Low Amp MPPT vs High Amp PWM

Fosten · August 2010

Hello all, first time poster here! I stumbled upon these forums a few months back, and just wanted to thank everyone involved. This is a great resource for the solar community! You folks seem to have great experience and know what you're talking about! Maybe you can point me in the right direction. My family owns a cabin in the mountains of Pennsylvania, which is located several miles outside of town and off-grid. Until now, our PV system has allowed us to run small appliances (saw, drill, coffeemaker, microwave, etc) with great enjoyment. Recently we installed an 10.2a oil furnace, and are now looking into what upgrades we need to make in order to power it this winter.

Our current setup includes:

Solar Array: 175w (Harbor Freight 15w (x3) + Sun HS130 Polycrystline 130w)
Controller: 10a Harbor Freight
Inverter: 3300w/6000w 12v Mobile
Battery Bank: 283ah 12v (75ah 12v Everstart Marine + 208ah 6v US1800XC (x2))

Additional panels and batteries aside, the Harbor Freight controller seems to be the bottleneck in the system. My first thought was to look for more Amps, but then when I learned about MPPT, I began to see the benefits of that technology. The high Amp MPPT controllers seem well-liked, but I'm not sure if our system necessitates the expense at the moment. For a mid-range system, is it better to go with a low Amp MPPT or a high Amp PWM?

I am considering between these five controllers

Morningstar ProStar 30 solar charge controller with digital meter $202.86

Xantrex C35 Solar Charge Controller $113.05
+ Xantrex Digital Meter for C-type Controllers $94.05
=$207.10

Xantrex C60 60 Amp Solar Charge Controller $189.05
+ Xantrex Digital Meter for C-type Controllers $94.05
=$283.10

Morningstar TriStar 60 amp 12, 24, 48 volt PWM solar charge controller $202.86
Morningstar TriStar PWM & MPPT M-2 digital meter $95.47
=$298.33

Morningstar SunSaver 15 Amp MPPT Solar Charge Controller $238.43
+ Morningstar RM-1 Remote Digital Meter $89.05
=$327.48

With a 175w/283ah ratio, we are interested in adding more panels soon. If I understand correctly, for a full charge in direct sunlight, we could add at least 185w to the 30a PWM, 245w to the 35a PWM and 545w to the 60a PWM, plus additional panels to account for efficiency loss. But only 25w more actual panels to the 15a MPPT on a 12v system? Would any extra input simply be lost unless we diverted it to another controller? Then again, its not always sunny in Pennsylvania, so the extra boost an MPPT provides might be just what we need for winter conditions. Is the increased solar efficiency of an MPPT controller worth the trade-off of a lower Amp system? Is the added cost of a high amp MPPT worth it in the long run, even though it is more than what we currently need? Wishing for a 30a MPPT, are there plans for any new models to be released?

We may also be looking to add a hot water heater and a water pump in the future. If these are running on DC load, would a low amp controller be sufficient? Should we eventually convert over to a 24v system? When will we need to? What are the advantages? What other upgrades might benefit our system the most? Extra batteries would help us run for longer periods of time, but the it doesn't appear as if the recommended AGM batteries should be mixed with the FLA batteries we currently have, so we would likely need to overhaul the whole setup. Extra solar panels would give us a quicker recharge, but we'd like to choose a controller first so we can size the upgrades to the array. Can anyone suggest a good price on panels? Need to upgrade everywhere, but not sure where to invest the money. Any and all help would be appreciated. Thanks again for this community! Happy to be a new member!

BB. · August 2010

Re: Low Amp MPPT vs High Amp PWM

Welcome to the forum.

Your questions--First, we always tell people to measure their loads and figure out how their peak Watts/Amps and their overall watt*hours (or amp*hours @ what bank voltage) they want out of their system. Kill-a-Watt meter or DC Amp*Hour Meters are a good place to start with your measurements.

Based on the above, then we recommend that your battery capacity be around 3 days of no-sun and 50% maximum discharge capacity--or ~6x your daily load.

Then, for an off grid system--basically hours of sun per day * array wattage * 0.5 for overall system efficiency (off-grid system). For hours of sun per day--you may get >5 hours of full noontime equivalent sun for the summer, but 2 hours a day (on average) for winter sun--depending on where you live.

