Reed relays are often rated for 10kV and 5A but only 50W power. So, what is this 50W?

Power is voltage * current. With a power restriction like that, If one is high the other is low.

50W is 5A when switching 10V or 0.005A when switching 10,000V

Or 2A at 25V etc.

And it is contact ratings, nothing to do with coil consumption - most reed relays take very low coil power & bare reed switches do not even have a coil - eg. this one is rated 1500V and 3A with a 120VA maximum switched power value:

https://docs-emea.rs-online.com/webdocs/002e/09007...

Many switches & relays have limits in similar forms; the higher the voltage being switched, the more arcing is likely to occurs so the lower the safe switching current to give the rated life of the device.

Many also have different ratings when switching AC or DC.

It's called a "working area" limit and is common in many types of system, both electrical and mechanical - eg. torque and speed on a drive system or gearbox.

It's also applied to "on state" power in semiconductors - this give you an idea of the resulting graph shapes, with the area when both values would be high clipped off to limit the product:

http://rohmfs.rohm.com/en/products/images/faq/disc...

I do think the 10,000V you mention is an insulation rating rather than a switching rating, though I can't say without seeing the exact device - there are some types with very high ratings.

The 10KV is the breakdown voltage for when the contacts are open. The 5A is the maximum current when the contacts are closed.

Unless it's a huge relay, 50W does sound like a lot for the coil.

A relay has two ends, so if you're talking volts and amps there are two sets of each. The input end is the coil which energizes an electromagnet that pulls two contacts together to close a switch--that's the output end.

In this case I'm guessing the contacts (the output end) can handle 10kV and 5A, and the coil (the input end) takes 50W. Though 50W seems like a lot.