A while ago, I had a discussion with my girlfriend about best heating decisions for hot water to make tea.

Is it more cost-effective to heat 1 liter of water with an electric kettle or to start with warm water from the sink and then heat it?

So let’s do the math and explore whether it’s better to heat 1 liter of water directly with an electric kettle or if it’s cheaper to use warm tap water and then heat it further.

Assumptions

For this, we have a few assumptions:

• cost of electricity: 0,3€/kWh
• cost of gas heating: 0,1€/kWh
• tap water heat: 10°C
• water cost: freshwater: 1.86€/m³, wastewater: 2.02€/m³ -> 3.88€/m³

We assume, that taking water from the tap is about 36°C hot, as we do not want to let it run a long time, which would waste a lot of water. Of course, as water is not directly heated in the pipe, we have more waste water as we need to wait for the hot water at first. We assume, that we waste 2 Liter of water for that.

Delta Calculation

We will first calculate the difference in electricity cost, when using gas, to see what the raw potential would be:

1. Heating water directly in the kettle:

   • From 36°C to 100°C: This requires 0,074 kWh of electricity.
     • At 30 cents per kWh, this comes to 2.2 cents.
   • From 10°C to 100°C: This requires 0,105 kWh of electricity.
     • At 30 cents per kWh, this totals 3.15 cents.
   • Difference in cost: It’s only 0,95 cents more to heat water from a lower starting temperature (10°C).

2. Heating with a gas burner:

   • The gas burner heats the water slowly up to 36°C. During this process, we’re using 0,03 kWh of energy, which costs around 0,3 cents at 10 cents per kWh.
   • However, you need extra water because you’re using warm tap water first. In total, 2 extra liters of water flow through the tap.
   • With 0,388 cents per liter. Total extra water cost for this method is 0,776 cents for 2 liters.
   • Total gas cost is 0,3 cents for the burner to heat the water up to 36°C (0,1€/kWh*0,003kWh). This excludes the lost heat of the one liter waste water which is harder to calculate.

Thus, heating with a gas + electricity combination leads to an additional 1.076 cents when factoring in the gas and extra water used.

Cost Breakdown

Electric kettle (using cold tap water): 3,538 cents

• Water: 0,388 cents
• Electricity: 3,15 cents

Using gas to heat the water: 3,664 cents

• Electricity: 2,2 cents
• Gas: 0,3 cents
• Water: 0,38 cents
• Extra water: 0,776 cents

Conclusion

When comparing the costs, it turns out that heating water directly with an electric kettle is slightly cheaper. The difference is small, but with the ongoing rise in gas prices and potential future reductions in electricity costs, using solely the electric kettle is better most of the time.

Of course, the 2 litre waste water is quite much, and we did not take heating of the waste water into consideration, but thats okay

What’s interesting is that water costs play a significant role here. The extra water used in the gas method adds a notable amount to the overall cost.

In the end, while the difference is small, electricity is currently the more better option, if you’re heating water in small quantities. For your individual calculation, you can use

Water Heating Cost Calculator