One thing that is critical to understand in these discussions is the concept of power. I'll start with an example, a simple 120V 120W light bulb. The power used by a 120W light bulb is 120 watts. Simple enough. The voltage is 120V. The current is 120W/120V = 1A. So, my 120W light bulb uses 1A of current at 120V.

Okay, so now suppose you want to run that same light bulb off of a 12V battery bank. For that you would use a 12V to 120V inverter. Commonly available no problem. You should by an inverter that supplies at least 120W. Okay, no problem, we'll buy a 200W inverter.

Okay, time for the big concept, Power out = Power in. Actually that is way simplified and never true. Any time you transform power from one voltage to another you loss a little because nothing is perfect. So, you will often hear people speak of the inverters efficiency. For arguments sake, let's suppose our 200W inverter is 90% efficient. That means you will only get out 90% of what you put in.

So, if I need 120W out for my light bulb, I will need 120/0.9 in or 133W. Now remember, that power is coming from 12V batteries, so the current required from the batteries is then 133W/12V = 11.1A. Whoa, wait one second. Above I said that light bulb only needed 1A, but now I am saying our inverter needs 11A. That is the price you pay for converting from 12V DC to 120V AC, roughly 10x.

Okay, so why is this 11A important. Well, if you are using solar, you will need at least 11A solar panel. Actually solar panels are sold in watts, so you will need 133W solar panel to run that one little light bulb. If you are using batteries, you will need to consider the capacity of the batteries. Battery capacity is rated in Amp-Hours. So, if I want to run that 120W light bulb off batteries for 10 hours, I will need a battery with 11A * 10h = 110 amp-hours. Okay, that's not to bad, a typical deep cycle battery can provide 250 AH, so I can run my light bulb for 20 hours! Awesome.

But wait, I don't really want to run a light bulb, I want to run an airconditioner. Okay, so run the same math above, for an airconditioner. For example, my 15,000 BTU Carrier uses 13A at 120V. That's 1560W. That means my inverter will now need to be at least 2000W (startup loads are much higher than running loads). Whoa, that one expsensive inverter, but I gotta have AC and not run the genset, so I'll spring for it. But wait, what about the solar and the batteries? Okay, how much power does the inverter need. 1560/0.9 = 1733W. Okay, so I need at least 1733W of solar.

A 200W solar panel is roughly $1000 and is 4.5' x 3'. So I am going to need 9 of these panels to run my AC and it will take up about 15' x 8' of roof space. Whoa $9000 for solar to run 1 AC unit, that can't be right. Well, it is and it isn't, I'll come back to that in a bit.

Okay, what about batteries? Okay, so I need roughly 1800W of power, and at 12V that comes to 1800W/12V = 150A. Cool, my battery is 250 AH, so I can run my AC for 250/150 = 1 hour and 40 minutes. What, only 1 hour and 40 minutes? Yep, and then that battery will be completely dead, and need to be recharged. Well, I really want to run my AC for longer and I want to run 2 of them sometimes, so I'll get 4 really good deep cycle batteries. Whoa, that costs how much? Wow, this power stuff is getting exspensive.

Okay, so your batteries are dead and you need to recharge them. No problem, I'll use my $1000 200W solar panel. Keeping it simple, that solar panel can supply 16A at 12V to charge my batteries. (It will be worse than this for alot of reasons.) Again power out = power in. So, let's suppose I drained my 4 batteries using 150A for 4 hours (AC cycled on and off all day and night), for a total of 600AH. My solar cell can supply 16A, so it will take 600/16 = 38 hours to charge my batteries. Whoa! Okay, so I wil use a 100A charger off my genset, that will only take 6 hours. What 6 hours? Gee I could have just run my genset to run the AC's and only had to run it for 4 hours.

Okay, so some of that I say is exaggerated, but not by much. What you need to understand here is power out = power in. Watts out = Watts in or Amp-Hours out = Amp-Hours in. All this power stuff costs a huge amount of money. When figuring out how much power stuff you will need you need to add up all the watts of the stuff you are using, AC/Microwave/TV/Lights/etc and how long you are going to use it on average. First you can size your inverter or genset with the total Watts needed, then you can size your batteries/solar/chargers depending on how much stuff is used. If you really plan to boondock with as little genset as possible, you need to get the most efficient of everything, which is going to drive the cost way up. Getting rid of voltage conversions helps also, so a 12V fridge off batteries is better than a 120V fridge off an inverter off batteries.

Alot of people spend 3-5K on a genset, then another 3-5k on batteries, inverters, solar so they don't have to use the genset. For me personally, I just use the genset. It is very quiet, and we don't boondock, so we can get away with that.

Good luck and get your calculator out.