After you have "sized your system"--many folks make it 1.5 to 2x larger just to account for future growth.

But--as you have seen, solar pv energy is not cheap--so conservation usually ends up being your first choice as it is almost always cheaper to conserve a Watt*Hour than to generate one.

The rest of the system design ends up being more of a balancing act between the physical needs of your site vs the amount of money in your bank account.

PWM vs MPPT... There are two or three very good reasons to pick an MPPT controller over a PWM, and one reason not (that being the higher cost of a MPPT controller):

If your panels need to be a fair distance from solar array. You can run the solar array between 70 and 100 VDC (Vmp array) and run the high(er) voltage lower current back to the charge controller/battery shed and save a lot of money on copper wire... Especially if your bank is a low voltage one--such as 12 volts.
If your area gets very cold (sub freezing in the middle of the day). An MPPT controller can use the "extra energy" available from a solar array as Vmp rises with very cold weather (power=volts*amps).
With PWM, you have to match Vmp-array to Vbatt fairly closely for good efficiency (~17-18 volts Vmp for a 12 volt battery). With an MPPT type controller, you can purchase any Vmp Solar Panel and run it efficiently--For the most part, 100 watt and greater solar panels tend to be a lot less money per what and not to come in Vmp of 17.5 volt increments. So, with a MPPT controller you have more selection and possibly better overall costs for large arrays.
There is one other good reason to choose an MPPT controller over a PWM type... PWM controllers cannot control the maximum current from the solar array through them... So, you have to size the PWM controller to the maximum output current of the array and never exceed it. A MPPT type controller can actually control the maximum power (output current) through itself. So, you can over size the array somewhat and still have a safe and reliable installation. For example, most sites will not see more than 0.77-0.86 of the solar array wattage--So, you can size the array by 1/0.77 and for 98% of the day, not hit the controller/array limits (less than clear day, warm weather, dusty panels, etc.).

All About Charge Controllers
Read this page about power tracking controllers

If you are into data collection, the new MorningStar 60 amp MPPT charge controller even comes with an Internet Server for data.

Any of the models you have listed are good choices. Note that the 15 amp model does have a lower Voc (Voltage open circuit) than the other 45/60 amps MPPT charge controllers. For charge controllers, if they support a Remote Battery Temperature Sensor--I always recommend to purchase that option.

Assuming you have weather similar to Williamsport Penn (from PV Watts program), December can average ~2.5 hours of sun per day (rest of the "summer" is around 4.5-5+ hours of sun per day)... At some point, you have to decide how much solar power you need vs using the generator for backup during bad/weather/winter... Toss out the lowest 3 months, you get February as the next lowest at 3.6 hours of sun per day... So, a good point which to plan your solar array. Say you use 1kWHour per day in winter:

1,000 watt*hours * 1/(3.6 hours of sun * 0.52 system eff) = 534 watts of solar panel minimum for 9 months of the year

Another issue is the size of your backup genset--I like to recommend people get one that will power their AC battery charger and not get a very large (and nice) 8kw+ genset. The large gensets when run at less than 50% load tend to be very fuel efficient. Most off-grid solar setups can operate with a 1.5-4 kW genset to charge their battery bank.

One thing to think about sizing your solar system vs using a genset it how much of the time the cabin will be occupied. Solar tends to be more cost effective when run 9 months or more out of the year... If you are there for just a season (winter sports or summer lake use,summer weekends, etc.)--a smaller solar system with using a generator more tends to be a better use of money).

Mixing of batteries, especially different brands and/or types (flooded cell vs AGM) tends to be a bad idea. Batteries are better as matched sets so that they charge/discharge/share loads equally. And AGM charging requirements are different enough that they can be damaged if connected to a flooded cell set (sealed/AGMs are easy to destroy with "over charging" causing venting vs flooded cell where you just add distilled water once in a while).

Usually, you are better off just running the current set of batteries until the die--And at that point, you have a good idea of what your current loads are and can size the next bank accordingly.

Personally, I like to pick battery cells that are large enough to support one or possibly two to three parallel strings maximum. I recommend to run away from paralleling >3 "12 volt" batteries. Balancing the current among all those batteries can be a real pain. As well as checking electrolyte in the extra dozens of cells with massively paralleled banks.

