# Efficiency corrections

Registered Users Posts: 5
First, let me say that I really appreciate all the courteous and patient help from the real experts on this forum. You guys are something else!

My questions are about the "rules of thumb" I see here for derating the usable output from solar panels.

I think I understand the 77% figure I see; that's the estimate of solar panel rated output that actually gets provided through the controller to the batteries. Is that right?

What is the "52% end-to-end" efficiency? What does that take into account. And where does that number come from?

Also, the PVWatts web site uses, as a default, 77% as its DC-to-AC derate value. Is that the same as the 77% figure you guys use in your calculations. If so, should it somehow be backed out of calculations using the website PVWatts values to eliminate double-correction.

Example: Let's say I have 1,000 watts of solar panels, and a PVWatts website value of 3 KWh/sq m/day. What would be my actual usable power from the panels?

Would it be: 1,000 x 3.0 x 52% = 1560 watts

Or perhaps: 1,000 x 3.0/0.77 x 52% = 2025 watts

Or ????

• Banned Posts: 17,615 ✭✭✭
Re: Efficiency corrections

All good questions. Let's try for some good answers.

Yes the 77% derating is the average output of panels and an MPPT charge controller during the hours of equivalent good sun. So you may have a 1000 Watt array and 5 hours of good sun but instead of getting (1000 * 5) 5000 Watt hours from it you more realistically can expect (1000 * 0.77 * 5) 3850 Watt hours.

Curiously, the derating factor is the same in PV Watts because that is designed for grid-tie systems. Their AC output is roughly the same as the array & charge controller on an off-grid system because the technology is very similar. So that same 3850 Watt hours DC going into a battery bank on a off-grid system would be 3850 Watt hours AC from a GTI.

The 52% end-to-end derating is for off-grid systems and takes into account not only the 77% derating but also the additional losses of putting power into and taking it out of batteries as well as inverter consumption and conversion efficiency. So there we'd see an off-grid system with a 1000 Watt array and 5 hours of equivalent good sun be capable of (1000 * 0.52 * 5) 2600 Watt hours AC. This is a derating that can be improved by adjusting power use to take advantage of Watt hours the panels can produce once the batteries are full (and as charging requirements do not take full power) that would otherwise not be harvested.

They are both "typical" deratings and actual system performance can be affected by many factors particular to the installation such as high/low ambient temperatures, fog, insolation, and altitude.

Does that make it any clearer?
• Registered Users Posts: 5
Re: Efficiency corrections

So I can conclude that if I'm considering an RV with DC loads only supported by the battery bank - ie, no inverter - I should be able to do a lot better than the 52%.

I'm still a bit fuzzy on how to calculate PVWatts into the picture. If I use the website to find how much good sun for a given month and location, do I then need to derate that by another 77% when sizing my system?
• Banned Posts: 17,615 ✭✭✭
Re: Efficiency corrections

So I can conclude that if I'm considering an RV with DC loads only supported by the battery bank - ie, no inverter - I should be able to do a lot better than the 52%.

I'm still a bit fuzzy on how to calculate PVWatts into the picture. If I use the website to find how much good sun for a given month and location, do I then need to derate that by another 77% when sizing my system?

PV Watts uses historic weather data to determine how much insolation (hours of equivalent good sun) the panels receive. The 77% efficiency derating occurs 'after' the sun falls on the panels. This is included in the PV Watts calculator as the "DC to AC derate factor" and is a variable. No additional reduction is needed. Keep in mind that it is meant for grid-tie systems (hence the 77% default factor).

Some people go crazy on the calculations, figuring in every V-drop in wiring and different battery efficiencies, et cetera. This is not necessary unless you have some variation in design that would skew the efficiency at one point, such as a very long wire run from array to controller/GTI. For the most part the day-to-day operational changes will be a bigger factor in performance and so taking into account every tiny loss in the system is not going to make much difference. It's more to give you a reasonable expectation rather than an absolute guarantee of power production.
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Efficiency corrections

Yes you will be able to do better, how much?>> who knows as it depends on the type and length of those loads...

A wink and a nod to the guy that faithfully keeps track of his battery SG 8) as time goes on, it helps to find a failing cell or 2 as they go south..

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