Help me understand overpaneling

[Dzl]

I am in the process of designing a solar system for a live-in bus build. I haven't figured out my energy budget (and won't be able to do so accurately until further along in the build) but I am estimating a 200-400 AH battery bank @ 12v. For maximum parts compatibility and to reuse components I have from a previous vehicle I would like to stick to 12v. Unfortunately that means that the bottleneck based on my budget will be the solar charge controller (currently I'm planning to use the Victron 100v/50a smartsolar controller).

Because of the relatively low cost of solar panels compared to other components such as the charge controller, and the fact that it seems they rarely operate at maximum output, I'm attracted to the idea of overpaneling, but I'm having trouble finding information or guidelines on how to properly do so (victron has a good explanation here). Based on the datasheet of the charge controller I'm looking at, I believe I can safely overpanel as long as my array doesn't exceed the controllers Max VOC (open circuit voltage) of 100V or the lsc (short circuit current) of 60A. If I'm understanding this correctly (and assuming that the short circuit voltage refers to the input side of the controller not the converted 12v output) that means the theoretical maximum array size would be 6000 watts (way more than my needs of <1000 watts).

Is my understanding of how to calculate safe overpaneling correct? are there other safety considerations?

Also from the standpoint of efficiency, I wonder what a reasonable amount of overpaneling would be? 125%-150% charge controller output capacity feels like it might be a reasonable number in order to account for efficiencies incurred from flat roof mount panels, heat, etc. Is there a 'rule of thumb' or a formula people use to determine this?

 

[Steve_S]

You have it pretty well understood. Victron like other Tier-1 products can handle a "BIT" of over panelling. It has nothing to do with downstream from the Charge Controller. Panels never produce the rated amount unless it's miraculously aimed properly and in perfect conditions, which never happens. Safe region is no more than 15% but better to reserve to 10% over. go too far and you see magic smoke and suffer a wallet injury.

 

[Dzl]

You have it pretty well understood. Victron like other Tier-1 products can handle a "BIT" of over panelling. It has nothing to do with downstream from the Charge Controller. Panels never produce the rated amount unless it's miraculously aimed properly and in perfect conditions, which never happens. Safe region is no more than 15% but better to reserve to 10% over. go too far and you see magic smoke and suffer a wallet injury.

Thanks Steve, this is helpful feedback. I do have one point of confusion regarding how the safe figure of 10-15% is derived. If I'm reading the Victon documentation correctly, their 100/50A controller can supposedly handle a maximum input of 60A @ 100V (6000 watts), but only output 50A @ 12V/24V (700/1400 watts).

If I'm understanding correctly that means that in theory the controller could theoretically handle an input that is 400% higher than its rated output at 24v or 800% higher at 12v. What accounts for the discrepancy between the 10-15% and the much higher numbers I am deriving? Is the 10-15 more of a safe rule of thumb across all brands and controllers? or am I musinderstanding something or overlooking some risk (I'm pretty new to this, there is a lot to wrap your head around).

 

[Steve_S]

I'm a "little" over-panelled because I have a fixed ground mount and with my locality it's needed. the 10-15% is to prevent harming your gear - safe zone as most Tier-1 controllers handle it (just differently). I do not use a Victron so I cannot speak to them specifically. For example, I have seen my panels hit very close to max output (lot has to do with the physical location and I'm just 5% over in my config) and saw the heat etc on my controller (Midnite Classic).

The numbers on the panel side VS batt side. You may be thinking Ohh look at the high DC Wattage & Volts BUT the trick in the controller is to downstep, regulate & actually amplify (amperage) the force. Ohhh that's terrible but it'll have to do... it's a long discussion, bottom line don't think of things like "theoretically handle an input that is 400% higher than its rated output at 24v or 800% higher at 12v" it doesn't work that way, you'll overload yourself and your gear.... honestly, apply KISS approach. It is extremely easy to get caught in details and minutia and become mentally overloaded with information & data. Hence why a lot of folks gravitate here & to Will's video's, to try & get answers & solutions. There's a heap'o phooey out there too.... .

 

[Mars]

I would not call it "over paneling". That is misleading. I have always referred to the ratio of the solar array DC wattage to the watt rating of the grid tied inverter or charge controller as the load factor. In the past I have designed systems with a target load factor of 120%. The last system I designed has a load factor of 143%. This system is an off grid system using four REC315NP modules configured as two strings of two modules in series. The charge controller is a Victron 100/30 100 Volt, 30 Amp with a maximum power rating of 880 Watts at 24V. 1260W array/ 880W = 143%.

