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Solar Thermal Useful Heat Calculator

Calculate useful daily heat from a supported net efficiency or apply a documented collector gain/loss curve at one operating point; report kWh and BTU.

Use this result well

Inputs that matter
Collector aperture area (m²), Collector-plane irradiation (kWh/m²/day), Net daily useful-heat efficiency (%), Water temperature rise (°C), and 6 more
Output to expect
Daily useful solar heat, Collector optical gain and thermal loss
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Daily useful solar heat

Calculate useful daily heat using collector aperture area, irradiation on the collector plane and a net daily efficiency supported for the actual temperature, flow and system boundary.

Useful heat = aperture m² × irradiation kWh/m²/day × net daily efficiency; 1 kWh = 3412.141633 BTU (IT).

Collector optical gain and thermal loss

Apply a documented first-order collector curve at one operating point. The temperature reference and heat-loss coefficient must match the same collector test model and operating range.

Useful power = max[0, aperture area × (effective optical efficiency × irradiance − loss coefficient × (curve fluid temperature − ambient temperature))].

Updated: September 2026

Example Scenarios

Try this example to see the calculation, then enter actual documented values and preserve the measurement conditions. A failed or unresolved comparison is part of the record.

→ 8 kWh useful heat/day

Try this example to see the calculation, then enter actual documented values and preserve the measurement conditions. A failed or unresolved comparison is part of the record.

→ 1,920 W useful collector heat

FAQ

Calculate useful daily heat using collector aperture area, irradiation on the collector plane and a net daily efficiency supported for the actual temperature, flow and system boundary. Apply a documented first-order collector curve at one operating point. The temperature reference and heat-loss coefficient must match the same collector test model and operating range.

Use the exact equipment data, measurement point, operating conditions and units described in the selected mode. Record source documents, dates and unresolved conditions in Sources and assumptions. Built-in examples illustrate arithmetic; they are not manufacturer ratings or approved designs.

Use Save planning case for an explicit local history entry. Copy MD keeps current inputs and notes; Download CSV exports the result breakdown. Export portable copies before clearing storage and save a separate case after changing equipment, conditions or measurements.

About Solar Thermal Useful Heat Calculator

Calculate useful daily heat using collector aperture area, irradiation on the collector plane and a net daily efficiency supported for the actual temperature, flow and system boundary. Apply a documented first-order collector curve at one operating point. The temperature reference and heat-loss coefficient must match the same collector test model and operating range.