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Solar Water Heater

Flat-plate / ETC collector area for hot-water demand.

Design

Inputs (SI units)

Household or facility occupancy driving hot water demand.

Typical residential use 40–60 L/person/day; commercial/hotel higher.

Mains/borewell water temperature, varies seasonally.

Delivery temperature at the tap/tank — above 60 °C carries a scald risk.

Zero-loss efficiency intercept from the collector's efficiency curve (SRCC/EN 12975 test).

First-order heat loss coefficient — flat plate ≈4–6, evacuated tube ≈1–2 W/m²·K.

Average daytime irradiance on the collector plane used for the efficiency-curve correction.

Site daily insolation on the collector plane.

Aperture area of one standard collector panel / ETC module.

Available irradiation on a poor day as a % of the design day, for backup heater sizing.

Results

Collector aperture area — selected
6 m² (3 units)
Collector aperture area — theoretical
4.24 m²
Storage tank — selected standard
250 L
Storage tank — theoretical
250 L
Daily thermal energy demand
9.3 kWh/day
Solar fraction achieved
141.3%
Collector operating efficiency
42.1%
Backup heater rating
2.02 kW
Daily backup energy (low-radiation day)
4.04 kWh
✓ PASS — Delivery temperature 60 °C is below the 60 °C scald-risk threshold.
✓ PASS — Solar fraction of 141% meets a good design target (≥60%).
✓ PASS — Collector operating efficiency 42.1% is acceptable.
✓ PASS — Selected tank (250 L) is a sensible 1–2× multiple of daily demand.

Engineering Recommendations

  • →Insulate all pipework and the storage tank to IS 15300 / EN 12897 standards to limit standby losses.
  • →Install a thermostatic mixing valve where delivery exceeds 60 °C.
  • →Provide a pressure relief valve and freeze/stagnation protection strategy appropriate to the climate.
  • →Engineering-grade preliminary calculation. Final design must be verified against project-specific site conditions, manufacturer datasheets, applicable standards and utility requirements.

Detailed Calculation Log

Documented assumptions: water density 1 kg/L, specific heat c = 4.186 kJ/kg·K, tank standby losses neglected in the primary demand energy (captured via collector loss coefficient a₁).
Demand 200 L/day at ΔT 40 °C requires 9.3 kWh/day thermal energy.
Collector efficiency curve corrected for ΔT and irradiance to 42.1% (from η₀ = 65%).

Save & Load Project

Designs are stored privately in this browser — nothing is uploaded.

Engineering Formula

  • E = m × Cp × ΔT (Cp=4.186 kJ/kg·K)
  • A = E / (G × η × 3600) [m²]

Sensible heat law m·Cp·ΔT. Collector area equals daily energy divided by useful insolation, converted to kWh.

Step-by-step Calculation

  1. 1.Temperature riseΔT = T_hot − T_cold40 °C
  2. 2.Daily hot water demandV = people × L/person/day200 L/day
  3. 3.Thermal energy demandQ = m·c·ΔT9.3 kWh/day
  4. 4.Collector instantaneous efficiencyη = η₀ − a₁·ΔT / G_irr42.1%
  5. 5.Required aperture areaA = Q / (G × η)4.24 m²
  6. 6.Collector count (theoretical)A / A_module2.12 units
  7. 7.Collector count (selected, rounded up)ceil(A / A_module)3 units
  8. 8.Solar fraction achievedSF = E_delivered / Q_demand141.3%
  9. 9.Storage tank size (theoretical)1.25 × daily demand250 L
  10. 10.Storage tank size (selected standard)nextStd(STD_TANK_L, 250)250 L
  11. 11.Backup shortfall on low-radiation dayQ − (E_delivered × 40%)4.04 kWh
  12. 12.Backup heater ratingshortfall / heating window (2 h)2.02 kW

How to use this calculator: Solar Water Heater

Flat-plate / ETC collector area for hot-water demand. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Confirm that the Solar Water Heater matches the quantity or design check you need.
  2. 2Enter Number of people / users, Hot water use per person, and Cold water inlet temperature using the units printed beside each field.
  3. 3Check every value before calculating, especially decimal points and measurement units.
  4. 4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
  5. 5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

InputExample valueWhy it matters
Number of people / users4 personsHousehold or facility occupancy driving hot water demand.
Hot water use per person50 L/person/dayTypical residential use 40–60 L/person/day; commercial/hotel higher.
Cold water inlet temperature20 °CMains/borewell water temperature, varies seasonally.
Required hot water temperature60 °CDelivery temperature at the tap/tank — above 60 °C carries a scald risk.
Collector optical efficiency (η₀)65 %Zero-loss efficiency intercept from the collector's efficiency curve (SRCC/EN 12975 test).
Collector loss coefficient (a₁)4 W/m²·KFirst-order heat loss coefficient — flat plate ≈4–6, evacuated tube ≈1–2 W/m²·K.
Average solar irradiance700 W/m²Average daytime irradiance on the collector plane used for the efficiency-curve correction.
Daily solar irradiation5.2 kWh/m²/daySite daily insolation on the collector plane.
Standard collector module area2 m²Aperture area of one standard collector panel / ETC module.
Low-radiation day fraction40 % of full sunAvailable irradiation on a poor day as a % of the design day, for backup heater sizing.