One of the absolutely critical parameters that you need to pay attention to is Voc. Note that that Voc show in module specifications is at Standard Test Conditions (STC). At low temperatures Voc will be significantly higher. At -25C (-13F) Voc will be 13.5% higher per the temperature coefficient (-2.7%/C) of the REC315NP modules. 40x13.5% = 5.4 volts so two modules in series is 90.8V at -25C.

With a MPPT charge controller, the charge controller will move off the maximum power point if the current starts to exceed the capacity of the charge controller. This will limit the power output and is a condition known as "clipping". With grid tied systems at a load factor of 120% in Northern New Mexico, clipping is insignificant (less than 0.5% of the annual energy output available from the array).

Mars

Here is a copy of the REC315NP module data for reference:

 

[Thor]

Just a quick note.

I have a victron smart solar mppt 100/30 with a 24v system that generates a maximum of around 800 watts to the batteries. I've been running 12 100 watt panels in a 4s3p arrangement for over a year without any problems.

I've also run 16 100 watt panels in a 4s4p arrangement with a victron smart solar mppt 100/20 with the same 24v system for a few weeks without any problems as a test.

The panels are east/west facing and some shading at times during the day, so that's why I use the extra panels, but everything works fine.

 

[radial1951]

I am in the process of designing a solar system for a live-in bus build... Based on the datasheet of the charge controller I'm looking at, I believe I can safely overpanel as long as my array doesn't exceed the controllers Max VOC (open circuit voltage) of 100V or the lsc (short circuit current) of 60A. If I'm understanding this correctly (and assuming that the short circuit voltage refers to the input side of the controller not the converted 12v output) that means the theoretical maximum array size would be 6000 watts (way more than my needs of <1000 watts).
Is my understanding of how to calculate safe overpaneling correct? are there other safety considerations?

Dzl, you are correct. Sorry, but I think some of the above posts are missing the point and could perhaps read the Victron information. I have been researching exactly this matter for some time. The Victron 100/50 at 12v has a nominal 700W input power limit. You can connect as big a PV array as you like as long as the total panel specs do NOT exceed the total Voc and Isc values permitted by the 100/50 ie 100v and 60A. You obviously have read the relevant pages on the Victron website, so just be careful how you arrange the panels as far as Series and Parallel strings are concerned. I would actually like to know if the Renogy Rover 60 has a similar ability to be over panelled...
Regards, Ross.
See https://www.victronenergy.com/blog/...ar-modules-to-the-new-mppt-charge-regulators/

 

[Stupid Monkey]

My MPPT advises 20% over the max power out, and says you can double it.
It just ignores excess current.

I have a 30A 12v MPPT and ~700watts solar works well for uk cloudy skies and no problems on sunny days.

ASBO/ Kaise 30A DC/DC and MPPT combined similar to that renogy van Unit Will did a video on the other week

 

[krby]

Complete solar novice here. It seems like there should be some sort of "dump load" for solar. I'm imagining a very "over paneled" PV array, that can still produce ok power in less than perfect conditions. But then a device between the PV an the charge controller that is set to divert anything above a specified current (you'd still have to manage voltage to keep things easy). So if you MPPT is 100V and 60A max, you could panel so that you could hit 150A in perfect conditions and this made-up dump-load device would draw off everything above 60A.

It'll have the same problems as a wind dump load, if it's a big resistor it's going to have to be BIG and have to keep cool or heat water, or something. I'm literally thinking this thru right now, so maybe this exists or (more likely) is a dumb idea.

 

[gnubie]

A decent solar charge controller won't care if there are more amps available from the array than it is rated for, it'll just go into current limiting and the array voltage will rise up but never exceed Voc. You will be throwing away capacity but nothing bad will happen. With wind voltage will continue to rise as the generator/alternator RPM increases so you either have a really really over-engineered charge controller to cope with the potentially really large swings in voltage, or you have a dump load to burn off excess production.

If you want to do something with excess solar production the easiest, but not totally easy, thing to do IMO is to throw it at water heating.

 

[krby]

A decent solar charge controller won't care if there are more amps available from the array than it is rated for, it'll just go into current limiting and the array voltage will rise up but never exceed Voc. You will be throwing away capacity but nothing bad will happen.

