Panel Sizing

[PbdBlue]

I have a Kid in my RV with two 160 watt 12 volt panels in series. Considering upgrading to residential panels. When using the Kid sizing calculator do you use the STC specs or the NOTC? Seems like NOTC would be closer to real life. Also I can get 255 watt poly panels at a very attractive price. For about $90 more per panel I can get a similar monocrystalline panel. Is it worth the extra dollars for the mono?

[Vic]

Hi PbdBlue,

Use STC specs for the PVs when using the String Sizer,  it does the deratings.
EDIT:  However,  why not run STC,   AND then use NOCT values,  as the Sizer does temperature derating on STC values,  so  anticipated Array Power,  and Charge currents are too optimistic,  IMO<.

On RVs,  often with limited roof space,  Mono PVs will use a bit less space,  as thery are a bit more efficient than are Polys.

As you know,  most PVs in the 250 W range have 60-cells.  12 V PVs usually have 36 cells.   So,  it is difficult to use 60 cell PVs along with the 36 cell PVs,  as their Vmp voltages are too far apart ...  you probably know this.

60 cell PVs are usually a much better value,  if you can use them with your RV roof space,  and so on.

Good Luck,  Vic

[PbdBlue]

Thanks. I intend to remove the 12 volt panels. RV has ample room for two 60 cell pv's. I can get 255 watt poly pv's locally for ~$185. Similar mono panels are running ~$265 and up. Leaning towards the poly panels but just trying to get a feel for how big a difference there is performance wise poly vs mono.  I'm planning on running the panels in parallel to minimize shade effect.

[Vic]

PbdBlue,

The difference twix Mono and Poly PVs is one or two percent-ish greater PV production for the Monos,  per unit area ...  something like that.  Not a big deal.   Especially when you can buy Polys locally.

Just try to buy the best quality that you can.

Some RVers have limited space,  and want as much power production possible,   then Monos may help a bit.   Just getting PVs to fit available space can be a bit of a puzzle,  as well ...   as you know.

Will look at the approximate power differences between Mono and Poly to see what things look like,   now.

Vic

[PbdBlue]

This is the panel I am considering. The sizing tool shows excessive panel wattage which I expected. The charge amperage is > than the 30A spec at 34.6A but I expect you'll never actually see near that much in real life. Also says that 1.2 Kid's are required with a footnote that indicates 1.2 is the cutoff for adding a second Kid. I'd really like to run the panels in parallel since many times with an RV one or the other panel may be shaded. Thoughts?

USOL-255-60P
Maximum Power Pmpp [W] 255
Power tolerance
0~ + 3% Open Circuit Voltage - Voc (V) 37.5
Short Circuit Current - Isc (A) 8.88
Maximum Power Voltage - Vmpp (V) 30.1
Maximum Power Current - Impp (A)8.47
Module efficiency 15.70%

Here is the sizing tool result:

PV Module Data
Power 255 Watts
VOC 37.5 Volts
VMP 30.1 Volts
ISC 8.88 Amps
IMP 8.47 Amps
VOC Temp Coef % 0.33 C
VMP Temp Coef % 0.45 Amps
Environmental Data
Coldest Ambient Temperature -22 F
Hottest Ambient Temperature 104 F
Nominal Battery Volts 12 Volts
PV Array
Number Of Modules In Series 1
Number Of Parallel Strings 2
Total Modules 2
Rated PV Array Power 510 Watts
Anticipated Array Power @ 104 F 476 Watts
Rated PV Array Current 16.94 Amps
Battery Charging Current @ 14.4V 35.4 Amps
VMP (Maximum Power Point Voltage 30.1 Volts
VOC (Open Circuit Voltage) 37.5 Volts
VMP @ -22 F 37.5 Volts
VOC @ -22 F 44.3 Volts
Charge Controller
Max Operating Voltage 150
Max Non Operating 162
VOC (HyperVOC) @
48V Nominal Bat Voltage
Maximum Number Of 3
Modules In Series
Configuration
Max Allowable Output 30
Current Per KID
Based On This Current
Max Allowable Wattage 432
Per KID Based On
This Configuration
Present PV Array Wattage 510
Of This Configuration
Design Check
Max VOC OK
Temp The MNKID Enters HyperVOC -1559.2 FÃ,°
Array Power EXCESSIVE
(Wattage)
KIDs Required 1.2

[Vic]

PbdBlue,

Thanks for the PV info.

The String Sizer is really calculating the Maximum that might be reasonable expected from the data that you entered.

A rough rule of thumb for what one might expect from a PV array is to multiply the STC PV power by 0.77.  This factor takes into consideration heating of the PVs from  mid-day full sin,  and has a reduction from CC inefficiencies.

For RVs,  the factor may well need to be about 65 - 70% of STC to try to consider that many RVs have PVs dead-flat on the roof,  with oppr ventilation around the PVs   ...  IMO.

Your thought about running two PVs in parallel due to possible shading,  is a good one.

Personally,  think that your KID will not really be over-PVed.

My opinions,  Good Luck,  Vic

[PbdBlue]

That sounds about right. I rarely see more than 200 watts out of my current 320 watt array. And you are spot on about the flat mounting. I think I'm going to go with the new panels. The local guy is running a special $350 for 2 which seems like a very good deal. Thanks for your insight. I really appreciate it.