How is usage data calculated?

In its simplest form, the calculated energy used for heating and hot water is shown below.

Summary:

Energy used
Homely calculates this from the figures below.

Heat delivered
The difference between flow and return, which are both provided by the heat pump.

CoP
Provided by the manufacturer, often from the heat pump.

Details of the calculation:

Heat delivered (q) 
q = m · c · ΔT

q = heat delivered (Watts, i.e. Joules per second)
m = mass flow rate of water (kg/s)
c = specific heat capacity of water (≈ 4180 J/kg·°C)
ΔT = usually flow − return (°C), 

Example calculation:

Flow rate = 20 L/min
(convert to mass flow)
20 L/min ÷ 60 = 0.333 L/s
m ≈ 0.333 kg/s

Flow = 50°C
Return = 45°C
ΔT = 5°C

∴ Heat delivered

q = 0.333 × 4180 × 5
q = 6,960W ≈ 7kW

CoP
Heat Output / Electric Input

Manufacturers publish CoP tables against conditions like:

Real conditions usually fall between table points, Homely interpolates CoP between nearest values

Example after interpolation:

Outdoor = say 5°C
Flow = 50°C
Interpolated CoP ≈ 3.0

Electricity used
q / CoP

Example calculation:

q ≈ 7.0 kW
CoP ≈ 3.0

Electricity used ≈ 7.0 / 3.0 = 2.33 kW