Power available on 20amp circuit?

How did you determine the power draws of your various devices? And what kind of PC do you have that drawing a continuous 1500W??

But yes, a 20A breaker would correspond to 2400W in your case. I'm guessing the 80% load recommendation is so that you won't end up tripping your breaker accidently during normal use. If your breaker really is 20A and you're really drawing as much power as you claim, it must not be very sensitive.

With a 20 Amp breaker at the panel, that circuit definitely should be able to allow up to 20 amps to flow in the circuit. That requires that the cable to the garage (and, really, all other household circuits) be of the correct gauge (12/2), but I am sure it is that way - nobody would install, nor could they get inspection approval - if it were not done properly. You claim 117.8 VAC, I presume by measurement. So that's 2355 W max.

My guess first is that the power consumption numbers you quote for the two computers that run continuously are not correct, and the actual consumption is lower. How did you get those numbers? You see, they total 2260W. Plus, on top of that, the deep freeze switches itself on and off. Now, the deep freeze may consume 90W average (how do you know that is accurate?), but its problem is that starting up the compressor in it takes MUCH more than "average" current for a few seconds, and that usually will trip a breaker already near its max load.

But if all those load numbers are correct and the system is operating at 99.7% of breaker limit with no trips on freezer start-up (because the breaker is designed to tolerate short-term surges), you have NO spare capacity left and are constantly on the edge of tripping the breaker. THEN you say you anticipate using an electric heater that might add 200W to the load. (By the way, 200W is a pretty small heater - are you sure about that?) Most certainly that would overload the breaker.

You need to re-examine all the load numbers you have posted to verify. If they are correct, you will not be able to use all those items at once.

Why has your house not burned down? Because the design is proper. When a breaker and circuit wiring and outlet fixtures are specified for use at 20 amps, that means it is SURELY safe to use up to that load with NO risk of excess heating and fire. That is, assuming no flaws like loose or oxidized connections have developed in the circuit since it was installed. the breaker should trip and shut off the circuit long before excessive overheating happens.

if the panel is in the garage and there space for a new breaker. if you have a friend that a licensed electrical person. pick up some wire and outlets and boxes. add a new line in the garage and move the frige over to the new line. if the breaker not tripping put your hand on it if it warm then you may be near the max of the breaker. with frig motors they pull a lot of amps when starting then when running use less amps. same goes for laser printer.

If the Kilawatt meter is reporting the 1300w PSU is drawing 1480w, I would suspect it is not accurate. I don't know how a PSU can draw MORE current than its maximum rating. Test the Kilawatt meter on something you know the actual wattage, like a 100w light bulb, or a electric space heater that has a stated wattage (I would test this on a different circuit in the house).

I have no reason to doubt the info from a Kill-A-Watt meter. So, maybe the figures for those computers are correct. In that case, they alone put almost a full load on the circuit.

By the way, here's how to look at the Watt ratings of a computer PSU. The rated Watts for one is the MAX it can supply to the computer, and usually the computer actually uses less than that. Those numbers are AFTER the normal efficiency losses in the PSU. So for a PSU that can supply, say, 1300W and has an Efficiency rating of 80% or better, its max consumption from the wall outlet is 1300/0.8 = 1625 W. Your other PC has TWO PSU's? Both in use simultaneously? Well, then maybe the Kill-A-Watt readings of average consumption of 1480W from the first and 780W from the second are reasonable.

For the deep freeze things are not so simple. The lowest Kill-A-Watt readings are 10 to 20 W, and that is really VERY low for any operating freezer. Those MAY be the not-running condition, in which case they represent the power used solely for the control circuits and indicator lights. The 24-hour AVERAGE of 90 W does not help, I think, because so much of that time the unit is not running and consuming only 10-20W. You need to know the short-term reading on the meter when the compressor is running (not during the start-up peak), because that running condition lasts for many minutes and helps create the maximum load on the circuit.

I see now why the heater uses relatively low electrical power.

I assumed the breaker panel you showed in your picture is NOT in the garage - it is the main panel in your house. The single cable from that Breaker #9 to the garage is the only cable feeding power to the garage area. So it is NOT feasible to add a breaker at the garage end and try to create a new circuit with its own breaker that way. You would need to add the breaker at the main panel and run a new 12/2 cable from there to the garage to establish a new circuit there. Then you could split the loads in the garage to reduce the two individual circuit loads. To do this you do need help from a qualified electrician who can examine your main panel and ensure it can accept this additional circuit and load, then do the installation work.

Your idea is viable, and you have recognized an important question. You will need to determine which circuit (that is, which panel breaker) the proposed outside outlet is on, and then what ELSE is on that same circuit already. Look also for one more thing while doing that. I don't know about where you live, but in my area the electrical code requires that all exterior outlets be controlled by a switch mounted inside the building as a safety measure. That way if something goes wrong with the equipment connected to that outlet, there is a remote and safe place to turn it off. So look for such a switch, because you may want to be very careful about shutting off power to one of those computers.

Technically, the extension cord is not the ideal solution, and might be considered illegal. That is because you are not supposed to use extension cords left exposed to weather and especially to possible damage (lying on the ground) as a major way to get power to something. But that's a grey area. You might consider carefully, though, some way to ensure the cord is mechanically protected from damage.