Connecting mis-matched panels in series?

[Woodpens]

I started a small installation last summer on my backyard shed to mainly charge my lawn tractor battery. Started with a Renogy kit of 50watt Mono panel, Renogy 10A PMW controller and one deep cycle 12v battery for storage. Worked well and I wanted to expand a bit to be able to use it for a few hours of power outages for the house, so I added another battery in parallel to double the storage. Then I upgraded to a Renogy Elite 20A MPPT controller and added another Renogy 100w mono panel that I hooked in series to increase voltage. Everything seems to be working well and I am showing 44v at the panel on sunny days. My question is the two panels have different specs. The 50w has open current of 21.8V and optimum current of 2.9A. The 100w panel has open circuit of 24.3v and optimum current of 4.93A
Am I in any danger with this system being mismatched?
Would I be better to take out the 50w panel and replace with another 100W? I am thinking of adding another 100w panel to what I have now making it 250w. I appreciate any suggestions, thanks!

[SunEagle]

Hello Woodpens and welcome to solar panel talk.

Wiring different panel specs in the same system generally limits the output to the weakest panel. Depending on your battery system Ah rating will determine how many watts you need to use. The rule of thumb is to get a C/10 charge rate where C = Ah rating of your battery system. Since you did not provide the Ah rating I can't say if a second 100 watt panel will work but I can say the using both the 50W and 100W panel is not a good idea.

[Woodpens]

Testing update, I ran a 900watt, (7.5A )heater today and my panel was putting out 42V and around 8amps charging so it was alm,ost maintaining in direct sun. I’m thinking adding another 100w panel in series and I will be good for that load in emergencies?

[SunEagle]

Testing update, I ran a 900watt, (7.5A )heater today and my panel was putting out 42V and around 8amps charging so it was alm,ost maintaining in direct sun. I’m thinking adding another 100w panel in series and I will be good for that load in emergencies?

The problem with solar for emergencies is that for most the time the power is needed usually means it is at night or very cloudy.

[Woodpens]

Very true, I have a gas generator if I need a larger load or at night, just want to be able to run a few things without starting up the generator if possible. Thanks

[SunEagle]

Very true, I have a gas generator if I need a larger load or at night, just want to be able to run a few things without starting up the generator if possible. Thanks

Having a solar/battery system works great if your load is sized for it but IMO emergency power is best provided by a generator. Noise can always be reduced with a sound barrier box.

[Woodpens]

I bought another Renogy 100w panel and tried three different ways to connect it. Tried running the two 100w panels in series by themselves with the 50w disconnected, then the same with addition of the 50w panel with a combiner in parallel, then all three panels in series. That yielded the best result: 65volts at the controller, and charging my batteries at 12 amps in full sun, this was pulling a 3.3 amp load for my shop window air conditioner (new Midea 10,000BTU very efficient unit) I was able to maintain the battery voltage in full sun with this load! I have room to add one more 100w panel on my shop roof to max out this system and I am very impressed about this small backup system that has multiple uses! Thanks for all the info on this board.

[SunEagle]

I bought another Renogy 100w panel and tried three different ways to connect it. Tried running the two 100w panels in series by themselves with the 50w disconnected, then the same with addition of the 50w panel with a combiner in parallel, then all three panels in series. That yielded the best result: 65volts at the controller, and charging my batteries at 12 amps in full sun, this was pulling a 3.3 amp load for my shop window air conditioner (new Midea 10,000BTU very efficient unit) I was able to maintain the battery voltage in full sun with this load! I have room to add one more 100w panel on my shop roof to max out this system and I am very impressed about this small backup system that has multiple uses! Thanks for all the info on this board.

Sounds like you are making your system work for you. But depending on what your loads are, 250watts and a small battery will not last long with a few days of clouds.

[wolfkeeper]

I started a small installation last summer on my backyard shed to mainly charge my lawn tractor battery. Started with a Renogy kit of 50watt Mono panel, Renogy 10A PMW controller and one deep cycle 12v battery for storage. Worked well and I wanted to expand a bit to be able to use it for a few hours of power outages for the house, so I added another battery in parallel to double the storage. Then I upgraded to a Renogy Elite 20A MPPT controller and added another Renogy 100w mono panel that I hooked in series to increase voltage. Everything seems to be working well and I am showing 44v at the panel on sunny days. My question is the two panels have different specs. The 50w has open current of 21.8V and optimum current of 2.9A. The 100w panel has open circuit of 24.3v and optimum current of 4.93A
Am I in any danger with this system being mismatched?
Would I be better to take out the 50w panel and replace with another 100W? I am thinking of adding another 100w panel to what I have now making it 250w. I appreciate any suggestions, thanks!

With solar panels in series, it doesn't matter if they're different power or different voltages, they really need to be the same or very nearly the same optimum current. If they're not, the smallest current one will throttle all the others.

From what you've said you'd almost certainly be better off connecting two identical 50 watt panels in parallel, and then connecting the 100W panel(s) in series with the 50 watt panels. That should more or less double the power.