Battery runtime is a simple division: capacity divided by draw. Both sides change over time, which is why a laptop that lasted eight hours new may manage three after two years.
The screen is usually the largest single consumer. Brightness has a disproportionate effect, and a bright display alone can halve runtime.
Background work is the hidden half. Sync clients, indexing, updates and browser tabs keep the processor awake, and a processor that never reaches its low-power state costs far more than one that idles.
Battery health is the ceiling you cannot argue with. Cells lose capacity with charge cycles and heat, and a worn battery reports a full charge that simply contains less energy.