usefulweb.site Bill Detective

Money-saving & comparison

Bill Detective

Spot spikes, price changes and calculation errors across your utility bills.

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Billed total may differ from consumption × unit price + charges by this much (rounding, taxes included in prices) before it is flagged.

Billing periods (required)

Start and end dates are both counted as billing days. Fixed charge = total standing/base charge for the period. Leave Total billed empty to skip the arithmetic check for a row.

Billing periods
#PeriodStart dateEnd dateConsumptionUnit priceFixed / standing chargeOther chargesTotal billed
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Try an example: Monthly electricity with a winter spike and a price rise · Quarterly water bills

How it works

Enter several billing periods of an electricity, gas, water or data bill. Bill Detective normalises consumption per day, flags unusual periods (outliers), unit-price and standing-charge changes, and totals that do not add up, each with a plain-English explanation.

  • Days = end date − start date + 1 (both dates count). Consumption per day = consumption ÷ days. Periods are sorted by start date.
  • Computed total = consumption × unit price + fixed charge + other charges. A mismatch is flagged when |total billed − computed| > tolerance % × computed (minimum 0.01).
  • Consumption change: per-day consumption vs. the previous period; flagged when the absolute change exceeds the threshold (default 25%).
  • Above average: per-day consumption more than the threshold above the average of all periods.
  • Outliers (IQR rule, only with ≥ 4 periods): per-day value above Q3 + 1.5 × IQR or below Q1 − 1.5 × IQR, quartiles by linear interpolation.
  • Outliers (median rule): per-day value more than X% (default 30%) above the median per-day value.
  • Unit price change: any change vs. the previous period. Fixed charge change: both the fixed charge per period and the fixed charge per day differ from the previous period by more than 1% (so a flat monthly fee over months of different length, or a fixed daily rate, is not flagged).
  • Gaps and overlaps between consecutive periods are reported as information.

For scripts and AI agents

  • JSON API: POST https://billdetective.usefulweb.site/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://billdetective.usefulweb.site/mcp · tool name bill_detective
Example request
curl -s -X POST https://billdetective.usefulweb.site/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{…see OpenAPI examples…}'
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