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Solar thermal hot water: practical guide
This guide explains how to use the Count.ie solar thermal hot water. Compare potential collector yield with annual hot-water demand. Follow the inputs, method and worked example below, then compare your own scenario.
Guide written .
Open the Solar thermal hot water calculatorWhat to prepare
- Use meter readings, bills, equipment specifications and actual tariff periods where possible.
- Separate power in kW from energy in kWh and daily use from annual use.
- Identify whether efficiency, standing charges and losses are included in the chosen model.
Understand the inputs
The calculation method
Demand uses water heat capacity. Usable solar yield = area × specific yield × usability; contribution is capped at annual demand.
Worked example
Illustrative inputs and their result.
Example inputs
- Collector area (m²)
- 4
- Annual collector yield (kWh/m²)
- 400
- Usable fraction of yield (%)
- 75
- Daily hot-water demand (litres)
- 150
- Temperature increase (°C)
- 45
- Days per year
- 365
- Backup heater efficiency (%)
- 90
- Backup input energy price (€/kWh)
- 0.1
Calculated example result
- Solar Fraction Percent
- 41.89%
- Annual Fuel Saving
- €133.33
How to interpret the result
Energy consumption and money saved are different outputs. Results based on entered prices or efficiencies are scenarios, not a performance guarantee, installer quote or measured emissions assessment.
Use the same demand and time horizon for each option. Test a less favourable tariff, lower efficiency or lower use to see how sensitive the estimated saving is.
Common mistakes to avoid
- Do not count the same efficiency loss twice.
- Keep standing charges and unit charges separate when the model distinguishes them.
- Check seasonal demand, export restrictions and tariff assumptions before extrapolating.
Assumptions and sources
Annual energy matching does not model seasonal storage or daily mismatches. The usability input should account for excess summer yield, storage and system losses.
If your case falls outside this scope, use a more suitable calculator or contact us about an unclear method.