So when you repair a generator, your basic repair speed without any perks or toolboxes is 1 charge per second(c/s). It takes 80 charges to repair a generator and thus takes 80 seconds.

However, when you repair with another survivor there is a repair speed penalty of 10% per survivor on all survivors. Since repair speed bonuses are always additive these always means a penalty of .1 c/s per survivor.

Repair Potential Rating is the charges required to repair 5 generator (400) divided by the entire teams repair speed. A lower number is better because it means all gens get done faster.

Repair Potential Loss is the difference of repair speed without the penalty and the actual repair speed.

In reality you will rarely have 4 c/s team speed because someone will be getting chased
Additionally because there's only a limited number of generators on the map and generators only get completed in 80 charge bursts it will literally always be better to multi repair a generator when there is less generators left then people available to repair generators.
The only exception to this is if the killer is patrolling gens hard or 3 genning at the late game because then you have to force the killer to bounce gen to gen over and over again (AkA Juggling) so they can't stop both being repaired at a given time.

1 Survivor on a generator

Individual Generator Repair Speed: 1 c/s
Generator Repair Time: 80 seconds
Repair Potential Lost: 0 c/s
Team Repair Speed: 4 c/s
Repair Potential Rating: 100


2 Survivors on a generator

Individual Generator Repair Speed: 1.8 c/s
Generator Repair Time: 44.4 seconds
Repair Potential Lost: .2 c/s
2 Generators being doubled
Team Repair Speed: 3.6 c/s
Repair Potential Rating: 111.1
1 Generator being doubled
Team Repair Speed: 3.8 c/s
Repair Potential Rating: 105.3

3 Survivors on a generator

Individual Generator Repair Speed: 2.4 c/s
Generator Repair Time: 33.3 seconds
Repair Potential Lost: .6 c/s
Team Repair Speed: 3.4 c/s
Repair Potential Rating: 117.6

4 survivors on a generator

Individual Generator Repair Speed: 2.8 c/s
Gen Repair Time: 28.6 seconds
Repair Potential Lost: 1.2 c/s
Repair Potential Rating: 142.9

Break Down using the Prove Thyself Perk

1 Survivor on a generator

Individual Generator Repair Speed: 1 c/s
Generator Repair Time: 80 seconds
Repair Potential Lost: 0 c/s
Team Repair Speed: 4 c/s
Repair Potential Rating: 100


2 Survivors on a generator

Individual Generator Repair Speed: 2.0 c/s
Generator Repair Time: 40 seconds
Repair Potential Lost: 0 c/s
2 Generators being doubled
Team Repair Speed: 4.0 c/s
Repair Potential Rating: 100
1 Generator being doubled
Team Repair Speed: 4.0 c/s
Repair Potential Rating: 100

3 Survivors on a generator

Individual Generator Repair Speed: 2.7 c/s
Generator Repair Time: 29.6 seconds
Repair Potential Lost: .3 c/s
Team Repair Speed: 3.7 c/s
Repair Potential Rating: 108.1

4 survivors on a generator

Individual Generator Repair Speed: 3.2 c/s
Gen Repair Time: 25 seconds
Repair Potential Lost: .8 c/s
Repair Potential Rating: 125

by repairing a generator cooperatively, you trade total time to get all gens done for individual generator repair speed, however there is a diminishing return to this for each additional survivor.

In a regular game, repairing a generator with 3 or more people is almost never a good idea. The diminishing return on a 4th person repairing a generator is so bad that your overall speed gen repair speed is actually less than having 3 survivors and the individual time is only down less than 5 seconds from just doing it with 3 people. Never 4 man repair

Repairing a Generator with 3 people comes with a significant diminishing return, while it can be smart in certain situations and is almost always good in the late game since there is not very many generators to repair left, doing this early on is almost always a bad idea as the killer interrupting the generator is devastating to your repair progress. And the additional 11 seconds over just repairing the gen with 2 people instead usually won't make a difference.

Repairing the Generator with 2 people is where things start to get valuable. You are not sacrificing very much time at all in terms of the entire games repair speed however you bring your individual repair speed down by more than 30 seconds which is great.

Repairing a generator with 1 person while it has the best overall game time, a generator that takes 80 seconds to complete is much more likely to get interrupted by the killer before you can finish it. This is better than 3 or 4 manning. Don't waste your time looking for someone else to 2 man with, but 2 manning is better in MOST situations.

Now if someone is running prove thyself, this is when gen repair speeds start to get crazy. If someone is running prove thyself 4 manning is still usually a bad idea since the killer is going to be patrolling gens. 3 manning is pretty good with prove thyself. There are some situations where its better not too since it still only saves you 11 seconds compared to 2 manning however its way better than 3 manning without prove thyself since the entire game time impact is basically nothing.

Additionally with prove thyself, 2 manning is literally only an upgrade over 1 manning a generator. The only reason not to 2 man with prove thyself is you don't know where anyone is or the killer keeps interrupting the gens you're trying to work and you need to juggle him. You still shouldn't waste time looking for people, if you REALLY want to use prove thyself then use bond since you'll find people without even trying.

Also to note: Since you repair faster with multiple people, you're going to have more travel time between gens overall, since you're spending less time on gens individually. You are also going to be more likely to 3 gen yourself since people just tend to ooga booga see gen fix gen instead of thinking about gen placement when repairing.

Thanks to Happy Clown for the colored Prove Thyself Icon.

More info about those here