In general, a higher voltage battery bank is better. Fewer parallel battery connections (and fuses, and cabling, and cells) plus lower current (smaller gauge wiring, smaller hardware, etc.).

My rough rule of thumb--about 1,200 watts maximum for a 12 volt bank, 2,400 watts maximum for a 24 volt bank. For example, a 1,200 watt 12 volt inverter would need a circuit/fusing capable of:

1,200 watts * 1/0.85 invt eff * 1/10.5 batt cutoff voltage * 1.25 NEC safety margin = 168 amp minimum wiring/fusing

All that done with a maximum of about 1.0 volt drop from the battery to the inverter...

Also, if you look at charge controllers, they are rated at maximum output current... So a 12/24/48 volt 45 amp charger can support:

14.4 volts * 45 amps = 648 watts into battery bank
28.8 volts * 45 amps = 1,296 watts
57.6 volts * 45 amps = 2,592 watts

So, you can see a higher voltage battery bank lets you use the same single charger for 2-4x as many solar panels.

Probably enough in this post... Questions?

-Bill

Fosten · August 2010

Re: Low Amp MPPT vs High Amp PWM

Your points in favor of MPPT are quite valid. With voltage regulation and equalization for sub-freezing temperatures (which we have a lot of!), I can see that MPPT does a lot more than just increase overall efficiency. However, since the 15a MPPT can only handle 200w on a 12v system, it wouldn't be much use to us. If we added even one more panel, we'd have to convert to a 24v system. And to accomplish that, we'd need to replace our 12v-only inverter and buy at least two more batteries. Would running the two 6v and the 12v in series to produce 24v be a good/bad idea since they are different batteries?

Moving into the next tier of controllers, a couple different models come to mind.

Morningstar TriStar60 $624 = 864w 12v capacity
Morningstar TriStar45 $524 = 648w 12v capacity
Outback FM60 $540 = 864w 12v capacity
Xantrex XW60 $529 = 864w 12v capacity
Outback MX60 $450 = 864w 12v capacity
Rougue MPT-3024 $300 = 432w 12v capacity

Reviews of the Rogue seem to be positive, it is reasonably priced, has a 432w 12v capacity, and would allow us to add one more panel before winter. Next summer we could convert to 24v with 864w capacity and then add an additional 499w of panels. This would give us at least two or three years use out of the Rogue before we'd have to think about upgrading our controller again and by then the technology may be even further advanced with newer models on the market. Are there any other limitations I might run into with the Rogue? What size controller does Midnight Solar have coming soon?

Since we already have a SUN HS-130 panel, I'm thinking we might as well stick with the same brand. The SUN190s seem to be a good value, but I'm not sure which of these two panels would be better for my setup.

130w 23.5 Voc / 19.6 Vmp SUN HS130
+
190w 32.8 Voc / 26.7 Vmp SUN HS-190-RL
or
190w 21.50 Voc / 17.4 Vmp SUN SV-T-190

You mentioned that an MPPT would help with matching voltages, so with this increased flexibility should I always look purchase higher voltage panels? Is the difference between the two SUNs simply 12v vs 24v? I seem to be confused on the differences between Voc vs Vmp vs Vdc vs Imp. It was mentioned on the Rogue thread that the system voltage is limited to 24 vdc in/out, would the SUN HS-190 still be ok? Would I be better off pairing my SUN130 with another SUN130 in series? For what benefit would I want to wire my panels in series? How do these SUN panels compare to the more expensive Kyocera, Evergreen, BP, or Mitsubishi models? I had previously considered the 42.0 Voc / 30.0 Vmp Uni-Solar PVL-124 but 197.1" seemed too long for a physically compact array if I had any future plans to assemble a sun tracker with an Arduino board.

