There was a patch a couple weeks ago that changed this. I haven’t nailed it down exactly but I think I have enough information to describe closely enough how it works now.
Power coils now reduce BOTH spark cost and time to produce. The multiplier they use applies to both spark and production time for a given input power. For example 2300 power is needed to reduce spark cost of gold grapples by 50%. The time it takes to produce is also halved. The following talks about how to get said multiplier that applies to both spark and production time.
The best multiplier you can get is 0.4675, which means 46.75% of the original spark cost and production time. adding a power coil above the recipe requirement reduces the multiplier by roughly 5% of the remaining amount. This reduces to roughly 4% after 1000 power, 3% after 2000 power etc. This means there are diminishing returns due to the reduction relative to the remaining multiplier, and the shrinking of the reduction itself.
For example, with a green dot every 500 power, you can see it eventually goes flat:
When there is no power requirement for the recipe, the first power coil seems to reduce the multiplier to about 0.82 (almost 20% just with 100 power), then continue at the 5% reduction stated above. Whereas recipes that require power start at the 5% reduction stated above.
The power requirement seems to have a non-linear affect on the maximum number of power coils needed to achieve the 0.4675 multiplier. For example a recipe requiring 0 power, 500 power, or 1000 power all require 2300 power more than their requirement to achieve a 0.50 multiplier, and ~4600 power more than their requirement to achieve the 0.4675 multiplier. Whereas a recipe needing 1500 power achieves a 0.52 multiplier at 2300 power above it’s requirement and a recipe needing 3000 power achieves a 0.55 multiplier at 2300 power above it’s requirement.
Sorry this is not as specific as my previous post, but the calculation appears to be much more complex now. I’m sure someone can figure it out, but I think for now this is good enough.
Bottom Line
If a recipe does not require power, 100 power will reduce your spark and production time to 80%. Otherwise it will reduce it to 95%
2300 power above the recipe requirement gets you about 50% spark cost and production time. Add a couple hundred more for recipes needing above 1000 power.
4600 power above the recipe requirement gets you about 45% spark cost and production time. Add a few hundred more for recipes needing above 1000 power.