Consumption and space, know what you need for both

When adding a solar panel system to your build, it is important to find out exactly what your consumption needs are and how well that works with much space you have for panels and batteries. It is a good rule of thumb to double the power of your system output to what your needs are. So, if your lights, refrigerator, fan, and laptop require a system to operate at 300W, then you should have 600W of total panel power.



With this setup, your batteries will charge during the day while you use your system.



The same logic goes for calculating the total capacitance of your system.



Capacitance: the ability of a system to store an electric charge

How to calculate wattage and amperage

Power is measured in Watts

VOLTS x AMPS = WATTS

32 in LCD: 12 Volts x 3 Amps = 36 Watts

Cell Phone Charger: 12 Volts x 2.1 Amps = 25.2 Watts



Calculating battery capacitance needs is just as simple. To start, pull the technical information for each piece hooked up to your system and find its individual current needs.



Current is measured in Amps (A), amp hours (Ah) are measured by a simple calculation.

AMPS x Hours used = Ah



Usage is recorded per hour (Ah), so if something runs at 5 Amps, that means it uses 5 amps per hour.



Some common amp examples:



Maxxair Fan: 4.5A

Desktop PC: 5A

Inverter: .8A on stand by

Cell Phone: 2.1A

TV (32in LCD): 3A

Lights: 3A per circuit



What you need to install a solar system on an RV

I recently had a client who knew he wanted to add a solar panel kit to his 35 ft Fleetwood Bounder Recreation Vehicle but didn’t know much more than that. He and I met, and he showed me all of the things currently using electric in his RV: lights, fans, TV, and computer. Everything else ran on propane. His total hourly needs were around 18.4 Amps (the list above are his actual items and amp usage figures). Multiply that total by 12 hours for overnight usage to get the total capacitance needs of 200Ah. Generally, you do not want to drain your batteries below 50% so a system of 400Ah was perfect for his needs.



The parts needed for this project came in just under $2,700 and all can be bought on Amazon.



This RV had an existing PV system that was old, broken, outdated and in need of a replacement. It was running off of 2-6V 235Ah batteries wired in series making it a 12V system at 235Ah and it would not hold a charge overnight without turning on the generator.



The batteries I choose are Renogy’s 12V 200Ah Deep Cycle Gel. The main benefit of using gel batteries over all other types is that they can be mounted in any position and don’t need to be vented–which can super important when working in tiny spaces in a van or RV. The overall battery life is double that of an AMG or Lithium and the cells won’t be damaged until depleted by more than 30%, compared to the 50% marker of the other types.



Parallel: 2-12V 200Ah batteries is 12V and 400Ah

Series: 2- 12V 200Ah batteries is 24V and 200Ah



The benefit of having a 24VDC over 12VDC is that 24V converts to 110/120VAC more efficiently than 12VDC. And something that needs 4 amps to run on 12V will need 2 amps on 24V. Send me a message if you have questions about your electrical system.



What you need to know before you start installing the solar panels on your RV

For this build, I installed a 1500W Kreiger 110VAC inverter. Again, I calculated his max power usage at any given point and doubled it. For less than $140 all his AC needs were met. In the same storage bay that I mounted the inverter, fuse, and cutoff off switch I added the 40A charge controller. Unless you are going to be running power tools or a microwave, you can stick to lower power and cheaper inverters.



Before hooking everything up, you need to know if you are going to have a 24V system or 12V. Wiring everything in series gives you a 24V system and 12V in parallel. When wiring in parallel you need branch connectors to reduce all leads to one positive and one negative. I needed 10 MC4 branch connectors to wire in 6 panels and 2 sets of extensions. Then I ran those through a Roof Cable Entry and through the cabin to the charge controller under the floor.



How to mount solar panels on an RV roof

Mounting solar panels to the roof of an RV can pose its own problems, but space is not one of them. The roof was made of thin plywood on top of 4 inches of Styrofoam, SELF-TAPPING SCREWS WILL NOT WORK ON THIS MATERIAL. I used RV roof tape for leaks and whatnot. Once the glue melted under the sun I couldn’t remove the tape without cutting and yanking it off.



Because I’m full of good ideas, I hooked the outputs of the onboard and auxiliary generators to the battery bank to give it that extra boost it might need on a cloudy weekend. I also ran a 0 AWG cable from the Isolator Relay in the fuse panel to the battery to charge the batteries off of the alternator while the engine is running. Adding an isolator relay is definitely worth the little work that it adds.



On a partly cloudy day under in-direct sunlight, he is able to run his lights, fan, and TV while still generating more than double what his general usage is. This means that his PV system is charging his batteries faster than he can drain them.



Not everybody is going to need the same size system on their build. Spend a little extra time and do the math before jumping in and buying a kit. Understanding exactly how much power, or capacitance, you need will save you a lot of money in the short and long term.



If you need help figuring out what system would work best for you, or you live in Southern California and want to hire me to add a PV system to your RV or campervan, send me a message. My contact information is on the Contact page.

