Capacitor for complete electrical system not just air?

For unclean power nothing beats an isolation transformer. For keeping an even keel with off-voltage power nothing beats a ferro-resonant line tamer. Connect an isolation transformer to a line tamer and I have something I am not afraid to connect a tube-type MacIntosh stereo system to.

If you want clean power no matter what is on the other side, there are three options:

1: As stated above, Victron has a converter/inverter which can run off two power supplies, so one can be on a generator, switch to shore power, and have no dropouts due to the batteries.

2: Magnum Energy has a 3000 watt hybrid inverter/converter that adds voltage if the incoming AC is not up to spec.

3: You can always add an online UPS (note, not standby, online.) Of course, the downside of this is that it comes with its own heavy batteries, and costs about the same as either #1 or #2. To boot, it generally assumes dirty power is an exception, rather a rule, so might autofry early.

My next rig, I'm looking at option #2. That way, it doesn't matter how dirty the shore power is, all the sensitive stuff gets clean voltage until the house batteries are discharged.

There is such a thing as a power factor correction capacitor, To get the right one requires someone analyze your system and the proper size depends on what is turned on.. Power factor is not an issue with water heaters and lights but it is with motors and may be with "electronics" .. However I doubt that fixing it will help you much.

Now.. if you are trying to use a generator that is a tad small to power A/C or Microwave (Which sounds like your issue) then there is a device you should look at.

This is a battery/inverter box, not unlike a UPS only way bigger, but it works a bit different too...

This one monitors power flow and when it sees a high demand (IE: A/C starting surge) it kicks in and "boosts" the power from the generator by adding some from it's inverter. Kind of like paralleling two Honda 2000i's but only when needed. Then once the A/C shuts off and the generator has "Excess capacity" again it drops back to batter re-charge mode to get ready for the next cycle.

I think Victor makes one, but it's been a few years since I looked at 'em so I can not even .. Some days I'm good. Some days GREAT, Today just good...

Google: Generator Boost System

Yahama 3000 watt generator with 500 watt boost system

I was not able to find the one I was looking for.. but this gives you the idea.

ELI the ICE man

Just wanted to throw that out there!

Chris Bryant is right. Capacitors provide basically 2 duties.

They can store energy like a battery and discharge relative to the load. A big stereo system would use this implementation.

They also provide phase angle shift to start a inductance motor in a specific direction (then referred to as start capacitor). They are in a circuit only during start-up and provide a second magnetic field when starting a motor. After the motor begins rotation usually a centrifugal switch will take them out of the circuit and the motor will continue to run just fine.

Without a start capacitor (or other start circuit assistance) a single phase motor doesn't necessarily know which direction to rotate if the rotor stops between fields of the windings.

A DC cap like those used on big car stereos, works rather differently than an AC cap. Like Chris said, with 120v AC it's about altering phase. The big 12vdc caps used with car stereos do indeed build charge up to whatever voltage the alternator is delivering and when the voltage sags due to say, a big bass hit on a 1000w stereo, the cap dumps it's energy into the circuit helping hold up the voltage. (and by 1000w, I mean a real 1000w amplifier, not a Walmart 1000w amp)

^^^^ +1 for Chris

Otherwise you are looking for a Victron Multiplus inverter for about $1,700.

http://www.jamestowndistributors.com/userportal/show_product.do?pid=61576

People always think a start capacitor sits there and builds up a huge charge, and that is what starts the compressor. While it does do that to a certain extent, that really isn't its purpose in motor starting- rather it is to change the phase a bit- delay the cycle. Think about trying to start pedaling a bicycle with the pedal at the very top of the travel. It's much easier to start if it is halfway down. That is what the start capacitor does.
We have used them in the 12 volt side- on old ferroresonant converters they were used to eliminate the battery.

They do make them - Here is one example (no longer available) designed to power a auto "Monster" audio system.

Here is an article on combining Ultra Capacitors with batteries to power a vehicle.

There is such a thing as an "Automatic Power Factor Corrector" that will do the job for you.

How many THOUSANDS of dollars do you want o spend?