MPPT size help

[idiggplants]

ok, so we are replacing our pwm controller on our small off grid setup. we currently have a 30a pwm controller charging a 100ah 12v battery being powered by 2x 100w panels ran in parallel.

we are replacing the controller with an mppt for more efficiency. i would like to leave room for expansion. but i also read that mppt controllers, if oversized, are less efficient? id like to allow for 300-400w of panels. would a renogy 20a charge controller handle 400w?

would we be better off switching the panels to series to up the voltage and lower our line loss? we currently have 10awg wires from the panels to the cc. about a 40' run.

next question... is renogy good to go? i know it isnt morningstar, but if it gets the job done, thats all i care about. how about hqst?

[idiggplants]

here is the mppt im thinking about getting:

https://smile.amazon.com/Renogy-Monocrystalline-Solar-Starter-Controller/dp/B01DECKI02/ref=sr_1_2?s=lawn-garden&ie=UTF8&qid=1487164433&sr=1-2&keywords=renogy+100w+mppt

or

https://smile.amazon.com/dp/B01CAXO4C4/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=QHKTR36X6BT3&coliid=I3V7NBCD5AGKPB&psc=1

the existing panels and pwm controllers are all renogy and have been performing wonderfully.

[SunEagle]

IMO both of those "kits' are over priced.

The renogy "tracer" CC is considered a true MPPT type but does not have the ability to change charging voltage settings.

I am unfamiliar with the other CC.

The amount of allowed input wattage for 12volt charging is usually limited to the Amp rating. So a quick calculation would be to multiply the battery voltage by the amp rating. 12v x 20amp = 240watts. So I would seriously doubt a 20Amp CC can handle 400watts charging a 12volt battery.

[Mike90250]

400w of PV, would produce 33A @ 12V [ Wattage/Voltage=Amps ] So there would be times that the 30A controller would be over paneled. Some models may not gracefully handle that sort of over power well. Morningstar & Midnight Solar both make good mid-sized models of MPPT controllers. Tracer may work, but I have no experience with that brand,

[idiggplants]

if im not mistaken, you can get an add on display/controller to change charging voltage settings, as well as get info on how many ah have came in/out, current load draw, etc.. our existing controller has that ability, but we have never used it. is it that important?

if they are overpriced, im definitely open to hearing where i can find a 100w panel and a 20/40a mppt charge controller for close to that price.

so we would need the 40a mppt? is that going to be too big for 200w of panels?

[SunEagle]

if im not mistaken, you can get an add on display/controller to change charging voltage settings, as well as get info on how many ah have came in/out, current load draw, etc.. our existing controller has that ability, but we have never used it. is it that important?

if they are overpriced, im definitely open to hearing where i can find a 100w panel and a 20/40a mppt charge controller for close to that price.

so we would need the 40a mppt? is that going to be too big for 200w of panels?

First off purchasing 2 x 100watt low wattage panels usually cost more per watt than a single 200watt panels.

Next if you go above a 200watt system using a PWM CC will usually waste some of that wattage so an MPPT is a better investment.

The losses from oversizing an MPPT is minimal to what you would lose using a PWM CC so I would not sweat going with a 40A even if you only load it to 25A.

Finally you really should check the manual of a CC to see what ability you have to adjust the settings. Even if it comes with a control panel that lets you access the setting does not mean it allows you to make the changes you want. Unfortunately the higher priced CC's also provide you more flexibility in it's setup.

So the saying goes "you get what you pay for" and when it comes to solar pv system hardware the less you spend the lower the quality and shorter the life.

[idiggplants]

First off purchasing 2 x 100watt low wattage panels usually cost more per watt than a single 200watt panels.

Next if you go above a 200watt system using a PWM CC will usually waste some of that wattage so an MPPT is a better investment.

The losses from oversizing an MPPT is minimal to what you would lose using a PWM CC so I would not sweat going with a 40A even if you only load it to 25A.

Finally you really should check the manual of a CC to see what ability you have to adjust the settings. Even if it comes with a control panel that lets you access the setting does not mean it allows you to make the changes you want. Unfortunately the higher priced CC's also provide you more flexibility in it's setup.

So the saying goes "you get what you pay for" and when it comes to solar pv system hardware the less you spend the lower the quality and shorter the life.

well, the kit would be going for a shed which only requires 100w of panel. super low usage. no point in getting a 200w panel for it. none at all. and there is also no point in an mppt setup for it either.

what settings would we need to adjust? we are talking about a $170 CC vs a $400+ CC. all the default settings on my pwm have been quite sufficient...

its been quite a while since ive agreed with the "you get what you pay for" mantra. especially with electronics. especially when they come with 5 year warranties. in 5 years, by the time the warranty is up, i expect the tech to be out dated and a fraction of the price it is now anyways.

those are my thoughts anyways. i dont mean to sound unappreciative with your input.

[SunEagle]

well, the kit would be going for a shed which only requires 100w of panel. super low usage. no point in getting a 200w panel for it. none at all. and there is also no point in an mppt setup for it either.

what settings would we need to adjust? we are talking about a $170 CC vs a $400+ CC. all the default settings on my pwm have been quite sufficient...

its been quite a while since ive agreed with the "you get what you pay for" mantra. especially with electronics. especially when they come with 5 year warranties. in 5 years, by the time the warranty is up, i expect the tech to be out dated and a fraction of the price it is now anyways.

those are my thoughts anyways. i dont mean to sound unappreciative with your input.

Well you were asking about expanding a system to allow up to 400watts so I gave you some ideas which direction to go.

If 100watts is enough for your application then ok but you battery really should not be bigger than say 65Ah with only ~ 5.5amps of charging from that 100w panel.

For your 100Ah battery you will need 200 watts of panels but I guess your shed won't be able to handle that much.