Additionally, the Kill-a-Watt and DC ampmeter seem like handy devices, but will these controllers give me the same data? We do not have a backup generator or AC battery charger and have not looked into this at all, any recommendations? As far as remote battery temperature sensors, both the controller and batteries are located indoors in the same room with no temperature difference, would we still need one of these? I feel as if I'm getting closer to a decision, which is good news, because if recent weather is any indication, old man winter is right around the corner. Thanks again for all the help!

niel · August 2010

Re: Low Amp MPPT vs High Amp PWM

i hate to be a further kill-joy, but the nec can come into play with an 80% ruling on the controllers too. a 60a mppt cc can be held back by mr wiles to 48a even though they self regulate the currents to not exceed the 60a rated cc capacity.
do note that some controllers are ul, nrtl, etc. certified and some aren't. this does cost a lot of $ for that certification and although the rogue is a fine mppt controller, he just isn't big enough to front the costs of certification.

mikeo · August 2010

Re: Low Amp MPPT vs High Amp PWM

i hate to be a further kill-joy, but the nec can come into play with an 80% ruling on the controllers too.

I don't think this is any problem in this particular situation, all the panels are non UL rated so why should the controller be?

niel · August 2010

Re: Low Amp MPPT vs High Amp PWM

mikeo wrote: »

I don't think this is any problem in this particular situation, all the panels are non UL rated so why should the controller be?

i agree with you on that and i don't see the need for the nec to restrict controllers to 80% when applicable and i would not bother observing that either in an off grid setup. be aware that this has no bearing on the cc certification itself, but is part of nec rules which can be made to apply to off grid setups too even if only by your insurance company or possibly a stinker for an inspector going by and seeing something he did not approve.
anyhow, just saying.

BB. · August 2010

Re: Low Amp MPPT vs High Amp PWM

Fosten wrote: »

Your points in favor of MPPT a...However, since the 15a MPPT can only handle 200w on a 12v system, it wouldn't be much use to us. If we added even one more panel, we'd have to convert to a 24v system. And to accomplish that, we'd need to replace our 12v-only inverter and buy at least two more batteries. Would running the two 6v and the 12v in series to produce 24v be a good/bad idea since they are different batteries?

Now you see why "expanding" a solar PV system is difficult (aka expensive).

It is important to know your loads (peak Watts, Watt*Hour per day, how much sun you have--all by season). If you under design the system, it costs a lot to build out one the meets your needs.

Conversely, an over designed system is very expensive in its own right.

Battery miss-match (age, capacity, brand, type) all go in the mushy hand waving of not a good idea.

Roughly, adding new batteries to an existing string--The new batteries may "wear out" quicker and my rule of thumb is the new batteries will last about as long as the existing batteries. So if you have a bank with only a few years left on it--you probably don't want to replace one failed battery at a time.

Adding miss-matched batteries in series is an issue. Ideally, AH capacity of a series string should match. Otherwise, the weakest/smallest cell it the string will discharge first and can actually be "reverse charged" (change cell polarity)--that is usually an instant death sentence for that cell/battery.

And you could add battery strings in parallel. Mixing different chemistry/brands/AGM vs flooded cell/etc. all operate at slightly different voltages during charging and discharging. Very roughly, a 0.1 volt difference is a 10% difference in state of charge between two 12 volt batteries.

Or Flooded Cell which can be charged to 14.6 or higher (over 15 volts for equalization) will cause an AGM/Sealed battery to vent hydrogen and electrolyte--which will cause an early death for the AGM/Sealed cell.

If you have 16 cells, and one dies while there should be still years left in the others--Yea, go ahead and replace the one or two batteries with same make/model and you will be OK... You will probably replace all 16 a few years down the road--and perhaps keep the newer ones off to the side as charged spares or for a friend's bank if they have a failure.

Reviews of the Rogue seem to be positive, it is reasonably priced, has a 432w 12v capacity, and would allow us to add one more panel before winter. Next summer we could convert to 24v with 864w capacity and then add an additional 499w of panels. This would give us at least two or three years use out of the Rogue before we'd have to think about upgrading our controller again and by then the technology may be even further advanced with newer models on the market. Are there any other limitations I might run into with the Rogue? What size controller does Midnight Solar have coming soon?

Realistically, all of the MPPT charge controller are pretty efficient and all will do the basic job of charging your battery bank.

If you have a long run between the solar array and the charge controller/battery bank--a Higher Voltage MPPT controller is probably a better choice.

