Whats the most efficient solar setup I can have within 50 amps?

robts · January 2019

Hey everyone, excited to have found a friendly forum to ask some beginner questions as this is my first foray into solar.

I'm currently working on a cargo trailer conversion and hope to add some solar to charge my batteries while going down the road, or at campsites with no shore power.

The reasoning for the 50A limit is that the heart of my electrical system is a Samlex EVO 3012. For those unfamiliar with this Samlex line, its designed to be a charger, inverter, and transfer switch all in one, and also contains connections for an external solar charger. Connecting a solar charger to the EVO instead of directly to the battery will run any current from the solar charger parallel to current coming in from the grid/shore or a generator, and reduce load on the grid/generator for charging purposes.

This charging input is limited to 13-16VDC, and 50 amps. I know I could simply connect a charger directly to the battery and avoid the 50A limit, but my solar needs are very small. We have a 180Ah LiFePO4 battery bank at 12V, and estimate our daily energy needs while camping to be 120Ah/day unreserved, and 80Ah/day while being conservative. I'm also able to very finely program the EVO units charging parameters to be specific to our battery, and I'd prefer to have just one charging computer balancing a LiFePO4 battery, and not two. This also opens up my options for a charge controller without all the bells and whistles because the EVO is handling the delicate charging parameters.

So here's the summary and my question:

- I have about a 7' x 7' space on top of my trailer where panels can be installed.
- Whatever array I set up is limited to 50A at 13-16VDC running between the charge controller and the inverter.
- I'd prefer a setup that includes parallel wiring over series, because camping tends to include unpredictable shade, and I'd like to maximize my potential.

Whats the best I can do within a 50 amp limit?

I've also been wondering if a charge controllers amp rating refers to whats coming into the charger, or whats going out? For instance, if I have 600 watts of nominal 24v panels, they could be putting around 16 amps into an MPPT charger in good conditions (600w/36v = 16.66a). If that MPPT charger converts down the voltage to the 16v my inverters solar connection can handle, its 37.5a (600w/16v = 37.5a). Would that mean I'd need a 20a charger, or a 40a charger?

And lastly, when disconnecting a PV array, is it appropriate to have just a switch/breaker on the positive line before current gets to the charger? I'm still planning my wiring and am noticing just breaking the positive line would be cheaper than a complete disconnect for both poles.

Thanks to anyone for helping me figure this out.

mike95490 · January 2019

Welcome.
If you have a MPPT controller, and connect it's output to another device input ( your Samlex ) and the Samlex attempts to regulate the battery charge, that will drive any MPPT controller nuts. Likely to drive a PWM controller nuts too. But you are welcome to try it anyway and report back how it works.

50A @ 16V = 800w of solar harvest (about 1,000 w of panels). If you are planning on 37A, you need a controller rated at 40A output..

MPPT controllers are rated at output amps
PWM controllers are rated at input amps

You could use a Morningstar MPPT -60, and program it to limit charge amps to 40A. The -60 has a nice internal web server, they are silent and rugged. And need air for cooling.

i use a DC input breaker on the solar side of all my MPPT controllers, if you need to reboot the controller, the solar goes OFF first, and ON after the reboot finishes

robts · January 2019

mike95490, Thanks for your reply. I'll e-mail Samlex tech support to see what they say about how solar chargers behave on the external port. Perhaps they have recommended settings for getting it to play well, which might point me to a MPPT controller that can also be programmed specifically.

Amp ratings of the two types of controller makes sense. I probably should have known that one logically, and I'll probably just make a small breaker to disconnect both poles on the PV side. I did most of my initial research through blogs and websites that talk about adding solar to an RV/van electrical system, and nearly all of those guys only break the positive side of the PV circuit, same as you would a battery switch, but all the resources I check out that detail larger systems always break both poles to disconnect solar. I'm going to assume then that its safer to break both.

