Potence is around 2.5 times stronger than Strength
How strong is the regular Strength?
- As @eimyr told you already, p. 261 of the V20 Core Book give a detailed explanation of the way Strength works. Basically, if your Strength matches the action you want to take (look at the table on this page), you can take the action in question.
- If your Strength is not enough, you can still try to take the action, but the result is not guaranteed. You roll your Willpower (not Strength!) against Difficulty 9, and each success advances you by one step on the table. For example, a character with a rather average Strength of 2 can break through wooden doors rather reliably if they have a Willpower score of 10 (yielding them an average of 1 success each time they roll against Difficulty 9).
- Take note: you can also spend Willpower on that roll per normal rules outlined on pp. 266-267 to get an automatic success on that roll and prevent a botch.
- Pages 96-97 also give some extra explanation on "how strong the regular Strength is", mainly through listing deadlift potential of a given character -- again, presumably without a roll, with the same ability to spend and to roll Willpower to lift more the normally allowed.
- Page 259 lists one's carrying capacity based on their Strength and the rules of carrying too much. It does not list any rolls and ways to affect this, but as a Storyteller I would allow a player to roll and spend Willpower like described on p. 261, albeit with the result taking action for one Scene.
- Time is complicated in the World of Darkness. For the rules of time, check p. 254.
- When you roll Strength against a given difficulty, which is virtually always 6, each die you throw has 50% chance to give you +1 Success (rolling 6+), 10% chance to roll -1 Success (rolling 1), and 40% chance not to change the result (rolling 2, 3, 4 or 5). *On average, that's 0.5*1+10%*(-1)+40%0=0.4. So, one dot of Strength is worth 0.4 Successes on a typical roll. This usage of Strength will be needed to actually compare Strength to Potence.
- Sometimes (like in case of breaking a door with brute force, p. 261) the difficulty can change. As a rule of thumb, the expected amount of successes per die is (10-Difficulty)*0.1, which is 0.2 per die for the Difficulty 8 and 0.3 per die for the Difficulty 7. Jumping only requires a roll against Difficulty 3 (0.7 Successes per dot).
- Specialty, as described on page 96, gives you an extra Success for each die showing 10. On average, it gives you +0.1 Success per die (as there is a 10% chance to get an extra +1). This gives a 25% increase compared to a roll against normal Difficulty (6). So, for example, a sportsman specializing in the aforementioned deadlift and a Strength score of 4 should certainly lift as someone with a Strength score of 5. This guy still hits, punches and grapples with the Strength of 4, only the deadlift score is improved.
How strong is Potence?
That's the point where editions matter.
- In Revised, each dot of Potence just gave you a plain +1 Success on any Strength roll. It is unknown if they can be lost with botch dice or effectively prevent failed and/or botched Strength rolls -- ask your Storyteller.
- Page 192 of V20 Core has an explanation of the general way to use Potence. In short:
- Each dot is a plain +1 increase in Strength (effective Strength=Strength+Potence).
- If you spend 1 Blood Point as a reflexive action, for one round instead of the extra Strength dots you get from Potence, you get automatic successes as in Revised edition (see above).
Many people around my area (Moscow, Russia) agree that Potence was overnerfed in V20 and use the rules from Revised edition.
So, how does Potence compare to Strength?
Now that we have learned Potence gives a plain +1 Success per dot in any Strength-related roll and that Strength gives an average +0.4 Successes in such rolls, we can say that Potence dots are (1/0.4=2.5) times stronger, 2.5 times more valuable.
But how strong a character with Potence actually is?
I personally calculate character's effective lifting score using this formula: 2.5*Potence+Strength. Let's look at some examples.
- A weak schoolgirl with Strength score of 1 gets ghouled by a Toreador pervert and automatically gets Potence 1. She couldn't lift her own weight before, but now her effective lifting score is 2.5+1=3.5, so she can easily lift 175 lbs (around 75 kg) with no roll involved.
- A street brawler with Strength of 3 becomes a Brujah Shovelhead and gets 4(!) dots of Potence and has an effective lifting score of 2.5*4+3=13. That's damn a lot! According to page 261, he can easily "throw a station wagon" or lift 4000 lbs (1815 kg), and spending Willpower would allow to "throw a van"!
- A deadlift specialist from the previous example wants to cheat on a competition that does not have doping control. He buys a "cool new medicine" from blackmarket, which turns out to be vampire blood. Strength of 4 and Potence 1 give a total score of 6.5 (throwing a motorcycle or deadlift of around 850 lbs/385 kg with no roll). A successful Willpower roll adds +1 to this score, but the guy also spends a Willpower point on the roll, and gets a total of +1. It's a hard question when to apply the the increase from specialy, but if we apply it now, to the total score of 8.5, which should allow a lift of 1100 lbs/500 kg, that's lifting score of 10.625, or more than 1750 lbs/or 795 kg. Impressive!
OK, OK, got that. But what about the "physical might beyond mortal bounds"?
First we need to understand "mortal bounds". Humans can't get any Attribute above 5, under no circumstances. Strength of 5, perhaps with a Specialty at something, is all that one can achieve. Spending Willpower can increase this result, so the best one can hope for is deadlifting score of 7*1.25=8.75, a bit more than 1100 lbs/500 kg. That's close to the stuff we see in the world records
If we use the Revised rules, just one dot in Potence is enough to overcome "mortal bouns" if your regular Strength is 3+, because 3+2.5=5.5, and 5.5>5. If you spend Blood Points to increase your Strength, that's achievable earlier. But two dots of Potence actually those "mortal bounds" even for someone with Strength of just one. I guess, both Potence 1 and Potence 2 count as "the lowest ranks", as they actually are two literal lowest ranks of Potence.
Cool. Do I have to do all those calculations when I actually play?
No, definitely don't waste play time on that. Precalculate some important stuff, give some rough bonuses or penalties if the related stats change during the game, built cheat sheets for your NPCs.
While numbers are fun, especially in oWoD, it's not a game about numbers.