Here’s another take on it, I’ll try to describe things as simply as possible.
So go refill your coffee cup, make a sandwich, go to the bathroom, then sit down and get comfortable. This may be a long reply - [emoji23]
So let’s set this example - You have this RV. With its original standard power arrangement of:
120v plug to feed the 120v side of your power panel. The 120v a/c side feeds a set of circuit breakers (or fuses if your rv is really old). Those circuits feed the air conditioning, microwave, 120v side of refrigerator, etc. all your 120v sockets that were shipped in the rv as originally built.
In that “standard” arrangement there would be a “Converter” that is taking power from the 120v a/c source (it may be on its own circuit or possibly just on the same circuit as your 120v receptacles) it is almost always located in the power panel (Probably in the lower section). It’s job is converting the incoming 120v to feed 12v d/c output to the 12v side of the distribution panel to power all the 12v items in the RV while you are hooked to 120v shore power. This same 12v circuit in the power panel is also connected to the house batteries. This is keeping your 12v house battery (or batteries if you have more than one) topped off - or fully charged.
On a side note if your standard 12v converter is rated at 30 amps or 60 amps (either is a common size) but for my example we’ll use 60: then whenever you are on shore power and using your 12v systems at a rate of less the 60amps load which ever your system is rated at, it will continue to charge your house batteries. If you have a larger load that is over the rated converter output, say over 60 amps in this example , maybe your running every light, you have a 12v tv, your running water (and not hooked to shore water) so your pump is running - in other words, every 12v device is on. Then you may be using more 12v power (more amp draw) more than 60amps in this example or more than your converter is supplying.
Then in that scenario you will be also pulling from your house batteries at the same time to supplement the demand and not be charging your battery.
That is an extreme case but I just wanted to illustrate how your 12v standard system is charging while on shore power.
So far all a normal standard RV setup.
Now let’s add in the option of solar.
Let’s say you add 4 - 100 watt panels on the roof. You are boondocking (not plugged in to 120v) to power your batteries and 12v side of power panel.
You are running off your solar and battery bank.
Using the standard power conversion formula: Watts = Volts times Amps
Just like in high school algebra you can flip that around: Amps = Watts divided by Volts
So on a sunny day your 400 Watts of solar is putting out about 33.3 Amps of 12v power. 400 / 12 = 33.3
In this example you should have at least a 40 amp charge controller. (Always larger than the output of your solar array. Now your batteries are receiving 33 Amps (less any line loss in wiring or the power the charge controller needs to operate etc.) so you will likely be getting something less that 33 amps actually feeding your batteries and 12v side of power panel.
While in this sunny condition, you should be able to use 33amps of 12v power load and not drain batteries, again like above greater than the solar supply - at this point >33 and you would be pulling batteries down. Less than <33 and the surplus should be charging batteries. Any charge controller worth having will have a display showing this condition, usually by showing a + or - , or maybe a happy face when charging battery bank & frown when draining bank.
So now you want to power up a few of your 120v appliances. Maybe your laptop, phone charger, coffee pot etc.
You can use the same formula to size those loads.
1st off forget trying to run air conditioning, you would need an extremely large solar system and many batteries for that. Also any heating appliance will consume a lot of power.
So in order to power these 120v items while you’re out in the boondocks you will need an “inverter” that will take in 12v power from your battery bank and supply 120v power out for these devices.
Your inverter will also consume some amount of power itself for internal circuits while doing this conversion. (Remember it has to change over from direct current to alternating current so even if you don’t have anything 120v plugged or on, as long as your inverter is powered it’s using some small amount of power.
So if you have a 1000 watt inverter and you max it out, say you have your 400 watt coffee maker on, your 200 watt tv, computer using 100, and a 120v lamp, phone chargers, whatever (these numbers are not real) Just examples, but let’s say your using your full 1000 watt inverter power.
If you are powering 1000 Watts of load:
1000 divided by 12 = 83 Amps
Over twice (almost 3 times) the power being supplied by your solar array.
And that is just using straight math. Not taking into account line losses, transformer losses, start up surged (like for motors, like on a blender etc.).
AND you still have the other 12v circuits if on like your 12v pump, or lights, or vent fan etc. all also pulling power from your battery bank.
Per the Radarc electronics web site
www.redarc.com you really should use a multiplier of .85 when sizing your inverter.
In other-words, use a 1000watt inverter for an 850watt load.
There’s a lot of math to consider when sizing your solar system.
Someone earlier addressed the issue of powering your standard rv “converter” from your inverter! Hopefully now it should be clear that if you tried to just power your 120v side of your standard rv power panel from your inverter, that you would create a power consuming loop whereas your inverter is transforming 12v to 120v while powering your converter that is taking 120v to convert to 12v to feed battery - all the while each device is consuming power itself! So through line losses and device consumption you are loosing!!!!
This not to say you can’t power your onboard 120v systems using your inverter, but you would need to eliminate (or disconnect) your converter from the system.
Most people take a simpler approach and just have dedicated 120v plugs that are isolated to just the inverter to eliminate potential crossover.
Another option: there are some solar charge controllers out there that can also receive a 120v input to charge batteries from shore power. If your charge controller was large enough ( say 80 amps maybe larger) then there would be the option to just eliminate the standard converter all together since it’s function could be replaced by your multi power type charge controller. This example assumes that you have a basic 60 amp converter. As long as your charge controller is big enough to power your needs on the 12v side of your power panel.
Naturally efforts to minimize that need would greatly help reduce 12v power requirements. Example replacing all your 12v incandescent lights to the way more efficient LED lights is a big step. Really nothing you can do about your water pump but the good thing is it’s intermittent. Running your refrigerator on gas while boondocking will make a big difference (the 120v part on an ammonia refrigerator is a heating element) remember heating devices like coffee pots, hair dryers, space heaters will all consume a lot of power, so anything you can remove from the inverter to gas will help use less power from battery bank or solar charge requirements.
One thing I’m not so sure about are the effects (good or bad) or potential problems if any about having 2 separate charging sources for your battery bank.
Like for example: your solar charge controller trying to charge your batteries while at the same time your standard rv system 12v converter is also trying to charge your batteries at the same time (assuming you have them all connected together).
Are there any consequences to this arraignment?
Comments welcome on that!
So now that this post is almost as long as a war & piece novel, I’m sure I still haven’t covered near enough. I’m sure there will be a few complaints or corrections or criticisms about my rant. And I always welcome those. Besides my fingers are getting calluses!
But hopefully this helps someone understand things a little better.
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