So after a bit of what I hope isn't broken math, I've decided to size something between 400-600 watts. That amount of panels will fit well on my roof, and my battery is only 180Ah (LiFePO4), and I'm estimating that we'll need 120Ah a day at the high end. Heres my extremely rough estimates for my options:

Based on a 100W panel operating at 6A in full sun (at 16V), and producing 20-25Ah a day, and x 1.25 for a safety factor in sizing the controller:

400W (4 x 6 x 1.25 = 30A) - 35A MPPT

500W (5 x 6 x 1.25 = 37.5A) - 40A MPPT

600W (6 x 6 x 1.25 = 45A) - 50A MPPT

Is this a reasonable estimate for sizing the array for my battery? If my math isn't too far out of line, 400W could produce between 80-100Ah a day, which is slightly below my needs but cheaper to set up, and 600W could produce between 120-150Ah a day, which meets my needs, so it will probably be the path I take.

I've been looking at the Victron series of MPPT controllers. They seem to be priced right in the middle between the more industrial brands and the cheaper brands and I've read good things. I also already use their battery monitor, so it would be handy to be able to monitor both through the same app on my phone.

Unless I'm trying to save money, is there any reason 600W of panels and a 50A MPPT controller wouldn't work for me?

mike95490 · January 2019

On my 3 large systems, only the + side has breakers, The minus is grounded. If you don't use a grounded system then you need dual pole breakers

Does Victron allow you to configure your own voltage setpoints and absorb times ?

You don't want to configure a system that only meets your needs on good days, because even a slightly bad day is a lot worse harvest.

robts · January 2019

Makes sense on the grounding, I'll have a ground bar in the trailer so that might be an easy solution.

And yes, Victron's "smart" controllers allow changing of all settings and creation of user defined charging "presets". I wouldn't mind recommendations for a 50A MPPT controller, but from what I've researched, any controller under $300 seems to have poor reliability ratings or exaggerated specs, and anything from well known companies priced above Victron's 100/50 ($320) is priced a good bit higher.

Would there be a significant change in performance between 24v panels on MPPT vs 12v panels on PWM?

Seems like 600W is where I should be, and though I know 2x 300W, 24V panels and an MPPT controller could be considered ideal, I could also go with 4x 160W, 12V panels and a PWM controller which would be cheaper if the performance difference wasn't significant.

As far as I know, PWM controllers aren't programmable, but I heard back from tech at Samlex yesterday and was told that I shouldn't worry about how solar chargers react on the EVO units solar port. They always pass through current and the programming priority is handled by the internal computer of the inverter/charger. Programming an MPPT in this instance would simply mean being able to tell it to keep pushing current until the EVO unit shuts off charging when the correct parameters are met.

So for me and my small mobile system, does PWM make more sense? I'm not sure of a good resource to calculate the efficiency differences coming from panels of the same wattage.

mike95490 · January 2019

Morningstar and Trace PWM controllers are adjustable.
Trace has knobs inside, Morningstar requires a computer cable and their free MSview software

https://www.solar-electric.com/c40.html or https://www.solar-electric.com/tracc6060amp.html
https://www.solar-electric.com/tr60amp12244.html

and the 30A brat, cheap enough to use 2 systems, but I don't know about adjustments
https://www.solar-electric.com/midnite-solar-brat-pwm-solar-charge-controller.html

robts · January 2019

On a 600w system going to a 12v battery, would I see a significant difference between 24v panels and an MPPT or 12v panels and a PWM controller?

Its the only decision I'm stuck on now, price for the panels seems to be comparable depending on where they're purchased from, so the difference seems to be about $150-200 between a 50-60A PWM vs 50-60A MPPT.

If the efficiency difference is noticeable that price is justifiable to me, otherwise a PWM makes more sense.

Photowhit · January 2019

robts said:

On a 600w system going to a 12v battery, would I see a significant difference between 24v panels and an MPPT or 12v panels and a PWM controller?

Normally you would see about a 10% difference. A lot of the newer 12 volt nominal panels have a VMP of 18.9 volts so you might see an addition 5% (15% total).

robts said:

If the efficiency difference is noticeable that price is justifiable to me, otherwise a PWM makes more sense.

I think since your position to the sun will be compromised you might prefer a MPPT type for a bit higher performance from your limited area, that's a $$$ vs performance choice so all yours!

robts · January 2019

Photowhit, sound advice. I forgot to factor in that the top of a trailer is much less predictable than stationary panels, might as well maximize my opportunity if the difference comes down to $150.

Between you and mike I think I'm all set.

Thanks for all your help!

Whats the most efficient solar setup I can have within 50 amps?

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