What's the point of the radiation gauge when using power armor?

Power armor provides powerful radiation resistance, but it does not make you immune. As such, you will still take radiation damage, though very minimal. Unless you have a perk or a modification that removes radiation from food and water, this is also a potential source of harmful radiation.

As LessPop_MoreFizz mentions, it is also helpful for identifying radiation around the player. This should hopefully ensure you do not get out of the armor in a highly radiated area, and suffer large amounts of radiation damage as a result.

Also bear in mind that different power armor suits provide varying radiation resistance. There are also modifications that provide increased endurance, which is the base stat used to determine your players radiation resistance.

The power armor radiation gauge simply gives you an idea of what radiation is present in the surrounding environment.

It's important to be aware that, while certain things like power armor and hazmat suits greatly reduce radiation damage, nothing will completely negate all rads. If your Geiger counter is clicking, you're probably taking some amount of radiation damage.

So, this provides you two things:

• General awareness of when radiation damage is being taken.
• An idea of where not to step out of your power armor. (Unless you're wearing a hazmat suit underneath.)

Another thing to be aware of is that damage and max HP reduction from rads is not based on static values in the same way most other damage is. Each rad absorbed takes out 0.1% of your maximum health, regardless of how high or low your natural maximum HP value is.

Rad absorption that's displayed in a numeric form is generally in terms of "rads per second". The greatest available radiation resistance (such as what you can get with power armor) reduces most sources of radiation to "<1" rads per second.

This means you'll usually have somewhere north of 16 minutes and 40 seconds worth of survival time, before you'll have to use Aids or die, while the Geiger counter inside your power armor is clicking away. Certainly, this should be more than enough for most scenarios. But it's still important to be aware that it is a finite period.

One particularly annoying bit about the power suit's radiation gauge however is that it doesn't actually show you how much damage you're taking. Usually, it's extremely low ("<1 rads") but it would be nice to actually see that value so you can be certain.

The suit's HUD also doesn't give you a clear indicator of how much of your maximum health is currently unavailable due to radiation damage already taken. (Well, apparently there is an indicator but it's really easy to miss.) This can be particularly annoying when you are trying to judge how to effectively use your Aids to restore available HP or remove absorbed radiation.

To work around these limitations, you can go into VATS at any time by holding the normal VATS button for a half-second or so. (This works in and out of combat, regardless of whether there's anything around that can be targeted.) There, you'll see the normal health bar and radiation indicators as appropriate.