top of page


Water heating savings calculator


How much money can a heat pump or solar save on water heating?


Monthly outgoings can be hard to meet: mortgage, energy costs, food and many more. Water heating costs are fixed and cannot be changed unless we don't shower anymore or wash ourselves cold. In New Zealand, most households with three or more people with a water cylinder and electric element will spend per year above $1000 to heat their water. But you might be surprised that in spending money on more efficient water heating options you might save money every month. 


We have created a calculator that calculates your monthly mortgage and water heating payments. While putting a heat pump or solar hot water heating system will increase your monthly mortgage payments, there is a good chance that they will more than offset those costs with the savings on your monthly water heating bill.


Our calculator is designed for private homes right now. To help you with your commercial project or any other site around the globe, here is the formula for you.


Required Energy = daily amount of litres of water x 1.16 x (Required temperature – freshwater temperature)


For example:

Required Energy = 300 litres x 1.16 x (65 degrees C – 12 degrees C) = 18444 Watt-hours = 18.44 kWh


Finally, you need to multiply the result by your price per kWh and then multiply that result by 365 days. The result gives you your annual cost.

Different climates, materials and living stiles require different products for best results. Get in touch for a free consultation- we help you find a water heating solution that works best for you according to science, not a marketing message.

Solar systems save on average 70% off your water heating energy costs (if you have a roof that faces the midday sun and is not shaded). Heat pumps on average save you 65% off your water heating costs.


Having all of those numbers, you can now calculate your payback period based on the quote you have for a hot water heat pump or solar water heating system.



Energy Savings Calculator

How does this calculator work?

You can pick your amount of mortgage, years, interest rate and chose your current electric water heating size. Then, decide if you would like to see the results for either solar or a heat pump. We have taken average pricing and efficiency rates of heat pumps and solar hot water heating systems.


We have taken here only water heating options that heat the water directly. We have here, for example, ignored solar photovoltaic (electric) panels to run the electric element in a water cylinder. While in general solar photovoltaic is less efficient than solar water heating (more panels required), they do have other advantages. In essence, solar photovoltaic usually has a 25-year warranty on panels (solar water heating ten years). If the water cylinder is up to temperature, then the electricity can be used to power other items in your household, utilising more of the available energy. Please use our other calculator for calculating your estimated savings on solar photovoltaic panels.

  • Assumptions on heat pump water heating

We have assumed a medium quality and priced heat pump costing about NZ$6k. The efficiency of such a product will save you on average, about 65% of your water heating costs every year. The exact figure for your house might be slightly better or worse, so might the price. We have based all our assumptions for weather conditions, costs and efficiency on official numbers by NIWA and Energywise. A heat pump lasts on average between 10 and 15 years at present according to multiple electricians we spoke with.


Savings gained by a heat pump are more reliable as solar. This is because a heat pump relies more on temperature than actual sunshine. A heat pump is most efficient, when the temperature is just above 14 degrees Celsius at the outdoor unit. The heat pump will heat your water to 55 degrees Celsius. Your electric element will kick in every few days to heat the water to or above 60 degrees Celsius to kill off any potential legionella.

  • Assumptions on solar water heating

To be efficient, solar water heating needs an in NZ north-facing roof and a mounting angle that equals the latitude of your location. Other mounting aspects or roof orientation make the solar work less efficient, with the most critical factor being the roof orientation. However, many roofs are facing north or are just slightly out. The solar industry and our personal experience would give solar in right conditions 60-80% saving off your annual water heating bill. We have assumed an average of 70% and an average price of NZ$7k. Again, our numbers for weather and costs are based on official figures of NIWA and Energywise. 


A solar hot water heating system lasts on average between 20 and 30 years according to multiple plumbers we spoke with. The savings for solar water heating vary within the year. Higher savings in summer (often close to 100% during sunny days) and lower savings in winter. If you belief that the sun will not shine as much in future as it did in the past, then solar is not for you. If you, however, belief that climate warming is real and the sun will shine more/ stronger, then with the right roof solar might be the option for you.


- We have assumed all showers to be 10 minutes long and with a flow of 11ltr per minute

- Our base cost for water heating is based on heating with an electric element and average numbers are taken off Energywise

bottom of page