kwh/yr
1.5 ratio = 1623 kwh/yr

I was expecting that more PV = more kWh/yr.

What am i missing?

standard module
fixed roof mount
14.08 loss’s
10 degree tilt
180 degrees

jay

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