The Simple Numbers Behind Solar Thermal
Solar thermal maths comes down to one simple question:
How much hot water heat can we make from sunlight?
Unlike solar panels, solar thermal systems do not generate electricity.
Instead, they collect heat from sunlight and use it to warm water in a hot water tank.
Because of this, solar thermal maths focuses on:
- Heat energy (kWh)
- Collector size (m²)
- Sunlight strength
- Heat losses
At first, this may sound technical.
However, the maths is simpler than it looks.
Let’s go through it step by step.
1. Power vs Energy
First, two units matter most.
kW (Kilowatts)
kW measures heat power.
In simple terms, this tells you how quickly heat is being collected.
kWh (Kilowatt-hours)
kWh measures heat energy over time.
In other words, this tells you how much heat you actually collect.
Key Rule
Energy = Power × Time
Or:
kWh = kW × Hours
Example
Suppose your solar thermal system delivers:
1.5 kW of heat for 2 hours
The heat collected would be:
1.5 × 2 = 3 kWh
So the system collects:
3 kWh of heat
This simple rule appears throughout solar thermal maths.
2. Sunlight on the Collector
Next, solar thermal collectors rely on sunlight.
The strength of sunlight is called solar irradiance.
This is measured in:
Watts per square metre (W/m²)
Collectors also have a size, measured in:
Square metres (m²)
A simple formula for sunlight hitting the collector is:
Sun Power = Sunlight × Collector Area
Example
Suppose:
- Sunlight = 600 W/m²
- Collector area = 4 m²
Calculation:
600 × 4 = 2,400 W
That equals:
2.4 kW
So, 2.4 kW of solar power is hitting the collector.
3. Collector Efficiency
However, collectors cannot turn all sunlight into useful heat.
Some energy is lost through:
- Reflection from the glass
- Heat escaping to outdoor air
- Heat loss in pipes
Because of this, we use efficiency.
A simple formula is:
Useful Heat = Sunlight × Area × Efficiency
Example
Suppose:
- Sunlight = 600 W/m²
- Area = 4 m²
- Efficiency = 50% (0.50)
Calculation:
600 × 4 × 0.50 = 1,200 W
That equals:
1.2 kW
So the system delivers about:
1.2 kW of useful heat
Efficiency changes depending on:
- Sunlight level
- Outdoor temperature
- Tank temperature
Even so, this formula gives a good estimate.
4. Daily Heat Collection
Now we can turn power into daily energy.
Using the main rule:
Energy = Power × Time
Suppose the system averages:
1.2 kW for 4 hours
Calculation:
1.2 × 4 = 4.8 kWh
So the system collects:
4.8 kWh of heat per day
That heat goes into the hot water tank.
5. How Much Water Can That Heat?
This is often the most useful calculation.
It answers a practical question:
How much hot water can the system produce?
A useful shortcut is:
Heating 1 litre of water by 1°C needs about:
0.00116 kWh
A simple formula is:
Heat = Water Volume × Temperature Rise × 0.00116
Example
Suppose you want to heat:
- 150 litres of water
- By 35°C
- For example, from 15°C to 50°C
Calculation:
150 × 35 × 0.00116 = 6.09 kWh
So you need about:
6.1 kWh of heat
Now compare this with daily solar heat.
If the system collected 4.8 kWh, it would heat most of the tank, but not all of it.
On sunnier days, it may heat the whole tank.
6. Solar Fraction
Solar thermal systems usually provide only part of your yearly hot water.
This share is called the solar fraction.
Formula:
Solar Fraction = (Solar Heat ÷ Total Hot Water Need) × 100
Example
Suppose a home needs:
2,000 kWh per year for hot water
And solar thermal provides:
1,000 kWh per year
Calculation:
(1,000 ÷ 2,000) × 100 = 50%
So solar thermal covers:
50% of yearly hot water demand
7. Estimating Yearly Output
A simple yearly estimate is:
Annual Heat = Collector Area × Annual Sunlight × Efficiency
This helps show what affects performance.
In simple terms:
- Bigger collector area → more heat
- Sunnier location → more heat
- More shading → less heat
- More heat loss → lower output
This is why similar systems can perform very differently on different roofs.
8. Why Output Changes Through the Year
Solar thermal output changes with the seasons.
In winter:
- Days are shorter
- Sunlight is weaker
- Heat loss is higher
As a result, winter output is lower.
In summer:
- Days are longer
- Sunlight is stronger
- Heat collection increases
Because of this, summer often produces much more hot water.
Spring and autumn usually sit somewhere in between.
In Short
Most solar thermal maths comes down to five main things:
- Sunlight strength
- Collector size
- System efficiency
- Heat collected over time
- Hot water demand
That is really all you need.
Once you understand these numbers, solar thermal maths becomes much easier.
And remember the key rule:
kW × hours = kWh