A tariff with a low unit rate and a high standing charge rewards heavy users. A tariff with a high unit rate and a low standing charge rewards light users — and for someone in a small flat who is out all day, or who has solar and imports very little, the difference can be over £100 a year. The crossover point is (standing₂ − standing₁) × 365 ÷ (unit₁ − unit₂), and this calculates it for you.
Read your meter rather than trusting an estimate, and read it on the same day you switch so the closing bill is accurate. Figures here exclude any exit fees, direct-debit discounts or smart-meter rewards, which you should add yourself.