Questions about how to properly size my RV solar system

SoCalKarl · July 2017

Hello. I am new to this forum and am looking to all of yours expertise to help me properly and correctly size my proposed solar system for my new RV. I was told that the participants of this website can provide a wealth of guidance and information as to how to properly size my system.

Where can I go or who can I call to find out what my solar system should be (i.e., battery size, inverter size, etc.) based on my project demand? My RV is a 170" Sprinter van that I will NOT be living in but will be camping and traveling in. Worst case scenario is that I would be in it for 5 days in a row at best. Typically it will be 3 to 4 days on the road and then time to hit the nearest hotel.

I have a list of the electrical equipment that will be in the van (which isn't much). It is my understanding that first I need to determine the total daily wattage of use to enable me to determine the size of my panel array, the charge controller, the inverter, the battery bank and wire sizings. Is this correct?

So what is my next step and where do I go?

Any guidance and assistance is greatly appreciated. Thank you.

Estragon · July 2017

Welcome to the forum.

Yes, it always starts with loads. You find out the wattage of each load and estimate run time daily. Multiply watts times hours for each load for watt-hours(WH), and add together for total daily WH.

Then we need to know how long you will need to go with no solar charging. Typically you would size things so you can handle a day or two of gloomy weather before you need to charge with generator or vehicle alternator.

With this, we divide wh by 12v and multiply by number of days to get amp-hour (AH) supply needs. For long battery life you want to avoid drawing down to less than 50% capacity, so batteries are sizes to AH times 2.

With that, we figure out how much solar is needed to recharge batteries. In a mobile application especially, this process gets iterative if space limitations mean not enough panel can be installed to charge properly, you need to make some trade-offs in WH requirements and/or battery AH to get to a balanced system.

TaoJones · July 2017

SoCalKarl, congratulations on the Sprinter van. If you're driving at least a few hours each day or two, the Sprinter's alternator will probably have topped-up your batteries by the time you reach your next destination - even if it's completely overcast and your panels aren't providing much charging. Even a short-duration drive will probably do the trick as long as you haven't seriously discharged the batteries before you set out.

bsolar · July 2017

in simple terms .. determine your combined wattage draw and get enough panel wattage to run it .. you'll want alittle overhead .. the charger needs to handle your panel output with alittle headroom, and the inverter should probably be sized double your wattage usage so it doesnt heat up .. to me batteries are a separate issue, and its more in how efficient you can be at night,my opinion is focusing on batteries is the 'wrong' way to build a system, but you need enough to get by on .. design and setup things so you dont need much at night is my advice ..

Horsefly · July 2017

bsolar said:

in simple terms .. determine your combined wattage draw and get enough panel wattage to run it .. you'll want alittle overhead .. the charger needs to handle your panel output with alittle headroom, and the inverter should probably be sized double your wattage usage so it doesnt heat up .. to me batteries are a separate issue, and its more in how efficient you can be at night,my opinion is focusing on batteries is the 'wrong' way to build a system, but you need enough to get by on .. design and setup things so you dont need much at night is my advice ..

I guess different people have different approaches, but I mostly disagree. To me, the battery bank is he heart of the system, and you build out both directions from there. You need to first determine how big of a bank you need (based on loads and assuming an 85% efficiency in the inverter), then work out from there. The inverter is sized for the instantaneous loads, the charge controller an PV array is sized accordingly to charge the battery bank.

My first take was to do an end-to-end: Determine the PV array size based on kWH/day and the end-to-end efficiency. But after doing all the math in several different scenarios, I realized that was the wrong way to look at it.

Questions about how to properly size my RV solar system

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