I've had my 18-panel (two groups of 9) PV (solar) electric system in service for about a year and recently I decided to expand it a bit after realizing that I could do so, myself, for roughly $1/watt, after tax incentives. An so it was done, with a bit of help from a friend of mine who is better at bending conduit than I: Another inverter and 18 more solar panels were set on the roof - all done using materials and techniques equal to or better than that which was originally done in terms of both quality and safety.
Adding to the old system:
The older inverter, a SunnyBoy SB 5000-TL, is rated for a nominal 5kW and with its 18 panels, 9 of each located on opposite faces of my east/west facing roof (the ridge line precisely oriented to true north-south) would only produce more than 3900 watts for only an hour or so around "local noon" on late spring/early fall summer days that were both exquisitely clear and very cool (e.g. below 70F, 21C) so I decided that the new inverter need not be a 5kW unit so I chose the newer - and significantly less expensive SunnyBoy SB3.8 - an inverter nominally rated at 3.8kW. The rated efficiencies of the two inverters were pretty much identical - both in the 97% range.
One reason for choosing this lower-power inverter was also to stay within the bounds of the rating of my main distribution panel. My older inverter, being rated for 5kW was (theoretically) capable of putting 22-25 amps onto the panel's bus, so a 30 amp breaker was used on that branch circuit while the new inverter, capable of about 16 amps needed only a 20 amp breaker. This combined, theoretical maximum of 50 amps (breaker current ratings, not practical, real-world current from the inverters and their panels!) was within the "120% rule" of my 125 amp distribution panel with its 100 amp breaker: 120% of 125 amps is 150 amps, so my ability to (theoretically) pull 100 amps from the utility and the combined capacitor of the two inverters (theoretically) being 50 amps was within this rating.
For panels I installed eighteen 295 watt Solarworld units - a slight upgrade over the older 285 watt Suniva modules already in place. In my calculations I determined that even with the new panels having approximately 3.5% more rated output (e.g. a peak of 5310 watts versus 5130 watts, assuming ideal temperature and illumination - the latter being impossible with the roof angles) that the new inverter would "clip"(e.g. it would hit its maximum output power while the panels were capable of even more power) only a dozen or two days per year - and this would occur for only an hour or so at most on each occasion. Since the ostensibly "oversized" panel array would be producing commensurately more power at times other than peak as well, I was not concerned about this occasional "clipping".
What was expected:
The two sets of panels, old and new, are located on the same roof, with the old being higher, nearer the ridge line and the new being just below. In my situation I get a bit of shading in the morning on the east side, but none on the west side and the geometry of the trees that do this cause the shading of both the new and old systems to be almost identical.
With this in mind, I would have expected the two systems to behave nearly identically.
But they don't!
Differences in produced power:
Having the ability to obtain graphs of each system over the course of a day I was surprised when the production of the two, while similar, showed some interesting differences as the chart below shows.
In this graph the blue line is the older SB5000TL inverter and the red line is the newer SB3.8 inverter. Ideally, one would expect that that the newer inverter, with its 295 watt panels, would be just a few percent higher than the older inverter with its 285 watt panels, but the difference is closer to 10%!
What might be the cause of this difference?
Several possible explanations come to mind:
Adding to the old system:
The older inverter, a SunnyBoy SB 5000-TL, is rated for a nominal 5kW and with its 18 panels, 9 of each located on opposite faces of my east/west facing roof (the ridge line precisely oriented to true north-south) would only produce more than 3900 watts for only an hour or so around "local noon" on late spring/early fall summer days that were both exquisitely clear and very cool (e.g. below 70F, 21C) so I decided that the new inverter need not be a 5kW unit so I chose the newer - and significantly less expensive SunnyBoy SB3.8 - an inverter nominally rated at 3.8kW. The rated efficiencies of the two inverters were pretty much identical - both in the 97% range.
One reason for choosing this lower-power inverter was also to stay within the bounds of the rating of my main distribution panel. My older inverter, being rated for 5kW was (theoretically) capable of putting 22-25 amps onto the panel's bus, so a 30 amp breaker was used on that branch circuit while the new inverter, capable of about 16 amps needed only a 20 amp breaker. This combined, theoretical maximum of 50 amps (breaker current ratings, not practical, real-world current from the inverters and their panels!) was within the "120% rule" of my 125 amp distribution panel with its 100 amp breaker: 120% of 125 amps is 150 amps, so my ability to (theoretically) pull 100 amps from the utility and the combined capacitor of the two inverters (theoretically) being 50 amps was within this rating.
For panels I installed eighteen 295 watt Solarworld units - a slight upgrade over the older 285 watt Suniva modules already in place. In my calculations I determined that even with the new panels having approximately 3.5% more rated output (e.g. a peak of 5310 watts versus 5130 watts, assuming ideal temperature and illumination - the latter being impossible with the roof angles) that the new inverter would "clip"(e.g. it would hit its maximum output power while the panels were capable of even more power) only a dozen or two days per year - and this would occur for only an hour or so at most on each occasion. Since the ostensibly "oversized" panel array would be producing commensurately more power at times other than peak as well, I was not concerned about this occasional "clipping".
What was expected:
The two sets of panels, old and new, are located on the same roof, with the old being higher, nearer the ridge line and the new being just below. In my situation I get a bit of shading in the morning on the east side, but none on the west side and the geometry of the trees that do this cause the shading of both the new and old systems to be almost identical.
With this in mind, I would have expected the two systems to behave nearly identically.
But they don't!
Differences in produced power:
Having the ability to obtain graphs of each system over the course of a day I was surprised when the production of the two, while similar, showed some interesting differences as the chart below shows.
 |
| The two systems, with nearly identical PV arrays. The production of the older SB5000 inverter with the eighteen 285 watt panels is represented by the blue line while the newer SB3.8 inverter with eighteen 295 watt panels is represented by the red line. |
What might be the cause of this difference?
Several possible explanations come to mind:
- The new panels are producing significantly more than their official ratings. A few percent would seem likely, but 10%? 'dunno - maybe...
- The older panels have degraded more than expected in the year that they have been in service.
- The two manufacturers rate their panels differently.
- There may be thermal differences. The "new" panels are lower on the roof and it is possible that the air being pulled in from the bottom by convection is cooler when it passes by the new panels, being warmer by the time it gets to the "old" panels. If we take at face value that 3.5 of the 10% difference is due to the rating - leaving 6.5% difference unaccounted, this would need only about a 13C average panel temperature difference
- The new panels don't heat as much as the old. The new panels, in the interstitial gap between individual cells and around the edges are white while the old panels are completely black, possibly reducing the amount of heating.
- The new inverter is better at optimizing the power from the panels than the old one.