Can't create energy unfortunately...
The voltage drops because of Ohm's Law. In your case:
Voltage droop in power line = square-root of power used times the resistance in the power line
If you see the lights dim occasionally, at those moments, there is too much load on the line.
Your only solution, conceptually, is to add more energy to the system.
A UPS stands for "Uninterruptible Power Supply". There are two basic types:
A true UPS in which power from the wall is converted and applied to a storage device, like a battery, and then the storage device is used to power anything connected to it's outlets.
An SPS ("Standby Power Supply") connects it's outlets to the power line directly and simply switches to a battery after it detects the power line is no longer providing sufficient power.
True UPS's work because the power always comes from their internal reserve with or without external power. SPS's detect-and-switch, which causes a bunch of problems in your scenario:
- There is a distinct switching transient that will result in brightness change
- It does not switch (by design) on power line transients
- Output (AC) power is typically of poor quality, which will manifest in a different effective light intensity
Unfortunately, true UPS's are very expensive and, given your interest in maximum outlet power ratings, I suspect that you will need a massive UPS to keep up with your power needs.
Virtually every "UPS" you find in the consumer electronics space is actually an SPS.
I would recommend that you just go completely off-grid and make your own power if you are so sensitive to light level. A 2.2kW capable generator runs around $600 (US).
You might also consider using a DC-to-AC inverter to use a bank of car batteries for this purpose.
Sounds like an interesting project!