I spent a bit of time doing science with the power coils on my workbench. I assume the other power coils work the same.
The formula for how much the construction time is reduced by power is as follows:
First 100 Power
For the first 100 power over the recipe requirement, the construction time is reduced by 50% (in addition to below).
Each 100 power above the recipe requirement
For each 100 power above the recipe requirement, INCLUDING the first 100 power, an additional bonus applies according to this formula:
Reduction percent (of base construction time) = 100 / ( 8 X <Recipe Power Requirement> )
Maximum time reduction
The minimum crafting time is 25% of the original crafting time. If the above criteria leads to a number less than 25%, then 25% is used instead. 25% of the original crafting time is the same as a 75% reduction.
Examples
T2 Power Coils
Lets say I’m mass crafting T2 power coils, and I have 2100 power hooked up to my workbench.
These need 1500 power to craft, so I’m over the requirement by 600. They take 45000 seconds to mass craft.
First I get a 50% bonus because I’m over the power requirement.
Next I do the formula above:
100 / ( 8 X 1500 ) = 0.8333%
Since I’m 600 power over the requirement I multiply this number by 6 and add it to my original 50% to get 55%
Since than number is less than 75%, I reduce the original crafting time by that amount:
45000 X (1 - 0.55) = 24750
seconds, which is 5 hours, 37 minutes, 30 seconds.
If you do this same thing in game, you’ll see the actual recipe requirement is 5 hours, 37 minutes, 31 seconds, so there’s some rounding going on somewhere, but it’s close enough.
Decorative Gem Blocks
This recipe requires 4500 power. and takes 6000 seconds to mass craft. Lets say I have 5000 power hooked up to my workbench, so I’m 500 power over the requirement.
Again, we get the initial 50%
Run the formula we get:
100 / ( 8 X 4500 ) = 0.2777%
since we’re 500 power over, multiply that by 5 to get 1.3888%
Add that to our 50 and reduce the time by that percent
6000 X (1 - 0.513888) = 2916
seconds, which is 48 minutes, 36 seconds.
This is exactly what is seen in game with the same setup.
No Power Requirement
When you run the formula for a recipe with no power requirement, you get infinity:
100 / ( 8 X 0 ) = Infinity
Since infinity is greater than 75%, you get the full reduction with the first 100 power.
Max Useful Power
To determine how many coils are required to obtain the 75% reduction in crafting time, use this formula:
3 X <Recipe Power Requirement> + 100
The extra 100 is because of the rounding errors described above, but it also makes the formula work for 0 power recipes.
so for example, T2 Power coils need 1500 power so:
3 X 1500 + 100 = 4600
so 4600 power is required to achieve a 25% construction time. This is the same as what I saw in game.
Hopefully I didn’t mess up my math anywhere =D. Let me know if you spot something wrong or inconsistent with in-game behaviour.