Formula inputs & variables for Solar Water Heater

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / inputUnitMeaning in this calculation
Number of people / userspersonsHousehold or facility occupancy driving hot water demand.
Hot water use per personL/person/dayTypical residential use 40–60 L/person/day; commercial/hotel higher.
Cold water inlet temperature°CMains/borewell water temperature, varies seasonally.
Required hot water temperature°CDelivery temperature at the tap/tank — above 60 °C carries a scald risk.
Collector optical efficiency (η₀)%Zero-loss efficiency intercept from the collector's efficiency curve (SRCC/EN 12975 test).
Collector loss coefficient (a₁)W/m²·KFirst-order heat loss coefficient — flat plate ≈4–6, evacuated tube ≈1–2 W/m²·K.
Average solar irradianceW/m²Average daytime irradiance on the collector plane used for the efficiency-curve correction.
Daily solar irradiationkWh/m²/daySite daily insolation on the collector plane.

How the Solar Water Heater works

The Solar Water Heater uses Number of people / users, Hot water use per person, Cold water inlet temperature, Required hot water temperature, Collector optical efficiency (η₀), Collector loss coefficient (a₁), Average solar irradiance, Daily solar irradiation, Standard collector module area, and Low-radiation day fraction to calculate Collector aperture area — selected, Collector aperture area — theoretical, Storage tank — selected standard, Storage tank — theoretical, Daily thermal energy demand, Solar fraction achieved, Collector operating efficiency, Backup heater rating, and Daily backup energy (low-radiation day). Its engine applies E = m × Cp × ΔT (Cp=4.186 kJ/kg·K); the worked values below come from that same live calculation rather than a separately typed example.

With Number of people / users 4 persons, Hot water use per person 50 L/person/day, Cold water inlet temperature 20 °C, Required hot water temperature 60 °C, Collector optical efficiency (η₀) 65 %, Collector loss coefficient (a₁) 4 W/m²·K, Average solar irradiance 700 W/m², Daily solar irradiation 5.2 kWh/m²/day, Standard collector module area 2 m², and Low-radiation day fraction 40 % of full sun, the main worked-example result is Collector aperture area — selected = 6 m² (3 units).

How each Solar Water Heater input is used

Number of people / users

The Solar Water Heater worked example uses Number of people / users = 4 persons. This value is passed directly into the calculation, with an allowed minimum 1 and maximum 500. Household or facility occupancy driving hot water demand.

Hot water use per person

The Solar Water Heater worked example uses Hot water use per person = 50 L/person/day. This value is passed directly into the calculation, with an allowed minimum 10 and maximum 150. Typical residential use 40–60 L/person/day; commercial/hotel higher.

Cold water inlet temperature

The Solar Water Heater worked example uses Cold water inlet temperature = 20 °C. This value is passed directly into the calculation, with an allowed minimum -5 and maximum 35. Mains/borewell water temperature, varies seasonally.

Required hot water temperature

The Solar Water Heater worked example uses Required hot water temperature = 60 °C. This value is passed directly into the calculation, with an allowed minimum 30 and maximum 90. Delivery temperature at the tap/tank — above 60 °C carries a scald risk.

Collector optical efficiency (η₀)

The Solar Water Heater worked example uses Collector optical efficiency (η₀) = 65 %. This value is passed directly into the calculation, with an allowed minimum 30 and maximum 85. Zero-loss efficiency intercept from the collector's efficiency curve (SRCC/EN 12975 test).

Collector loss coefficient (a₁)

The Solar Water Heater worked example uses Collector loss coefficient (a₁) = 4 W/m²·K. This value is passed directly into the calculation, with an allowed minimum 1 and maximum 10. First-order heat loss coefficient — flat plate ≈4–6, evacuated tube ≈1–2 W/m²·K.

Average solar irradiance

The Solar Water Heater worked example uses Average solar irradiance = 700 W/m². This value is passed directly into the calculation, with an allowed minimum 200 and maximum 1100. Average daytime irradiance on the collector plane used for the efficiency-curve correction.

Daily solar irradiation

The Solar Water Heater worked example uses Daily solar irradiation = 5.2 kWh/m²/day. This value is passed directly into the calculation, with an allowed minimum 2 and maximum 8. Site daily insolation on the collector plane.

