Checking a rate change partway through a bill
When a new rate starts during a billing period, all consumption should not automatically be priced at either the old or the new rate. First find out how the supplier splits usage across the effective date.
Two prices need two quantities
Find the tariff notice and separate charge lines. If readings identify 120 kWh before the change and 180 kWh after it, use those quantities. At fictional USD rates of 0.20 and 0.25/kWh, the energy charge is 120 × 0.20 + 180 × 0.25 = USD 69. Pricing all 300 kWh at the new rate gives USD 75, overstating this example by USD 6.
Day allocation is an assumption
Without interval readings, a provider may allocate usage under its published billing rules. Equal daily allocation for a 30-day, 300 kWh bill split after 12 days assigns 120 kWh to the first part. This matches the example only because daily use is assumed constant. A cold spell in the second part could make the allocation differ from actual timing. Ask which method was used rather than silently substituting your own.
Fees and allowances have their own rules
A daily charge follows the relevant day count. Monthly fees may have different proration rules. Do not reset a full monthly tier allowance at the rate change unless the tariff requires it. Tax and rounding may apply line by line.
Use a zero-fee calculator run per flat-rate segment and retain its dates. Add the billed fixed charges once, then apply the correct tax base. If quantities differ from the statement, investigate allocation first; changing a unit price will not correct the split.
Electricity & Water Bill Calculator
Worked example: Check a rate change with stable consumption
Related reading: How time-of-use electricity pricing changes your costs
Reference: EIA: electricity prices and time-based pricing.