Otherwise you are looking at options and what those mean to you:

Remote Battery Temperature Sensor--Very good idea to have
Remote Battery Voltage Sense--Very nice--I would recommend it (Only a couple MorningStar 45/60 amp MPPT charge controller currently have this option--that I am aware of--other controllers may too)
Digital Displays--pretty nice
Logs---Nice to have--but to a degree, output power is based on battery state of charge--so not exactly accurate as to how much power you could have generated
Logs, Networked--Nice. Some you have to get a dongle to connect to your laptop. One you can actually plug into your router directly (small webserver)
External Contact--Some you can program to turn on a battery vent fan when charging. Or program to turn on an optional load (like well pump) once the battery bank is fully charged.
Networking Controllers together--Some require an external controller/computer to adjust internal settings.
Perhaps the next controller will have a 250 VDC input option. Will be nice for some installations--but not everyone will need it.

My take-a-way is that there are a lot of nice choices out there right now--and if you can design your system to operate within the abilities of any of these controllers today (at the right price)--then do it now.

I don't think waiting is going to get you twice the solar PV power at 1/2 the price (unlike computers, TV's, cell phones).

Since we already have a SUN HS-130 panel, I'm thinking we might as well stick with the same brand. The SUN190s seem to be a good value, but I'm not sure which of these two panels would be better for my setup.

130w 23.5 Voc / 19.6 Vmp SUN HS130
+
190w 32.8 Voc / 26.7 Vmp SUN HS-190-RL
or
190w 21.50 Voc / 17.4 Vmp SUN SV-T-190

You mentioned that an MPPT would help with matching voltages, so with this increased flexibility should I always look purchase higher voltage panels? Is the difference between the two SUNs simply 12v vs 24v? I seem to be confused on the differences between Voc vs Vmp vs Vdc vs Imp. It was mentioned on the Rogue thread that the system voltage is limited to 24 vdc in/out, would the SUN HS-190 still be ok? Would I be better off pairing my SUN130 with another SUN130 in series? For what benefit would I want to wire my panels in series? How do these SUN panels compare to the more expensive Kyocera, Evergreen, BP, or Mitsubishi models? I had previously considered the 42.0 Voc / 30.0 Vmp Uni-Solar PVL-124 but 197.1" seemed too long for a physically compact array if I had any future plans to assemble a sun tracker with an Arduino board.

Where an MPPT controller can help in matching is you can have Vmp array >> Vbatt-charging and be better than 95% efficient. This allows you to use less expensive, >100 watt solar panels that do not match the typical 17.5 Vmp rating for a 12 volt battery bank.

It will not let you parallel a 17 volt panel with a 26 volt panel with a 40 volt panel--Each of these panels (or Vmp-array) will still need their own PWM or MPPT controller to match their individual Vmp/Imp setups.

Additionally, the Kill-a-Watt and DC ampmeter seem like handy devices, but will these controllers give me the same data? We do not have a backup generator or AC battery charger and have not looked into this at all, any recommendations? As far as remote battery temperature sensors, both the controller and batteries are located indoors in the same room with no temperature difference, would we still need one of these? I feel as if I'm getting closer to a decision, which is good news, because if recent weather is any indication, old man winter is right around the corner. Thanks again for all the help!

And you can add a Battery Monitor too.

Basically each device measures gives you a "piece of the picture":

Charge Controller Statitics: Tells you how much energy the charge controller has supplied (day, week, etc.).
Kill-a-Watt: How much AC power a 120 VAC load has consumed (total Watt*Hours)
DC Amp*Hour / Watt*Hour meter: How much a power a connected DC load (~12-60 volts) consumed
Battery Monitor: Tells you how much energy has gone out of or been replaced back in your battery bank. More or less equivalent to monitoring your fuel tank with a meter on the main fuel line---Totalizes how much fuel has left or been replaced back into the tank through that fuel line. Much more accurate than using a Volt Meter to monitor battery bank voltage. Not as accurate at a hydrometer. But certainly much easier for guests/kids/family to monitor state of charge with a digital meter.

So, each has its place in the system.

In my humble opinion, the Battery Monitor is a more critical tool in the system. It can help you prevent battery damage from undercharging (or "deficit charging") by simply checking the meter a couple times a day.

A few battery monitors have a programmable output that can be used to turn off a load, start a genset, set an alarm if the battery bank is undercharged and at risk of damage. Usually much more accurate than setting a voltage set point type alarm.

-Bill

Low Amp MPPT vs High Amp PWM

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