Everyone knows that hot water for showers and baths can be expensive, but just how much does it really cost, and how much might you save having shorter showers, or maybe having a bath instead?

The costs given here are a guide only. That is because there are several variables in the calculations such as the flow rate of the shower head, and how long you stay enjoying that lovely hot shower.

If you want to be even more accurate than this guide, you can measure the flow rate of your shower head by counting how many seconds it takes to fill a bucket. For example, a 10 litre bucket that takes 30 seconds to fill, is using 20 litres of water a minute. You might want to change that shower head for one that uses less water. “Economy” shower heads typically use less than 10 litres of water per minute, and can be purchased cheaply at a hardware store.

How much water bath tubs hold also varies from house to house. You can work out the exact amount by filling the bath to the level you like by measuring it out of the tap using a bucket of known capacity.

Spoiler alert: the cheapest shower or bath is a cold one – BRRRR!!!

The cost of hot water for a shower or bath has been calculated for this item at a bathing temperature of 40 degrees Celsius (C), and assumes the water has been heated from 8 degrees C (average cold water supply temperature over a year).  The amount of energy required to heat a litre of water to 40 degrees C is 0.04 Kwh (rounded up from 0.0372) – which in other energy measures is also equal to 133952J,  31893cal,  37Wh and 127 btu.

The unit cost of a kilowatt hour (Kwh) varies from customer to customer, but it is somewhere around 37 cents per kilowatt as of March 2025. This may seem to be outrageously expensive, but to prove its reasonableness simply divide the actual amount you pay by the number of kilowatts your energy account says you use, and you will get a similar answer to 37 cents per unit. About half the cost of delivered energy is the daily infrastructure charges (pipes and wires) and GST.

You now have the numbers you need to work out your own costs using your own water usage, and kilowatt costs.

Here are some examples. Substitute your own numbers to work out your exact situation. Note how you can simply manipulate the times within an example to work out other shower length scenarios.

Example 1

• Shower flow rate 6 litres per minute for 3 minutes = 18 litres x .037 kwh/l x 37 cents = 25 cents
• Shower flow rate 6 litres per minute for 5 minutes = 30 litres x .037 kwh/l x 37 cents = 41 cents
• Shower flow rate 6 litres per minute for 8 minutes = 48 litres x .037 kwh/l x 37 cents = 66 cents

Example 2

• Shower flow rate 10 litres per minute for 3 minutes =30 litres x .037 kwh x 37 cents = 41 cents
• Shower flow rate 10 litres per minute for 5 minutes = 50 litres x .037 kwh x 37 cents = 68 cents
• Shower flow rate 10 litres per minute for 8 minutes = 80 litres x .037 kwh x 37 cents = 110 cents

Example 3

• Shower flow rate 15 litres per minute for 3 minutes = 45 litres x .037 kwh x 37 cents = 62cents
• Shower flow rate 15 litres per minute for 5 minutes = 75 litres x .037 kwh x 37 cents = 103 cents
• Shower flow rate 15 litres per minute for 8 minutes = 120 litres x .037 kwh x 37 cents = 165 cents

Baths

A “standard” bath filled to about 200mm deep before you get into it, uses about 160 litres of water. 160 litres x .037 kwh x 37 cents /kwr = $2.20

Making Dollar Savings

Saving money on hot water is directly related to how many litres of hot water are used.

A low-flow shower head allows a reasonable length of shower with minimal water usage and can be a good solution for families where controlling usage might be difficult.

A “thank you” to Stu for sending this article to The Upper Hutt Connection

20/03/25