What startup surge Watts required for 20100btu Air Conditioner

to start with with an air conditioner running (2000 +watts ) plus a coffee machine (1500 watts ) you are looking at 3500 + watts of power used from the generator
the genset has to be of a minimum of 4500 watts to run full time with out over heating
never mind what the rating says on the side, that rating is surge rated and the duty cycle is around 500 watts less than that figure
So if your gen set is 3500 watts the power is at surge power and that will overheat the genset
Of course you haven't mentioned the rating of the set , but overloading is the main reason for over heating

On the compressor will be FLA, full load amps, starting, and RLA , run load amps, running amps.

Your question needs more detail. The term "contacts" refers to the size of switch components and have nothing to do with the amount of current they will carry. Watts is calculated by voltage times current in amperes, or amps. If your AC is running on 110Volts, then 40-watt contacts would be able to carry about .0.4 amps. That does not sound sufficient for an air conditioner, so more clarification is needed. I would expect an air conditioner to require more like 400 watts to operate.

It can but you are not going to do it off a single deep cycle battery, You will need to have some robust batteries to sustain it. But that will almost need to be the only thing on that inverter.

It might be better to spend a little more and get the Air conditioner that can operate on 12 volts or 120 with the flip of a switch.

Look at the data panel for the 15,000 BTU AC. It's current rating should be right around 12 Amps at 120VAC. Which is 1440 Watts. Startup is when there could be a problem. Especially when the compressor approaches a near lockup condition. Current draw can be double that of normal operation Even though it is within the surge rating of the inverter- the batteries have to be at their peak at all times.

Answer
Simple Answer is just to know two formulas

Ohm's Law
V = IR

Power
P = IV

From those you can calculate voltage, amps, watts, ohms

V = Voltage (volts)
I = Current (amps or amperes)
R = Resistance (ohm's)
P = Power (watts)

Just solve for what you are missing.

Voltage times amps being drawn shows watts produced.
Watts divided by voltage shows amps being drawn or watts
divided by amperage shows voltage applied.

The load may be too much for the generator. Remember that some loads, especially those with motors driving a machine (like air compressors) require much higher startup power than what is listed on the nameplate. General rule of thumb is to allow 2-3 times nameplate rating for generator rating. For example. If your air conditioner is rated as 800 watts, then you would want a genreator AT LEAST 2400 watts to start and run the compressor.

When they sit allot - sometimes the fan will stick.. Take the cover off,, turn it on and see if the fan is stuck.. Spin it by hand to free it up..

1.5kW

a 10000 due to the startup surge. also need start kits on any ac started by a generator.

Its the watts you plug in too like ither 30 watts vs 50 watts. 30 watts will cause your wires to get hot in the unit. just pull down your breaker box and look at those white ground wires.