Read both registers before comparing costs
A two-rate meter can show a lower or night register and a normal or day register. Record both readings and confirm which rate label applies; do not assume that a register number always means night usage.
Compare the whole tariff cost
The useful comparison includes day usage, night usage, both unit rates, both standing charges and the same date range. A cheaper off-peak rate does not automatically make the total tariff cheaper.
There is no universal break-even percentage
The night-use share at which a multi-rate tariff becomes cheaper depends on the rates, standing charges, usage period and the single-rate comparison. Use your own figures rather than a generic threshold.
Check whether the account and meter match
Compare the registers and rates on the bill with the meter display. If readings, labels or billed periods do not line up, ask the supplier to explain them before making a tariff decision.