Will changing from PWM to MPPT help with this small 200W system?

[pralinebuckets]

Hi all,
I have an off grid cabin and built a small solar system with mainly things I had from other projects or was able to get off of friends. It isn't working great, so I'm trying to think of ideas for how to cheaply make it better. Current system:

4x 6v Trojan FLA golf cart batteries in series for 24v
1x 2000w 24v to 120v inverter
2x 100w Renogy 12v solar panels in series
1x Renogy 20A PWM (voyager) charge controller

The area doesn't get a ton of full sun, so isn't reliably charging the batteries. I've been looking at whether an MPPT charge controller might boost the efficiency, but am worried that because I'm supplying a 24v system with roughly 24v (+/-), I'll still be limited by the panels not supplying enough voltage to the charge controller to really do much good. Is that a good assumption? And if I changed the batteries to be 12v with the same solar series setup, could I expect any more performance with the rest of the setup the same (inverter voltage requirements aside..).

 

[sunshine_eggo]

The area doesn't get a ton of full sun, so isn't reliably charging the batteries.

this is 1000% the issue.

I've been looking at whether an MPPT charge controller might boost the efficiency, but am worried that because I'm supplying a 24v system with roughly 24v (+/-), I'll still be limited by the panels not supplying enough voltage to the charge controller to really do much good. Is that a good assumption?

No, because it's not an issue now with PWM.

In ideal conditions an MPPT will produce 15-20% more power than PWM. MPPT is substantially better than PWM in cold conditions and may not be a substantial improvement in hot conditions.

And if I changed the batteries to be 12v with the same solar series setup, could I expect any more performance with the rest of the setup the same (inverter voltage requirements aside..).

No, and you would need a new inverter.

You need more PV or less usage. Consider that your inverter may be consuming a significant amount of power. They burn watts even when nothing is drawing a load. Consider turning it off when not in use.

 

[pralinebuckets]

The inverter rarely gets used and is off the majority of the time.
Obviously more sun would be better, but that's something I don't really have the ability to change. So given that limitation, it sounds like aside from the MPPT vs PWM difference my assumptions are mainly correct? I should get an MPPT controller, go down to 12v and if I need an inverter get a new one?

 

[Risky Rob]

Renogy sells a 20A mppt Rover for ~$90, and they are much more capable than the Wanderer. Maybe get 2 more panels, as well?

 

[pralinebuckets]

Yeah - that's a good idea.
I'm trying to avoid too much investment in this current system because eventually (once I'm able to cut down enough trees to support a larger solar array) I expect to migrate to a larger 48v system and most/all of what I use now won't be useful there.

 

[Rednecktek]

Yeah - that's a good idea.
I'm trying to avoid too much investment in this current system because eventually (once I'm able to cut down enough trees to support a larger solar array) I expect to migrate to a larger 48v system and most/all of what I use now won't be useful there.

The question you're going to need to ask yourself is "Is spending $100 on this system for maybe 10% more power a good value?"

From what I've seen and doodled, anything in the <300w range of panels it doesn't tend to be a good value. Yes, you can connect in series and maybe start generating power a little earlier and later into the day, but it increases the risk of shade nerfing the array.

If you can find a cheap (used) 20a MPPT locally, even if it's not Victron/MidNite/Outback then it might be worth it. Even the most basic of MPPT controllers will turn solar DC into battery DC and do you really need more than that with what you're working with now?

Pro Tip: If it's got USB ports on the front/side it's 99.999999% of the time a cheap chinese PWM with a lying sticker.

Best advice, if possible, check your local Craigslist/FacePage for a used MPPT controller and a couple more panels. Your existing controller should play just fine with a 3rd panel if you can swing it. If you've got physics issues, a MPPT and a couple used higher-wattage panels will be a much better investment.

In the future, if you do decide to spend the money on a MPPT controller, get the most amperage you can for the money. Being able to throw more panels on a system without having to drop another $100 on a larger SCC adds up in the future. Yes, you're planning on a larger 48v system with a huge array, but how long is that Actually going to take. I know I've been trying to get electricity to my camp for about 4 years now because of deployments, season, money, etc. I have almost all the equipment I need up there to make a 2400w 24v system to power the cabin... when I can go clear a LOT of scrub and trees.

So yeah, tl;dr and all...
If you can find a good deal then it's not going to hurt and buys you time. If you're not going to be able to increase the panel sizes then it's not going to be worth it.

 

[pralinebuckets]

Yeah that's a good point. I'm definitely considering replacing the two 100W panels with a couple 300-400W ones... but that does come at a higher cost.

 

[Rednecktek]

Yeah that's a good point. I'm definitely considering replacing the two 100W panels with a couple 300-400W ones... but that does come at a higher cost.

Whip out the tape measure first. Density doesn't drop much so a 200w panel is about 2x the square footage of a 100w. If you're going to be buying only a couple panels then Craigslist is usually your best bet, followed by Amazon. Getting used panels online really only makes sense if you're buying a pallet because the shipping costs are murder.

Avoid flexible panels, they're not great nor cheap per watt. If you're doing Amazon expect to spend about $1/w.

With larger panels a MPPT makes all the difference in the world.

 

[pralinebuckets]

Thanks! All good considerations

 

[sunshine_eggo]

The inverter rarely gets used and is off the majority of the time.
Obviously more sun would be better, but that's something I don't really have the ability to change.

No, but you can get more PV to harvest more of what little you have.

So given that limitation, it sounds like aside from the MPPT vs PWM difference my assumptions are mainly correct?

No.

I should get an MPPT controller, go down to 12v and if I need an inverter get a new one?

If you're good with getting only another 0-20% benefit, sure. Your money would be way better spent on more panels.

 

[pralinebuckets]

I went with a totally different option. I cannibalized a 50A xantrex freedom 10 inverter from a different system so that I can keep these batteries topped off with a generator.

 

[Substrate]

Heh, I was going to say you'll be paying for new GC batteries if you can't keep them fully charged regularly.

But now you are paying for fuel. Seems like you can't win. The hint here is in the future to go with LiFeP04 since you don't have to charge them to full to keep them healthy.

In the meantime, perhaps make sure that your wiring run from the panels to the controller are as short and as large of a gauge you can manage to avoid voltage drop.

 

[pralinebuckets]

Heh, I was going to say you'll be paying for new GC batteries if you can't keep them fully charged regularly.

But now you are paying for fuel. Seems like you can't win. The hint here is in the future to go with LiFeP04 since you don't have to charge them to full to keep them healthy.

In the meantime, perhaps make sure that your wiring run from the panels to the controller are as short and as large of a gauge you can manage to avoid voltage drop.

Eventually I’ll go LiFePO4 with a larger solar array. I’m building a cabin and at the moment the only need for power that cant be satisfied with a generator all of the time is for lighting, fans and a cell booster in the makeshift outhouse. This was built with off-casts from a mobile system where I converted to lithium.

The eventual solar location and cabin location are quite far from the shithouse, so no reason to upgrade this. Better to run it into the ground and when ready build the longer term system.