The power armor specs of Fallout 1 say that the microfusion power generator on the T51b generates up to 60kW. That's certainly reasonable for a powered armor. The question is the total amount of energy and how much power does it generate at each moment.

Normally, nuclear fusion is not something you can just turn on and off, but I'll assume that the microfusion technology of SCIENCE! is capable of quick start and stop procedures as well as output scaling. So we don't know how long the fuel lasts in the T51b under maximum output, but we can do a rough comparison with the output of the laser rifle.

Lemme quote myself here from a different thread on MFCs:

/edit: No, I don't know how a self-contained fusion generator could work. Let's see what the energy density of an MFC would be. The laser rifle can fire 12 shots from what I assume is a single MFC unit. Let's say that the laser rifle dispenses 5 kJ per shot, slightly more than a 7.62 mm NATO round. Consider an electrical-to-optical efficiency of 50%, so the total energy that the MFC contains is 120 kJ. The MFC seems to be the same size as a lantern battery, so its volume is about 490 mm³. So from this I think the energy density of one MFC is ~245 J/mm³ or 245 MJ/l, which is a more common unit for energy density. This is about seven times as much as a Zinc-air battery, although I'm not sure if I didn't do some hilarious mistake above.

Given that cold fusion is commonly thought to be less energetic than "hot" fusion or fission, I guess that would make sense. I'll check all the numbers later.

It's kinda hard to really estimate what the content of one MFC really is. Does one physical cell fully charge the laser rifle to twelve shots? Or does it make more sense to say that the ammunition counter in Fallout 1/2 is a bit more abstract? We'd have to differ between physical microfusion cells (which would then contain 120 kJ), and microfusion cell charges (which appear to be what the counter in your inventory in Fallout 1/2 shows you), which would be about 10 kJ per unit.

Let's try comparing it to the Highwayman. I'll assume it to be slightly less efficient than a Tesla Model S, because it's bulkier and less aerodynamic. 30 kWh/100 km means 1080 kJ/km if I'm not mistaken, so let's round it down to 1000 kJ/km because I'm a physicist and I like round numbers. Also slightly less than a Chevy Volt in pure electric mode, which takes 810 kJ/km, so my numbers make sense I think.

I have no clue what the range of the Highwayman is on flat terrain, but let's say it's 500 km.

So 100 units of MFC charge give you 500 km at 1000 kJ/km. So the total energy in the full tank is 500,000 kJ, meaning that a single MFC charge is equivalent to 5 MJ, or 5000 kJ. So a few orders of magnitude different from what the laser rifle tells us. Of course, if we assume a single MFC charge to be 5 MJ and still want a single laser blast from that to be around 5 kJ of total energy, we could also lower the efficiency of the laser, which would have to be at about 0.001 instead of 0.5 as I assumed above. That's quite crappy. The laser in Fallout 1 and 2 is red, so it might either be a diode laser or a HeNe laser. 0.001 (or 0.1%) is actually about the efficiency of a HeNe laser in real life, although no high power variants exist (because of the low efficiency).

If we'd assume one MFC charge to be 5 MJ, and, as in my quote above, one physical MF cell to contain twelve units (assumed from the laser rifle having twelve shots from one reload, which I assume means shoving one MFC into the gun like it's shown in the later games), the total energy content of an MFC would be 60 MJ. Thus the energy density of the MFC, which has a volume of 490 mm³, would be 12.25 GJ/l, which is much closer to nuclear fuel than to conventional batteries, but still magnitudes below it. Not too outlandish.

And 60 MJ would keep your T51b moving at full power for about 16 minutes. But I assume the microfusion pack of the T51b is bigger than the MFC and more geared towards slow and steady energy production, and the full 60 kW output is not used all the time. So, dunno, if you squint a bit and ignore the fact that Fallout is SCIENCE! which doesn't really need to make too much sense, it actually kinda works out. Microfusion cells and so on don't use "hot" nuclear fusion like we know it, but some sort of fictional "cold" fusion process. It probably can't be scaled up too much, and its energy density is a bit lower than nuclear fission. So it's a great technology for transportation and weapons, but large scale power plants in the MW and GW range for national power grids don't really work too well with it, so they need to rely on "classic" nuclear fission power.

Wow, that was a long and pointless post, but after thinking about all that stuff it occurs to me that despite Fallout not being written by scientists and probably just hammering in random numbers and buzzwords, it actually works out quite well. It's at least somewhat consistent. I'll have to ask some of the writers about scientific backgrounds if we can get one of them on the Podcast again...

Anyway, I think I'll write a full article about the reasonable aspects of Fallout's SCIENCE! for NMA o_O