What size do you need?

People use far more water than they realize. A typical 4 person home will use between 1,000 and 2,000 litres per day.

The amount of electricity consumed and the amount of water used in a shower is also far more than people realize. A typical high pressure shower will use 16+ litres per minute

The easy guide – PAYGS solar sizing

How Much Electricity Do You Use Heating Water?

You can quickly work out with 90%+ accuracy how much electricity you are using in heating water by:

Add up the number of minutes everyone in the house spends in the shower

Multiply the minutes by 18 ( a typical showerhead output) to arrive at the total amount of potable hot water in litres at 40 C being used.

Add 50 litres for general use

Divide the total litres by 36 to arrive at the approximate kWh consumption. (Approximately 1 kWh is consumed heating 36 litres to 40C using the specific heat of water formula- see below)

Minutes in Shower Potable hot water litres @40°C per minute Total litres @40°C used Add general home use litres @40°C Total litres @40°C used Litres ÷ 36 = kWh
10 18 180 50 230 6.39
12 18 216 50 266 7.39
14 18 252 50 302 8.39
16 18 288 50 338 9.39
18 18 324 50 374 10.39
20 18 360 50 410 11.39
22 18 396 50 446 12.39
24 18 432 50 482 13.39

Season Cold Water in at °C Hot Potable at 40°C Difference in °C Volume Litres kWh
Winter 11 40 29 36 1.21
Average 16 40 24 36 1.00
Summer 20 40 20 36 0.84

Hot Water from the Geyser and Potable Hot Water for Washing

The water in an electric geyser is normally heated to 60 °C

When mixed with cold water from the tap at 16 °C, the overall temperature is reduced to the temperature that is usual for people to wash at 40 °C. This is similar to lowering the temperature of a cup of coffee by adding cold milk.

The amount of potable hot water at 40 °C from electric geysers is shown below (assuming a single draw off) with the geyser at 60 °C and the cold water at 16 °C

Potable Hot Water Litres at 40 C from an Electric Geyser at 60 C

As the PAYGS systems are close to 100% efficient in replacing electricity by solar, the hot potable water will be the same as in an electric geyser (assuming 1 full tank)

BUT an electric geyser will turn on and off throughout the day!

With the solar geyser you need to size the solar system to the amount of potable water you are using in a 24 hour period

As in the ‘Easy Guide’ illustration the graph below shows the number of total minutes you will get from a PAYGS SWH system for 150l, 200l and 300l

Minutes in a Shower at 18l per minute from electric geyser