Size a solar water heater and estimate the energy it saves.
Solar Water Heater Calculator: Free Online Tool
The solar water heater calculator helps you choose the capacity of a solar hot water system and estimate the energy it saves. You enter the number of people, the hot water used per person per day, the required temperature rise and the collector area needed for every 100 litres. The tool returns the daily demand, the recommended standard tank size, the collector area, and the energy saved per day and per year.
Home owners, hostel and hotel managers, plumbers and building services engineers use this kind of estimate when they compare supplier proposals. Water heating is a large share of household energy use, and a solar heater can replace much of it with free heat from the sun. A simple sizing check shows whether the proposed tank and collector are reasonable, and gives you a realistic idea of the savings.
When to use this calculator
Use it when you plan a new house, a student hostel, a clinic or a small guest house and want to know what size of heater to install. It is also useful when comparing a 150 litre and a 200 litre offer, when you need to reserve roof space for the collectors, or when you want to show the client how many kilowatt hours of electricity or gas energy the system may replace each year. It also gives a quick check on quotations and site requests before work starts.
How to use the Solar Water Heater Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Choose the unit system first (feet / inches or metres / mm). The input labels change to match your choice.
- Enter your values in the input boxes: Number of people, Hot water per person per day (litres), Temperature rise (°C), Collector area per 100 litres (m²). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
Daily demand equals the number of people multiplied by the litres per person. The calculator then picks the next standard capacity from 50, 100, 150, 200, 250, 300, 500, 1000 and 2000 litres per day. Collector area is capacity divided by 100, multiplied by the area per 100 litres. Energy per day equals litres x temperature rise x 0.001163 kWh, and the yearly figure is that multiplied by 365. The default case is 4 people, 40 litres each, a 30 degrees C rise and 2 m² per 100 litres, which gives 160 litres demand, a 200 litre system and 4.00 m² of collector.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| Hot water need | |
|---|---|
| Daily demand | 160 litres |
| Recommended system | |
| System capacity | 200 litres per day |
| Collector area needed | 43.06 sq ft (4.00 m²) |
| Energy saved | |
| Energy per day | 5.6 kWh |
| Energy per year | 2,038 kWh |
Tips and common mistakes
- Use a realistic hot water figure per person, since showers, kitchen use and laundry vary a lot between households and between summer and winter.
- Set the temperature rise from your actual cold water temperature to the required hot temperature, not from a guessed value.
- Remember that the yearly saving assumes full supply every day, so real savings are lower on cloudy and rainy days.
- Check roof space and shading before ordering, because the collector area is given for the whole recommended capacity, not only for the daily demand.
- Ask the supplier whether the collector area per 100 litres matches the product type, as flat plate and evacuated tube models differ.
Frequently asked questions
How is the daily hot water demand found?
It is the number of people multiplied by the litres per person per day. With 4 people and 40 litres each, the demand is 160 litres. You can raise the litres figure for homes with heavy use.
Why is the recommended system bigger than the demand?
The calculator rounds the demand up to the next standard size so that you have enough hot water. For 160 litres it recommends a 200 litre per day system. Standard sizes are 50 up to 2000 litres.
What formula is used for the energy saved?
Energy in kWh equals litres x temperature rise x 0.001163. For 160 litres and a 30 degrees C rise, this is about 5.6 kWh per day, or roughly 2,038 kWh in a year at 365 days.
Are cloudy days included in the yearly saving?
No. The yearly figure assumes the heater supplies the full demand every day, so it is an upper estimate. Real savings are lower during cloudy or rainy weather when a backup heater is needed.
What does the collector area per 100 litres mean?
It is the collector surface needed for each 100 litres of system capacity. The default is 2 m² per 100 litres, so a 200 litre system needs 4.00 m². Use the value given by your supplier.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
