What is 'inverter efficiency at maximum power'?

[bernard]

I am following PVWatts version 1 Technical Reference.pdf file, and got stuck at point 10 (System Derates).
The derated DC power output is given as:

P'dc = Pdc * ξsys / ηinv

Pdc - DC power
ξsys - overall system derate (losses in the system due to nameplate direct current (DC) ratings, Inverter and transformer, mismatch, diodes and connections, DC wiring, alternating current (AC) wiring, soiling, system availability, shading, tracking error, and aging)

But what value should be assigned to inverter efficiency at maximum power (ηinv)?

Is inverter efficiency at maximum power (ηinv) in fact inverter's and transformer's combined efficiency in converting DC power to AC power:
inverters_weighted_efficien.jpg

Thank you for the reply.

[Sunking]

No one can answer that question accurately as it is equipment specific. For GTI 90%. But that is all you are going to get for napkin designs. Until you have selected equipment, you will never have the right answer. So for now worse case is 90%

[bernard]

Thank you for the reply Sunking. So for GTI solar products it's 90% ("GTI" is solar products manufacturer?)?

But as a term, can inverter efficiency at maximum power (ηinv) be in fact inverter's and transformer's combined efficiency in converting DC power to AC power?

[sensij]

Thank you for the reply Sunking. So for GTI solar products it's 90% ("GTI" is solar products manufacturer?)?

But as a term, can inverter efficiency at maximum power (ηinv) be in fact inverter's and transformer's combined efficiency in converting DC power to AC power?

GTI is not meant to be product specific in this case. In general, it stands for Grid Tie Inverter. The efficiency table you found is a fine reference for the value you are asking about. Sunking suggested 90% as a safe minimum efficiency, and you can see no values in that table are less than that.

Keep in mind not every inverter relies on a transformer, so your description is overly specific. Otherwise, you have the right idea.

[bernard]

Thank you sensij.
The reason why I posted this is because the part "at maximum power" confused me to.

I really owe you, for all the help you gave me.

[bcroe]

But what value should be assigned to inverter efficiency at maximum power (ηinv)?

I would use 96% for my inverters, some late models do even better. With a
physical layout, you can calculate the wiring losses. Bruce Roe

[bernard]

I would use 96% for my inverters, some late models do even better. With a
physical layout, you can calculate the wiring losses. Bruce Roe

Thank you Bruce.

[J.P.M.]

Looking at most inverter data sheets and product lit., a decent grid tie string inverter, if operating as part of a well designed system and close to full rating, will probably put out something in excess of 95% of what goes into it for most of its life.

FWIW, that seems to agree with stuff I've checked in my HOA. Also, my own inverter seems to run > 96% if I believe the #'s the system monitor pukes out and what I measure.

I wouldn't dispute the 90% #, except that it may be a bit more conservative than may be warranted by my experience.

[bernard]

Thank you once again J.P.M.

[Sunking]

I wouldn't dispute the 90% #, except that it may be a bit more conservative than may be warranted by my experience.

That is what engineers do. When you spec the equipment, then we fine tune it. What I would be cautious about is when manufactures specify an efficiency is at some percentage of the power rating most generally at full or 100% power. It would be very rare for one to run an inverter at 100%. Running at lower power levels in real world application, most inverters will have lower efficiency, thus why one should be conservative.

As an engineer it is much easier to explain why the system operates better than expected than it is to explain why it does not operate as expected. Conservative most everyone is happy happy, liberal and no one is happy.

[sensij]

Thank you sensij.
The reason why I posted this is because the part "at maximum power" confused me to.

I really owe you, for all the help you gave me.

You're welcome. If you haven't stumbled onto it yet, you can see part of the reason for the "at maximum power" qualifier by looking at an inverter data sheet. For example, the SB5000TL-US-22 datasheet actually plots the η value on page 3. You can see that although this model is listed at 97% in the table you found, the actual efficiency could vary from <96% to slightly over 97% depending on the system design and operating conditions.

[bernard]

You're welcome. If you haven't stumbled onto it yet, you can see part of the reason for the "at maximum power" qualifier by looking at an inverter data sheet. For example, the SB5000TL-US-22 datasheet actually plots the η value on page 3. You can see that although this model is listed at 97% in the table you found, the actual efficiency could vary from <96% to slightly over 97% depending on the system design and operating conditions.

Thank you once again sensij. This community is really lucky to have people like you.

[bcroe]

You're welcome. If you haven't stumbled onto it yet, you can see part of the reason for the "at maximum power" qualifier by looking at an inverter data sheet. For example, the SB5000TL-US-22 datasheet actually plots the η value on page 3. You can see that although this model is listed at 97% in the table you found, the actual efficiency could vary from <96% to slightly over 97% depending on the system design and operating conditions.

What I take away from the data sheet:
Efficiency is already 90% at 5% output, and 20% & 100% output efficiency are both above
97%. The 40% output best efficiency looks to be around 0.4 % better. Like everything
else, running close to highest possible voltage gives best efficiency. Bruce Roe