Standard collector module area

The Solar Water Heater worked example uses Standard collector module area = 2 m². This value is passed directly into the calculation, with an allowed minimum 1 and maximum 6. Aperture area of one standard collector panel / ETC module.

Low-radiation day fraction

The Solar Water Heater worked example uses Low-radiation day fraction = 40 % of full sun. This value is passed directly into the calculation, with an allowed minimum 0 and maximum 100. Available irradiation on a poor day as a % of the design day, for backup heater sizing.

Solar Water Heater formulas and result interpretation

Formula 1: relationship used

In the Solar Water Heater, E = m × Cp × ΔT (Cp=4.186 kJ/kg·K). The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Solar Water Heater, A = E / (G × η × 3600) [m²]. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Collector aperture area — selected

For the displayed Solar Water Heater worked example, Collector aperture area — selected is 6 m² (3 units). Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Collector aperture area — theoretical

For the displayed Solar Water Heater worked example, Collector aperture area — theoretical is 4.24 m². Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Storage tank — selected standard

For the displayed Solar Water Heater worked example, Storage tank — selected standard is 250 L. Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Storage tank — theoretical

For the displayed Solar Water Heater worked example, Storage tank — theoretical is 250 L. Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Daily thermal energy demand

For the displayed Solar Water Heater worked example, Daily thermal energy demand is 9.3 kWh/day. Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Solar fraction achieved

For the displayed Solar Water Heater worked example, Solar fraction achieved is 141.3%. Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Collector operating efficiency

For the displayed Solar Water Heater worked example, Collector operating efficiency is 42.1%. Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Backup heater rating

For the displayed Solar Water Heater worked example, Backup heater rating is 2.02 kW. Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Daily backup energy (low-radiation day)

For the displayed Solar Water Heater worked example, Daily backup energy (low-radiation day) is 4.04 kWh. Verify Number of people / users, Hot water use per person, and Cold water inlet temperature and their units before relying on this output.

Solar Water Heater accuracy, checks and limitations

  • Solar Water Heater units check: confirm Number of people / users (persons), Hot water use per person (L/person/day), Cold water inlet temperature (°C), Required hot water temperature (°C), Collector optical efficiency (η₀) (%), Collector loss coefficient (a₁) (W/m²·K), Average solar irradiance (W/m²), Daily solar irradiation (kWh/m²/day), Standard collector module area (m²), and Low-radiation day fraction (% of full sun) before calculating.
  • Solar Water Heater result check: compare Collector aperture area — selected, Collector aperture area — theoretical, Storage tank — selected standard, Storage tank — theoretical, Daily thermal energy demand, Solar fraction achieved, Collector operating efficiency, Backup heater rating, and Daily backup energy (low-radiation day) with the substituted formula steps and the displayed rounding precision.
  • Solar Water Heater: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Solar Water Heater

  • Do not mix units for Number of people / users (persons), Hot water use per person (L/person/day), Cold water inlet temperature (°C). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not replace the displayed E = m × Cp × ΔT (Cp=4.186 kJ/kg·K) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Collector aperture area — selected = 6 m² (3 units) from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Solar Water Heater is useful

Solar Water Heater is designed for cases where Number of people / users, Hot water use per person, Cold water inlet temperature, Required hot water temperature are known and you need Collector aperture area — selected, Collector aperture area — theoretical, Storage tank — selected standard. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is E = m × Cp × ΔT (Cp=4.186 kJ/kg·K). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Number of people / users and Hot water use per person: what changes the answer

The worked example uses Number of people / users = 4 persons, Hot water use per person = 50 L/person/day, Cold water inlet temperature = 20 °C, Required hot water temperature = 60 °C. With those values, Collector aperture area — selected is 6 m² (3 units). Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Number of people / users (persons): Household or facility occupancy driving hot water demand. Hot water use per person (L/person/day): Typical residential use 40–60 L/person/day; commercial/hotel higher. Cold water inlet temperature (°C): Mains/borewell water temperature, varies seasonally. Required hot water temperature (°C): Delivery temperature at the tap/tank — above 60 °C carries a scald risk.

How to sanity-check a Solar Water Heater result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Collector aperture area — selected, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Solar Generation Calculator

Solar Generation Calculator covers the same practical workflow from a related calculation angle, making it a useful cross-check after Solar Water Heater.

Open Solar Generation Calculator

Standards, source trail and limitations

References show the method used. Check the current local edition, amendments and project specification before a regulated decision.

Design Assumptions

  • • Water density ≈ 1 kg/L.
  • • Well-insulated tank (< 5% standby loss).

Engineering Tips

  • ◆ETC beats FPC in cold climates (> 4 °C dawn).
  • ◆Size tank 1.5–2× daily draw.

Warnings

  • ▲Add scale-arrestor in hard-water regions.

Standards & References

IS 12933 (FPC)IS 16368 (ETC)

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