Domestic hot water (DHW) accounts for 15–30% of household energy consumption. It is the second or third largest load after space heating and cooking. Three main technologies compete: solar thermal collectors, heat pump water heaters, and electric resistance tanks. Each has different economics, space requirements, and performance profiles.
The simplest and cheapest to install. A heating element (2,000–3,000 W) immersed in a storage tank. Converts 1 kWh of electricity into exactly 1 kWh of heat.
COP = 1.0 (no better than direct resistance can ever be)
An air-to-water heat pump extracts heat from ambient air and transfers it to the water. Consumes 1 kWh of electricity to deliver 2.5–4 kWh of heat.
COP = 2.5–4.0 depending on ambient temperature
Panels on the roof absorb solar radiation and transfer the heat to a fluid (water/glycol mixture) that heats the tank via a heat exchanger. Requires an electric or gas backup for cloudy periods.
Solar fraction = 50–70% of annual DHW needs (temperate climate), with backup for the remainder.
For a 4-person household (estimated DHW energy need: ~3,000–4,000 kWh of heat per year):
| Technology | Electricity input (kWh/yr) | Heat output (kWh/yr) | Notes |
|---|---|---|---|
| Resistance tank (200L) | 3,500 | 3,500 | COP 1.0, some standby losses |
| Resistance tank (300L) | 4,000 | 4,000 | Larger tank, more standby loss |
| Heat pump water heater | 900 | 3,200 | COP ~3.5, air temp dependent |
| Solar thermal + elec. backup | 500 | 3,200 | ~65% solar fraction |
| Solar thermal + HPWH backup | 250 | 3,200 | Combined best case |
A flat plate collector consists of a glass-covered insulated box containing a blackened absorber plate with fluid tubes. Efficiency: 55–75% of incident solar radiation converted to heat (at low temperature differential).
An evacuated tube collector uses vacuum-insulated glass tubes around the absorber. Higher efficiency at larger temperature differentials (better in cold climates). Cost: ~30% more than flat plate.
The solar fraction is the percentage of annual DHW heat delivered by the solar system (before backup):
| Climate | Flat plate collectors (4 m²) | Evacuated tube (3.5 m²) |
|---|---|---|
| South Europe (Seville, Rome) | 70–85% | 75–90% |
| Central Europe (Lyon, Berlin) | 55–70% | 60–75% |
| North Europe (London, Dublin) | 45–60% | 50–65% |
| Scandinavia | 35–50% | 45–60% |
For a 4-person household in Lyon with 55% solar fraction:
A HPWH uses the refrigeration cycle to extract heat from room air, compressing it to a higher temperature to heat water. Side effects:
| Ambient air temp | COP (typical) |
|---|---|
| -5°C | 1.5–2.0 |
| 0°C | 1.8–2.3 |
| 10°C | 2.5–3.0 |
| 15°C | 3.0–3.5 |
| 20°C | 3.5–4.0 |
| 25°C | 4.0–4.5 |
In cold climates, the HPWH installed in an unheated garage loses significant efficiency in winter. If installed in a heated space, it effectively cools that space, requiring more heating energy — partially negating the COP benefit.
A HPWH with a smart controller can be programmed to heat water preferentially during:
The tank acts as a thermal battery: cheap or free energy is stored as hot water, usable throughout the day.
Choose the right technology based on your specific situation:
Assumptions: 4-person household, Lyon (France), 3,500 kWh/yr DHW heat need, electricity €0.22/kWh with 3% annual escalation, gas €0.10/kWh with 3% escalation.
| Technology | Installed Cost |
|---|---|
| Electric resistance tank (300L) | €800–1,200 |
| Heat pump water heater (250L) | €1,800–2,800 |
| Solar thermal flat plate (4 m², 300L tank) | €3,500–5,500 |
| Solar thermal evacuated tube (3.5 m², 300L) | €4,500–6,500 |
| Solar thermal + HPWH backup | €5,000–7,500 |
| Technology | Electricity (kWh/yr) | Annual cost |
|---|---|---|
| Resistance tank | 4,000 | €880 |
| HPWH (COP 3.2) | 1,100 | €242 |
| Solar thermal + elec. backup | 1,440 | €317 |
| Solar thermal + HPWH | 480 | €106 |
| Technology | Install cost | 25-yr energy cost | 25-yr total |
|---|---|---|---|
| Resistance tank | €1,000 | €22,000 | €23,000 |
| HPWH | €2,300 | €6,100 | €8,400 |
| Solar thermal + elec. backup | €4,500 | €8,000 | €12,500 |
| Solar thermal + HPWH | €6,250 | €2,650 | €8,900 |
HPWH wins on lifetime cost for most temperate-climate households. Solar thermal + HPWH is nearly equal but costs more upfront. Solar thermal without HPWH backup is mid-range due to high backup electricity in cloudy months.
If you already have solar PV:
| Roof space (per m²) | Solar thermal | Solar PV (driving HPWH) |
|---|---|---|
| Annual heat yield | 350–500 kWh | 130–170 kWh electricity |
| Electricity saved (via HPWH) | — | 130–170 kWh (direct) |
| Effective heat produced | 350–500 kWh | 400–600 kWh (× COP 3.2) |
| Economic value (€0.22/kWh) | €77–110 | €88–132 |
Verdict: With a modern efficient HPWH, solar PV is competitive with or superior to solar thermal per m² of roof space — and provides additional flexibility (charges battery, powers other appliances).
Navigation: