Well, in ENT's "Regeneration", phase pistols were quite dangerous.
First seen
at the beginning, when two scientists run through tempest to get back to the medical bay as one of their comrades yells while being assaulted by a Borg drone. You have a large view of the complex as the male and female scientists run, and two shots are quickly seen lancing towards the sky from the medical bay. When the two scientists reach the medical "building", they pass in front of the metal wall and it shows two gaping and smoking fissures in it, over one meter long each.
Later on, when Reed is trying to increase the particle yield, as the proper level of energy to be applied to phase pistols by cross-polarizing their power cells, and he's fiddling with settings from 5 to 10 MJ, although the wording is a bit cryptic cause he's speaking of power, and they keep the pistol firing at a plate of alloy. Could be megajoules per second, which is quite extreme for hand weapons, or megajoules per whatever amount of time they use for a single burst for a complete test - but there are limits to that because I don't really seem them speaking of megajoules if they would have to keep firing at the same target for dozens and dozens of seconds.
Reed first says "increase the power another five megajoules."
Then seven, eight, nine and ten.
Would that mean they already were firing megajoules per burst? Another five MJ implies that, and possibly already firing at 5 MJ.
However, Reed also said that he "doubled the particle yield" and that "it may take a few seconds to recharge after each shot."
This reminds me of the Cardassian rifle from "Return to Grace", which had to recharge as well, although it was measured in milliseconds, but had a fixed power rated in megajoules as well. It was opposed to the Federation rifle which had a fully autonomous recharge, meaning that the Cardassian rifle had no capacity of a full autonomous recharge, and most likely had to rely on a power battery to be plugged to in order to get a full recharge, even if it was very fast (basically, you plug it and unplug it immediately).
This was my take on it, meaning the Cardassian simple infantry weapon had a max energy stock of 4.7 MJ.
So following this logic, with the more primitive phase pistols, the megajoule power rate could easily be a reference to the power capacity after a pistol would be fully charged.
That said, I say primitive, but since Reed says that it may take a couple seconds to recharge after each shot, it both means the pistol
can fire its entire charge in one burst (so that's some 10 MJ tops) and it is capable of autonomous recharge.
It's more than enough to match the large breaches in the wall at the beginning of the episode btw.
Quite safely, we could think the firepower would be in the high kilowatt, and possibly in the megawatt range, although there's no telling how long they could keep firing those weapons at such a rate (in total).
What's also impressive is that the Borg drones
didn't explode like mad when hit by such yields with pistols on maximum, and a single drone's shields easily tanked the maximum setting shots of a pistol and two rifles, before Reed made modifications. They later adapted to that. Now, it doesn't mean each drone produces that much power (they could remotely tap into the ship's power grid) but you get to know what you're up to anyway once aboard a Borg ship.