From the www.W5SWL.com FAQ of 27 Nov 2019
Surge Protector Gas Tube Wattage Ratings
Wattage vs Gas Tube Voltage
We get questions about "how much power does this protector handle" a lot. Most surge protector bodies or housings that hold the gas tubes will far exceed "legal limited" for ham radio use. Mother nature might turn the protector body or housing into a blob, but the odds are you won't. Next the real part of this question, one must consider the gas tube or rather the gas tube Voltage. What generally happens when lightning happens nearby, also called a EMP event, is that a Voltage rise in the transmission line will happen (the potential from the shield to RF center wire primarily). This plays nasty with your radio, popping the front end, finals and worse. In a gas tube surge protector device usually the gas tube connects to the shield and the center RF wire. The Voltage mentioned is actually across the gas tube and once the Voltage rating of the gas tube is exceeded it will arc and cause that Voltage to discharge. Effectively grounding out the Voltage. This is how the gas tube operates and protects your equipment. Gas tubes are made in various Voltages, your "high" transmitter power, high VSWR or a EMP event that causes the gas tube Voltage to be exceeded (this is based on some math) will cause the tube to arc and short the Voltage to ground (you do have your protector grounded well, right? And not just simply in line like a coax fitting without a ground?). Gas tubes can survive more than one arc, but with each arc that occurs the probably of that being the last arc will increase. So you need to replace gas tubes periodically. Some do this on a set routine or schedule, others do it when the tube shorts out (sometimes it shorts, sometimes it doesn't) and if it shorts out the VSWR goes high through the protector. If it doesn't short, things can appear pretty normal even if the gas tube is now bad. Others just replace the gas tubes when they think it is time.
The voltage in a transmission line at which a gas tube arcs is a math type thing and for us hams is designed around our nominal impedance of 50 Ohms expected. But it can be any impedance, just make sure the VSWR isn't high for your system design. The gas tube really doesn't care if the voltage comes from mother nature's EMP event or your rig's power (or your high(er) VSWR). So gas tubes are rated in Voltage to which will cause it to arc when that Voltage is exceeded. Low power does not take much of a high gas tube Voltage rating. Higher power does. Too low of a Voltage and your amp or even your basic rig may trip out as the tube arcs. Likewise, if you are not running high power and your system is a relatively low VSWR system, then running a lower Voltage gas tube give you a bit more protection as it will arc sooner from that nasty EMP event. You do not want to put a excessively high Voltage gas tube in your feedline as you simply are doing away with needed protection unless your system needs require it. Higher Voltage isn't better, size your gas tube to your need with a little margin as close as you can.
Gas tube protectors need to be either on a stable low or constant VSWR system such as beam or other antenna that doesn't have the VSWR rise as you tune or change frequency. You choose the gas tube Voltage based on your system needs. In these cases such as using a antenna that requires a tuner, you want the surge protector between the tuner and the rig. If you put the surge protector on the antenna side of the tuner, you will most probably cause the tube to arc as the VSWR will be high at that spot in your feedline (which is why you are using a tuner, right?).
So what you ask is the difference between a low priced surge protector that claims a certain gas tube Voltage and one that is more higher priced. Well, if you are talking about a China made device, quite a lot. Some are good, very many are not. Most of those foreign protectors are of various metals in design, and since the protector has tube cavity where the gas tube is placed and interacts with the RF path many of the cheap ones do have a higher insertion loss and they can raise your VSWR due to impedance mismatches. Further one does not really know if the gas tube is really rated like it is claimed. Good protectors will have a nice well tested design for the gas tube cavity and have a low insertion loss. Many, such as our very higher frequency designed protectors are so critical in design that the gas tube is not replaceable as to do that would alter the characteristics of the tube chamber and the impedance. Expect to pay more for a quality design, you don't want to skimp on the surge protector if you are using one to protect your expensive rig, right?
Two very common gas tube ratings:
600 Volt Tube
At 1:1 about 1600 Watts, at 1.5:1 about 1100 Watts, at 3.0:1 about 700 Watts
230 Volt Tube
At 1.1 about 250 Watts, at 1.5:1 about 175 Watts, at 3.0:1 about 100 Watts
There was a good write up in the July 2018 issue of CQ Magazine on surge protectors and different voltages. Our testing indicates the above general values of Wattage vs VSWR, which pretty much match the results shown in CQ. The ARRL has also had some dandy written pieces on surge protectors and gas tubes, check out for example the July/August issue of QEX. The moral of the story is, the higher your VSWR the lower the Watts the gas tube in your surge protector will tolerate before it arcs.