Hi all,

I'm using watts as "watt hours".

There are several ways to size a system. Here is one idea.

Power storage is almost always the weakest link in such a system.

Currently we have a choice between lead acid batteries and more exotic technologies. For example the battery that is in the Toyota Prius, which is nickel metal hydride stores a great deal more energy that a similar physically sized lead acid battery. The problem is that the Prius battery costs approximately $7000.00 retail. Don't get me wrong—I think they are marvelous—but they are simply out of my price range.

Most RV's run on 12 volt dc systems. We sure want more than just lighting any more, so the more batteries we have the more energy we can use.
Most "stock" RV's have room for two batteries. They may store about 1300 watts each.

Batteries are pretty well understood by the industry. The more gently they are discharged the longer they will last both in terms of lifespan and capacity. It is also known that if batteries are discharged to less than 50% of their capacity their lifespan will be shortened.

So, in a two battery system with 2600 watts available we can really only use 1300—unless there is some emergency that requires power to be available and no other source can be used.

How much is 1300 watts? A single 2 amp 12 volt incandescent bulb will use 24 watts per hour, so if one runs ten such bulbs 240 watts are consumed each hour. Or you could run a 2500 BTU electric heater for about one hour.
 
It may be prudent to find room for more than just two batteries, even if it means welding a shelf under the RV.

How much solar power is needed to keep a 1300 watt lead acid battery charged? Well, we need to look at what the makers suggest. All lead acid batteries need to be "equalized" from time to time. The current required to do this is 5 amps (60 watts) for two hours. So the minimum requirement for solar charging is 60 watts for every 1300 watts of battery (105 amps).

Batteries do accept higher charge rates than 60 watts. In fact they quite enjoy rates of about 150 watts (~12 amps). If you have 18 amps going "into" a battery the "extra" power is quite often dissipated by the cell "boiling". Batteries don't do well when treated that way, so every good solar system will have a charge controller.

From this we can suggest the maximum efficient charge rate should be 150 watts (or ~12 amps).
Find out how much "real estate" is available in an unshaded  area of the roof of the RV. Calculate how much solar power may be generated in "full sun".  Divide that number by 150 watts and that will give you the minimum number of batteries you should consider having. Divide that number again by 60 and that will be the maximum number of batteries that would be prudent.

12 volt batteries are a better choice than six volt batteries in series, if the total amp hour capacity (or watts of storage) are the same. Here are some reasons why.

Suppose you have storage space for only 3 batteries. Using twelve volt 105 amp hour batteries will give you a total capacity of 3900 watts. But for six volt batteries you would only be able to use multiples of 2—so the storage would be only 2600 watts because of the physical limitations of the storage space.

In order to produce 12 volts, six volt batteries must be wired in series. When corrosion starts to occur (it will, sooner or later) that increases the resistance—and decreases the amount of usable power. In a twelve volt battery that connection is internal and far less likely to corrode.

The price for 4 twelve volt batteries may well be cheaper than for 4 six volt batteries. Why? Because more of them are produced.

I spoke earlier of batteries lasting longer if they are used more gently. An 800 watt microwave may need 80 amps to operate. A single battery system could power it capacity wise for 0.22 of an hour without going below 50% of charge—except that deep discharge batteries don't "live long and prosper" under high loads.  This may be easier to see in a simple chart

1 battery @80 amps draw = 0.22 hour 80 amps from the battery

2 batteries @80 amps draw = 0.54 hour 40 amps from each battery

3 batteries @80 amps draw = .92 hour  26 amps from each battery

4 batteries @ 80 amps draw = 1.34 hours  20 amps from each battery

5 batteries @ 80 amps draw = 1.79 hours 16 amps from each battery

6 batteries @ 80 amps draw = 2.27 hours 13.3 amps from each battery

The first two lines represent very high discharge rates. Translation the batteries may not have a very long service life.

There is one other strange thing about lead acid cells. They don't tolerate 1 or 2% discharge values very well. If you use the batteries—try to use 5% or more of their capacity. before you charge them.

My new solar system is producing  over 1000 watts per clear day, and 500 watts on a cloudy day. I have only storage for 3900 watts. As I am a weekend warrior when I leave on a trip my "house" batteries are fully charged. Most weeks I suspect I'll have watts and watts that I simply can't store.

Conclusions

Minimum 60 watts of solar charging for every 100 amp hours of capacity

Maximum 150 watts of solar charging for every 100 amp hours of capacity.

Always use a charge controller.

If one uses a great deal of power during daylight hours a case can be made for more than 150 watts per 100 amp hours, if your pockets are deep enough.