Sizing Small Solar System

I would use 2 6 volt golf cart battery’s @ 215 amp hours and I would like to use 2 300 watt panels to charge it . 

Crystal said:

How many solar panel watts are needed for two 114amphour flooded batteries in a 12 volt system?

In general, you want to know your loads and build a system to run the loads.

There is a basic idea of having a minimal charging capacity of 5% of your battery bank if you won't be cycling your system daily. and 10-13% of the battery bank capacity if you will be cycling your system daily.

So you say you have 2-114 amp hour batteries. If I assume that that is 12 volt batteries, that would be 228ah battery bank at 12 volts.

So if you would want 5% of that capacity for weekend use or  .05x228=11.4amps charging or about 23 amps charging if cycled daily.

So now we have to look at solar panels, and charge controllers. A 100 watt solar panel puts out about 75 watts when it's hot during solar noon. This is the difference between the Standard Test conditions(STC) and Normal Operating Cell Temperatures (NOCT) values. The NOCT value will be about 75% of the STC value. Most of this loss will be in reduced voltage when the panel is warm/hot.

Now Charge controllers. There are 2 basic types a less expensive PWM type which will require solar panels that are in a narrow voltage range to work efficently and MPPT charge controllers which use all the available power/wattage efficiently, but they use a little more power and require a voltage of at least 30% higher than the charging voltage.

With PWM type charge controller, for 12 volt battery banks a VMP of 17.5-20 volts roughly(we call these 12 volt nominal panels). It's easy to figure this as you are looking at the current (amperage output under load) the IMP number and about 90% of that will pass through to the battery when charging, so a typical 100 watt (SOC)panel will output around 5 amps at 16-16.5volts, So for 5% you would be looking for roughly 2-120 watt, 12 volt nominal panels to produce 11.4 amps, double that for a 10% charge rate, so around 480 watts. 12 volt nominal panels top out at about 180 - 200 watts. With a PWM charge controller the panels will need to be connected in parallel so that the voltage stays the same and the amperage/current adds.

With MPPT type charge controller, for 12 volt battery bank, you can work backwards from what you need. You need 11.4 amps charging and you will charge at roughly 14.8 volts (assuming Flooded lead acid batteries, FLA) so 11.4amps x 14.8 volts = 169 watts but that's at NOCT so you would divide that by .75 = 225 watts of solar array. If it falls in a range that the charge controller can use, you could use a single 225 watt panel. Again double that to 450 watts for for daily cycling. Again the MPPT is a bit more efficient, but it also allows you to use the often cheaper higher wattage panels.

In addition, MPPT type charge controller can often save you money in setting up your system, If you intend daily cycling, you would need 4 120 watt panels for the PWM system, but if you have 2 or more panels, you should use a combiner box so if one panel has a short, the other panels won't over power it and possibly create a fire. So each panel would need a fuse or breaker. With a MPPT type system, you could use 2 panels in parallel without needing to use a combiner box. Which could be considerable savings about the same as the added expense of a MPPT type charge controller.

Now after all that... If your load exceeds the ability of the solar panels ability to recharge the batteries, none of this will matter. The system will always run short and you will hit alarms and destroy batteries. So I encourage you to figure out your loads and build a system to work with your loads.

If all that isn't enough, your environment will be a major factor to consider when building your system! If you live in the sunny southwest you may have nearly double the sun exposure than average parts of the US. Then again, if you live in Olympia Washington, you would average less than 1 1/2 hours of direct sun light a day Nov-Jan, and need a bigger solar array and more likely a generator.

Things to consider!

The question may be simple however answering it would require some information, this being what will the discharge be, the frequency of discharge and  geographic location. 

Crystal said:

Wow thanks all for your reply! 
Only needs to power a rv size dometic propane fridge (it requires power to ignite) and keep laptop/phone charged.
Our lights are on a seperate system that is smaller yet.

Perhaps even just one agm battery will do then will only need 100 watt panel? 
We do have a backup genny if need be.

Live very simply in a yurt cabin in midwest Minnesota. 

I put that number out for the same reason that others here have posted. If you only charged a cell phone you wouldn't even need that. You need to have an idea of what loads are going to be added to the system as well as some idea of how much sun you can expect. Solar is dismal in the winter for obvious reasons yet a small set of panels can run a lot IF you follow the rule: "You can only take out of the system what you put into it".

But ya, 300 watts would probably do what you need at least in the summer.