How Much Does Your Hot Water Cost?
How much does it actually cost to make hot water? An often asked question. Firstly it depends greatly on what system is used and the price of the fuel to run it! To answer this question properly, let’s look at it in more detail
The energy required to heat water is the same, regardless of what method is chosen. This is because the laws of physics dictate the energy requirement, not the type or brand of product. Every liquid has a “Specific Heat Capacity” value. In the case of water that value is 4.187kJ / kg K.
This means that 4.187 Kilojoules of heat energy is required to raise the temperature of one kg mass (one litre) of water by one degree C at standard temperature and pressure.
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The ability of any particular system to deliver this heat energy is governed by its efficiency. If a system requires twice as much energy to what can be extracted (in the form of hot water equivalent) then the system has an efficiency of only 50%.
Some common forms of water heating are:-
- Electricity
- Gas (Natural or LPG)
- Heat Pump
- Solar
- Combination of some of the above
The efficiency of systems within these groups can be generalised as follows:-
| System | Heater Type | Efficiency | |
| Electric | Storage | 70% – 80% | |
| Electric | Instantaneous | 99% | |
| Gas | Storage | 49% – 66% | |
| Gas | Continuous | 79% | |
| Solar | Storage with Elec Boost | 75% Elec Eff. Ave | Total Sys.249% |
| Solar | Storage with Gas Boost | 60% Gas Eff. Ave | TotalSys.200% |
| Quantum | Heat Pump Air Sourced | Day 325% | Ave309% |
| Night 289% |
* The above figures are based on average Sydney conditions.
From the above figures it will be evident that no system can exceed 100% efficiency unless it can get some of its energy input from a free source that does not cost any resource to produce.
In the case of a solar flat plate heater, that source is the sun (direct or diffuse radiation).
In the case of a Quantum Compact Heat Pump, that source is the heat energy in the environment (sun, wind, rain & air).
In the case of a Quantum Air Sourced Heat Pump, that source is the heat energy in the air.
The formula to calculate the energy required to produce hot water is:-
There are 3.6 MJ in a kWh
The cost to produce this energy requirement is arrived at by multiplying the MJ or kWh by the purchase price for the fuel.
e.g.
$0.0676 / kWh for off peak 2 electricity
$0.0127 / MJ for natural gas (small volume user)
The fuel costs per unit of energy are listed in the table below. These are average for the Sydney / Newcastle region of Australia (June 2000). Although fuel prices may have changed since time of writing or other regions may have different costs per kWh, the ratios are still the same.
| Fuel Tariffs | |
| Fuel | Tariff (Example only) |
| Electric Off Peak 1 | $0.0372 / kWh |
| Electric Off Peak 2 | $0.0676 / kWh |
| Electric Domestic | $0.1025 / kWh |
| Gas (Large Volume User) | $0.00962 / MJ |
| Gas (Small Volume User) | $0.01270 / MJ |
The following table shows the cost to produce 1000 litres of hot water at 45°C temperature rise, using the tariffs shown above for the various water heating methods previously mentioned.
| Cost Per 1000 Litres Of Hot Water | |
| System | Cost |
| Electric Storage Off Peak 1 | $2.78 |
| Electric Storage Off Peak 2 | $4.72 |
| Electric Storage Domestic Full Rate | $6.70 |
| Electric Instantaneous Domestic Full Rate | $5.42 |
| Gas Storage Standard Efficiency | $3.70 |
| Gas Storage High Efficiency | $2.75 |
| Gas Continuous High Efficiency | $2.29 |
| Solar Storage With Electric Boost Off Peak 2 | $1.42 |
| Solar Storage With Electric Boost Full Domestic Rate | $2.15 |
| Solar Storage With Gas Boost | $1.20 |
| Quantum Heat Pump Air Sourced Off Peak 2 | $1.14 |
While these tarriffs may change with time, the relative costs should remain similar. You may like to substitute the current costs in your area to produce exact figures.
Please check your local electricity Authority for applicable tarrifs in your area. Canberra residents should contact ACTEW.
As can be seen from this information, the Quantum Heat Pump Hot Water System provides the lowest production cost per 1000 litres of hot water.