A fixed monthly base charge is dollars you owe before your first kWh. Spread over a big bill it’s negligible; spread over a small one it dominates the effective per-kWh price.
Real example — TXU Energy Simple Rate 12. Its $9.95/mo base charge is invisible at high usage but pushes the effective price up sharply at low usage — the same fixed dollars divided by fewer kWh. A base charge is a bet that you use a lot.
If you use under ~500 kWh most months, a plan with a big base charge is quietly expensive even when its energy rate looks low.