Of course, duh. I have no excuse for that, thanks! 150A available from a source of power (the PV array) won't hurt a user of that power.

 

[Dzl]

Dzl, you are correct. Sorry, but I think some of the above posts are missing the point and could perhaps read the Victron information. I have been researching exactly this matter for some time. The Victron 100/50 at 12v has a nominal 700W input power limit. You can connect as big a PV array as you like as long as the total panel specs do NOT exceed the total Voc and Isc values permitted by the 100/50 ie 100v and 60A. You obviously have read the relevant pages on the Victron website, so just be careful how you arrange the panels as far as Series and Parallel strings are concerned. I would actually like to know if the Renogy Rover 60 has a similar ability to be over panelled...
Regards, Ross.
See https://www.victronenergy.com/blog/...ar-modules-to-the-new-mppt-charge-regulators/

Thank you Ross, this is exactly the clarity I needed! With the above information, it will be a lot easier to choose a control and determine an array size!

 

[Dzl]

Complete solar novice here. It seems like there should be some sort of "dump load" for solar. I'm imagining a very "over paneled" PV array, that can still produce ok power in less than perfect conditions. But then a device between the PV an the charge controller that is set to divert anything above a specified current (you'd still have to manage voltage to keep things easy). So if you MPPT is 100V and 60A max, you could panel so that you could hit 150A in perfect conditions and this made-up dump-load device would draw off everything above 60A.

It'll have the same problems as a wind dump load, if it's a big resistor it's going to have to be BIG and have to keep cool or heat water, or something. I'm literally thinking this thru right now, so maybe this exists or (more likely) is a dumb idea.

I think what you are describing is very similar to the ElectroDacus DMPPT controller. It only works well with medium to large systems (upwards of 2kw of panels I think). But essentially the system is designed to be highly over-paneled and dumps all the excess energy into 'thermal storage' and heating. The creator uses the excess energy to directly heat his slab floor and hot water. Its a really brilliantly simple design, but it makes so much sense, I recommend watching (rather long) video I linked to if you are interested in the concept.

 

[Steve_S]

The Electrodacus looks quite interesting and here is the main website for them: https://electrodacus.com/
Docs & Manuals are there too.
They are still doing Kickstarter and that should be kept in mind as well.
I have Radiant in-floor heating in the FPSF Slab but it is heated with an On-Demand LPG Heater (Takagi TH3M Condensing unit) and a separate unit for my Hot Water because the radiant system has Green Glycol in it (being in Canada you don't put water in your concrete floor, 'stuff happens'). No hot water tank here and with my powerhouse being 75' away, not like I can put one in there and plumb it back to here. Well I do have 2, 4" Conduits running to the powerhouse running 8' undergound but trying to run another 3/4" Pex line (it would have to be insulated as well) in there would be a challenge to say the least and I have nowhere to put a tank in the cabin (not designed to have one, so no space to allocate for it. )

 

[wutduk]

I am in the process of designing a solar system for a live-in bus build. I haven't figured out my energy budget (and won't be able to do so accurately until further along in the build) but I am estimating a 200-400 AH battery bank @ 12v. For maximum parts compatibility and to reuse components I have from a previous vehicle I would like to stick to 12v. Unfortunately that means that the bottleneck based on my budget will be the solar charge controller (currently I'm planning to use the Victron 100v/50a smartsolar controller).

Because of the relatively low cost of solar panels compared to other components such as the charge controller, and the fact that it seems they rarely operate at maximum output, I'm attracted to the idea of overpaneling, but I'm having trouble finding information or guidelines on how to properly do so (victron has a good explanation here). Based on the datasheet of the charge controller I'm looking at, I believe I can safely overpanel as long as my array doesn't exceed the controllers Max VOC (open circuit voltage) of 100V or the lsc (short circuit current) of 60A. If I'm understanding this correctly (and assuming that the short circuit voltage refers to the input side of the controller not the converted 12v output) that means the theoretical maximum array size would be 6000 watts (way more than my needs of <1000 watts).

Is my understanding of how to calculate safe overpaneling correct? are there other safety considerations?

Also from the standpoint of efficiency, I wonder what a reasonable amount of overpaneling would be? 125%-150% charge controller output capacity feels like it might be a reasonable number in order to account for efficiencies incurred from flat roof mount panels, heat, etc. Is there a 'rule of thumb' or a formula people use to determine this?

I was going through this very thing today for the same charge controller.