# Introduction to Deltas

Deltas are (usually interpolated) consumption values matching calendar months.

The basic example is an electricity meter with readings:
- 2017-12-31 23:00:00 - 820 kWh
- 2018-01-01 03:00:00 - 824 kWh
- ...
- 2018-01-31 23:00:00 - 978 kWh
- 2018-02-01 01:00:00 - 980 kWh

Readings are just samples of a continuous process. To obtain a delta for Jan '18, we first interpolate readings at the beginning and end of the month:

- 820 kWh + (824 kWh - 820 kWh) / 4h * 1h = 821 kWh
- 978 kWh + (980 kWh - 978 kWh) / 2h * 1h = 979 kWh

The difference is the delta for Jan '18: 979 kWh - 821 kWh = 158 kWh.