High Gas Usage Calculated For Shared Hot Water

[AnthonyGu]

Hi all,

I’m trying to understand a discrepancy between my gas usage (in MJ) and the theoretical energy required to heat water. The numbers don’t seem to line up, even after accounting for system efficiency.

Meter Data

Read Date: 23 Mar 2026

Read Type: Actual

Start Read: 1,887

End Read: 3,342

Usage: 1,455 units

Unit Size: 10 L (i.e. tens of litres)

Conversion Factor: 0.477937

Total Energy: 6,954 MJ

So total water volume used:

1,455 × 10 L = 14,550 L

Expected Energy to Heat Water

Assuming:

Starting temp = 15°C

Target temp = 60°C

Temperature rise = 45°C

Specific heat capacity of water = 4.186 kJ/kg·°C

1 L ≈ 1 kg

Energy required:

Energy (kJ) = 14,550 × 4.186 × 45

= 2,741,000 kJ ≈ 2,741 MJ

Accounting for System Efficiency

Assuming a 70% efficient gas storage hot water system:

Required input energy = 2,741 ÷ 0.70

= 3,916 MJ

Comparison

Calculated requirement (with efficiency): ~3,916 MJ

Metered usage: 6,954 MJ

This is ~77% higher than expected — nearly double the theoretical heating requirement.

Question

Why is the MJ figure from the meter/conversion factor so high?

Even allowing for:

Storage losses

Pipe losses

Standing heat loss

Inefficiencies

…it still seems significantly above what physics would suggest is required to heat that volume of water.

[ChrisE_AGL · Accepted solution]

Hi @AnthonyGu 

Thank you for your post. 

The MJ gas usage on your bill is total gas used, not just the useful gas used to heat the water tank. 

If you have other items in your house that use gas (Stove or heater), these would also be factored into your total usage. 

With your math, while there are some assumptions, other things to factor would be if you have a pilot light present it will consume a set amount itself, water mains are often closer to 8-12 degrees rather than the 15 mentioned. Depending on size and insulation of your tank can depend on your standing heat loss. 
The 70% system efficiency you mention is typically onl the steady-state combustion efficiency only, it doesn't account for the standing loss, cycling or if pilot light is used, and with seasonal changes can make efficiency closer to 45-60% depending on time of year. 

Thanks, Chris