There are two components to consider here...first is your power generation.
I have to throw the assumption out there that power consumption is going to decrease at a similar rate as our die off...a 90% population die off is also going to come with a sharp drop in our power consumption. A few sources:
Coal - coal power generation doesn't just need a few highly skilled operators to maintain...it also requires a distribution system to get the coal to the plant and a mining system to get the coal from the ground. About 40% of our power comes from coal...but if the population has died off so significantly, do we really need to depend on this?
Natural gas - same as coal really, you need to get the gas out of the ground and into the power station. about 25% of our power is derived from natural gas and once again fits into the 'if were not using that much power, do we really need to maintain this?' category.
Nuclear - The skill and knowledge level to build and maintain these, as well as properly mine and refine uranium makes nuclear rather difficult to maintain as well.
Renewable - Hydro, wind, and solar generation make up around 10% of our generation and have no inputs that we need to refine and bring to the generation site, nor is it that manpower intensive to maintain (solar panel creation might not be so).
Almost a backwards answer to your question...but I see a 50% drop in our population as an immediate threat to our power generation, and in a few methods...if production of coal drops with our population, or production of natural gas...then we get into issues. If our distribution system that brings the coal and gas to the power plants drops, then we get into issues as well. However is so many of us die off that we no longer have the need for 90% of our power generation, then we find ourselves capable of meeting our energy needs.
Part 2 is the power lines and distribution grid. This is a bit more abstract as it's very often extreme events (snow storms) that cause issues here, but there is a large amount of maintainence required here just to keep the power flowing. It's a risky job, but the education level here isn't quite what you need compared to operators. Distribution is pretty much hub and spoke...your power generation goes to a central facility that connects to a variety of substations and then down from there. I actually think this becomes the same situation as above...if our population drops low enough that the population has to relocate to a concentrated area, then we'll be fine...you only need to maintain the part of the grid you us after all. However, if our population dropped but not enough to warrent a mass relocation of people to a more central location, then we are in the situation where there might not be enough people to fully maintain a sprawled distribution system.
Not sure I expected this conclusion when writing this answer...but it would appear there is a couple thresholds that can be crossed. The first threshold is when we are no longer capable of maintaining our current usage needs through our existing methods of energy production. But there also appears to be a second threshold where our population has dropped low enough that we can depend on the less intensive power generation techniques to meet our demands. I'd say a 50% population drop will get us in trouble...by about 90% we start to reach a critical point where our power generation can easily be handled by less intensive methods.