To understand how many panels to install, first you have to calculate your energy needs. There are several ways to do it.
The first one is purely theoretical: you take your appliances one by one, estimate their daily hour usage and do the maths. For example: if your TV uses 90 Watts and you watch it 3 hours a day, then TV needs 270 W per day. The wattage can be found on the appliances. The final sum is the power that you would need from solar panels on a daily basis. However, this method is far from perfect — it's hard to predict how much time a certain device is going to be used on a given day.
Another way to find out how much energy you'll need from the sun is to simply go for a ride. The purpose of this trip is testing your battery, so avoid using AC-generators for charging it along the way. After a day or two, check how much load your battery still has. It can be checked via charge controller if you have one, or with a voltmeter which sometimes comes with an RV battery. For example, a fully charged 12 V battery has a voltage of 12.6 V or a little more, but when it's half discharged, the voltage drops to around 12.1 V. Example:
Let's say you have a fully charged 300 A/h battery, and after two days of driving around you are left with 50% of the initial charge. So, you've spent 150 A/h in 2 days which means the average daily energy consumption is 75 A/h.
If you're using lead-acid deep cycle batteries, there's no way you consume more than that. They are just not meant to be discharged deeper than 50%
of their load. This is not very convenient, which probably made lots of RV owners switch to lithium-ion batteries lately. They are more expensive than lead-acid batteries, but better in almost every way.
- Lithium-ion batteries occupy less space and weigh around 75% less than lead-acid batteries.
- Their acceptable Depth of Discharge can go up to 90%.
- Partial state of charge is less of a problem.
- It takes twice as much time to charge up a lead-acid battery than a lithium-ion battery of the same capacity.
It's important to note that lithium-ion battery capacity is measured either in Amp-hour, like lead-acid batteries, or in Kilowatt-hour. However, it is easy to translate one into the other: just divide kWh capacity of the battery by its voltage.