So what is your daily watt hour usage and what do you want to grow into?

[cabindave]

idiggplants,
If you do want to go to 400 you could get the 40amp MPPT tracer model from Renogy. They have a good website to order direct from and are really very helpful. I have had nothing but good luck with them. With that size you could probably hit close to 800w total. I have the 40 amp controller and am very happy with it. There are comments above that say the controller of that brand cannot switch charge voltages but that is not true. They auto recognize 12 or 24 volt banks. You just hook up your battery bank before you hook up your panels. The website has a downloads tab and manuals for every product. Definitely check those out before deciding.
There are people that are anti 100watt panels for their own reasons, but for small setups they have free shipping, and can be hard to beat. Obviously in a large setup they are often not ideal.

Reply if you have any other questions. I had an almost identical setup to what you have now. I since went to 24v and use a 24v to 12v 700w down-converter by Pyle for lighting, radio, etc.

I looked again, it seems it would be possible to squeeze your 400 watts in on the 20 amp model too if you needed to save $75. It would be the absolute max for the controller. To do this you would have to run the battery bank at 24 volts. The larger 40 amp controller would do the same wattage at 12 if you really wanted to do that.

[idiggplants]

Well you were asking about expanding a system to allow up to 400watts so I gave you some ideas which direction to go.

If 100watts is enough for your application then ok but you battery really should not be bigger than say 65Ah with only ~ 5.5amps of charging from that 100w panel.

For your 100Ah battery you will need 200 watts of panels but I guess your shed won't be able to handle that much.

So what is your daily watt hour usage and what do you want to grow into?

hmm. might be some confusion. the kit we would be getting would be split up...

• the cc would go onto the cabin... which is at 200w with a 100ah battery... would possibly go to 400w with 200ah battery, but that is doubtful. our power usage has been minimal for 20 years. dont plan on getting more luxurious.
• the 100w panel would go to the shed with the pwm controller. it woudl be going on a "shot" 100ah battery, just cause we have one... if that battery dies, we could get away with a 30ah battery.

[ButchDeal]

no point in getting a 200w panel for it. none at all. and there is also no point in an mppt setup for it either.

1) 200 plus watt modules are cheaper per watt than 100w 12v version.
2) You asked about MPPT and the subject of this thread is help with MPPT

[SunEagle]

hmm. might be some confusion. the kit we would be getting would be split up...

• the cc would go onto the cabin... which is at 200w with a 100ah battery... would possibly go to 400w with 200ah battery, but that is doubtful. our power usage has been minimal for 20 years. dont plan on getting more luxurious.
• the 100w panel would go to the shed with the pwm controller. it woudl be going on a "shot" 100ah battery, just cause we have one... if that battery dies, we could get away with a 30ah battery.

OK. Thanks for explaining your system a little better. It is hard to make good recommendations when we get bits and pieces from the OP.

Small systems can get away with PWM CC's. It is only when you go above 200 watts is when a PWM throws away some of the wattage based on how it works and going to an MPPT is a better solution as long you get a "real" MPPT and not those cheap knock offs you can find on eBay.

[idiggplants]

idiggplants,
If you do want to go to 400 you could get the 40amp MPPT tracer model from Renogy. They have a good website to order direct from and are really very helpful. I have had nothing but good luck with them. With that size you could probably hit close to 800w total. I have the 40 amp controller and am very happy with it. There are comments above that say the controller of that brand cannot switch charge voltages but that is not true. They auto recognize 12 or 24 volt banks. You just hook up your battery bank before you hook up your panels. The website has a downloads tab and manuals for every product. Definitely check those out before deciding.
There are people that are anti 100watt panels for their own reasons, but for small setups they have free shipping, and can be hard to beat. Obviously in a large setup they are often not ideal.

Reply if you have any other questions. I had an almost identical setup to what you have now. I since went to 24v and use a 24v to 12v 700w down-converter by Pyle for lighting, radio, etc.

I looked again, it seems it would be possible to squeeze your 400 watts in on the 20 amp model too if you needed to save $75. It would be the absolute max for the controller. To do this you would have to run the battery bank at 24 volts. The larger 40 amp controller would do the same wattage at 12 if you really wanted to do that.

thanks for that info. good to hear something positive from someone who has the controller.

next question is wiring... with our 2 pair of 10awg wire running out to our panels, will we be better off switching them from parallel to series? up the voltage for less line loss? that would leave the second set of wires for 2 more panels should we go that route in the future.

we do not have the option to run the bank at 24v. all the stuff in our cabin is setup like it is an RV... 12v water pump, 12v car radio... 12v sockets for car chargers... etc.

[cabindave]

Definitely go to series with the MPPT controller. Then you can easily get away with 10awg wire and have very little loss (provided you don't have a very long run. Check wiring size calculators, etc.)

You do have the option to run that bank at 24v if your batteries can be configured that way. Look this up. Pyle PSWNV720. I believe it would run at least 360watt continuous, more peak. We use a 24 volt battery on our setup, and the only thing that is 24v native is a mini-fridge that gets occasional summer use. Everything else is run off of a fuse panel connected to that rather inexpensive 24v to 12v converter. We also run a 12v radio, pumps, and lights.

To clarify if you want up to 400 watts into the small CC, then you would have to go 24v battery bank. The larger 40amp CC of the same brand could handle 400w panel input to charge a 12v bank directly. The 24v charge is more efficient at 400 watts (again, check the docs for graphs. They are near the end of the manual).

[idiggplants]

thats interesting. i didnt think a step down thing like that would be so cheap. is it efficient? i guess if we go 400w, we would also go with a second battery anyways, so we could put them in series.

what if we put our limit at 3x 100w panels? the 20a could handle that... but could we run 3 in series? cause otherwise they would have to be in parallel if im not mistaken?