What SIZE is your rooftop A/C unit? I agree that a 4kW genset should run it, but I want to add that our 15000-btu Coleman is a load on startup, even with its factory-installed start kit.
Have you verified that the 30A breaker on the side of your genny's control assembly box is good? If so...
I'd say start by checking to see if your A/C has any kind of start assist on it now. To run at all, it needs two capacitors. One about 7.5uF for the Fan and another around 45.0uF for Compressor. They may be separate (two 2-terminal capacitors) or combined (one 3-terminal cap). If there's more than that connected (to the 45uF portion) then there's a Start Kit (an additional Capacitor plus some kind of control device. Control might be part of or wired to the cap).
There's a pecking order in compressor start kits:
1. None
2. PTCR 2-wire (Positive Temperature Coefficient Resistor) - This is also called Time Based. PTCR heats up in an engineered amount of time, disconnecting the capacitor.
3. Electronic 2-wire - Best example is the SUPCO SPP6e which is very popular here on RV.net.
It claims to sense the actual startup status of the compressor and drop the start cap out of circuit at the optimal time.
4. Start Relay - This is what our Coleman Mach 15 has, from the factory. According to Chris Bryant, our resident RV Tech contributor, it performs its job a shade better than the SPP6e that he usually recommends.
5. Dometic specialized Hard Start Kit - I have only read about it but it usually comes up in a marine A/C context. I read an article about running marine A/C with a Honda eu2000 and this was the kit that did best.
(6???) The kit shown above looks good. I don't know where this 3-wire electronic unit would fit in. I looked at their webpage and read the FAQ's. Sometimes the subtle slams on competing products are valid, sometimes they're hype. They say that if a 2-wire assist fails, it'll take the compressor out but their 3-wire unit will not. I know neither if that's true nor how often if ever it happens. But I do believe that "failed shorted" can happen to virtually any electrical/electronic device. Both kits are available on Amazon and that might be a good place to start with ratings. I've had appliance repair shops complain that SUPCO stuff doesn't last very long and somebody mentioned that in a review of the 5-2-1 kit noted above. I put a SUPCO on a residential fridge/freezer 10 years ago and it's still running. So is that 5-2-1 review FACT or HYPE?
JOSE is a recent addition to the RV.net team and has turned out to be an impact player. Although I haven't heard of partial failure in an A/C cap, they do fail with age. So perhaps replacement AND a start kit is the way to go here. All this stuff can land on your doorstep from Amazon and others, cost less than $100 total. Less than a service callout or the first hour at a shop...
There are some DVOM's that can read Capacitance. I'm blessed to have one of those, but there are others combined with an Ammeter that do it as well.
Speaking of AMP's, the note above is absolutely correct. Measure the Amperage Draws. That's really where it's at, BUT not all Ammeters can read the startup spike in AMP's and get it right. There's a thread here on RV.net about Meters and there were a few around $60 that can read Capacitance, Frequency, and Inrush Amps. Some have AMP capability only for AC current. Getting all of the above in one that'll do DC AMP's, let alone Inrush DC Amps, could result in two meters instead of one and/or more expensive meters.
Again, be careful of people's comments. A 5-2-1 review said the device cut "Locked Rotor Amps from 60 to 20." A compressor that is LOCKED has NOT started! If he had an instrument that could catch Peak Inrush Amps and saw 60A before the device was installed and 20A after, that'd be another matter. Most meters, like my Harbor Freight cheapie, can read two of the numbers listed on the compressor's rating plate. Running Amps is one. And God Forbid, Locked Rotor AMPS (LRA) is the other. I say God Forbid because that means the compressor has NOT started. Maybe it's stuck internally, but they'll read LRA with a failed Run Capacitor.
A/C is so critical that I carry some tools, meters and parts. I can read Volts, Capacitance, and Run or Locked Amps. I can replace the Start and Run Caps and replace the Start Device (Relay and Capacitor in the case of our Coleman) with SPP6e.
Capacitors are easy to replace and cheap to "test by substitution" with a new one. Just do this first:
1. Disconnect AC Power, then
2. Short across the cap's terminals to be sure it isn't charged
