Solar Water Heating with PV vs Solar Thermal vs Heat Pump: Complete Comparison
Which is the best hot water system? Which is more economical? And can PV panels power a water heater instead of a solar thermal collector? These are among the most common questions we receive at our technical support.
If you objectively compare the 4 main technologies, electric water heater, classic solar thermal (collector), heat pump DHW, and SolBox (PV directly to electric water heater), the answer is clear for the vast majority of Greek homes.
For most owners, the best solution is SolBox: zero paperwork, zero extra roof weight, uses the existing water heater, and the longest overall system lifespan. The analysis below shows why.
Quick Overview of the 4 Technologies
| Technology | How It Works | Space | Paperwork |
|---|---|---|---|
| Electric | Resistance heats water in tank directly from grid | Bathroom / indoor | None |
| Solar Thermal | Thermal collector heats water/glycol transferred to rooftop tank | Rooftop (panels + tank) | None (building) |
| Heat Pump DHW | Extracts heat from air and transfers to integrated tank | Indoor / outdoor with ventilation | None |
| SolBox | PV panels feed DC directly into resistance of existing electric water heater via MPPT controller | Rooftop (panels only) | None (off-grid, no inverter) |
Analysis per Technology
1. Electric Water Heater
The simplest way to have hot water: a tank with electric resistance heating water from grid. Advantages in upfront cost and installation, but the most expensive operational solution because it burns retail electricity 24/7.
Advantages
- Lower upfront cost
- Simple installation
- Works 24/7
- Small footprint
Disadvantages
- Expensive operational cost (100% grid electricity)
- Shorter lifespan (8-12 years)
- Zero renewable energy use
2. Classic Solar Thermal Water Heater
The traditional solution in Greece for decades. Thermal collector on the roof heats liquid (water or antifreeze mixture) that circulates in a heat exchanger inside the tank. The tank is placed on the roof next to the collectors. Backup via electric resistance in the tank.
Performance in Greek conditions is very high in summer months. Proven technology with relatively long lifespan. Practical challenges relate to roof space, tank weight, and maintenance.
Advantages
- Very high summer efficiency
- Proven technology
- No installation permit
- Long lifespan (15-20 years)
Disadvantages
- Requires rooftop space for tank (~150-200 kg)
- Waterproofing/weight issues in old buildings
- Lower winter and cloudy weather performance
- When replacing, you throw out the entire system
3. Heat Pump DHW
Heat pump that extracts thermal energy from ambient air and transfers it to an integrated water tank. Uses grid electricity for the compressor, but with COP 2.5-4, that is, for every 1 kWh of electricity it produces 2.5-4 kWh of heat.
Ideal for homes without a roof or limited space for solar collectors. Works in all weather conditions, even at night. Requires attention to installation location due to noise and ventilation.
Advantages
- Very high efficiency (COP 2.5-4)
- Works 24/7, independent of sun
- Does not require rooftop space
- Good choice for high-consumption families
Disadvantages
- Higher upfront cost
- Compressor noise (~40-55 dB)
- Requires ventilation / outdoor installation
- Consumes grid electricity (COP not = free)
4. SolBox (PV Directly to Electric Water Heater)
SolBox is a DC MPPT controller that connects PV panels directly to the resistance of the existing electric water heater. Uses Maximum Power Point Tracking technology to pull maximum current from panels in any sunlight condition, and converts it to heat inside the tank.
The system is standalone (off-grid): no inverter, no grid connection to HEDNO, no paperwork required (net-metering, permits, etc.). Uses your existing electric water heater, does not replace your tank. In v2 (in development), an external relay is added for automatic backup from an alternative heat source (e.g., boiler) when there is no sunlight.
Advantages
- No inverter, no paperwork, no permit
- Uses existing water heater, zero replacement
- Only panels on the roof (no tank/weight)
- Modular, add panels whenever you want
- MPPT for maximum efficiency at every moment
- Parallel operation with grid, when no sun the water heater works normally
- Safe low-voltage DC (not high-voltage AC)
- Works even during blackouts, autonomous system
- Greek design and technical support
- Zero maintenance after installation
- Panels 20+ years performance warranty
- Compatible with future expansion (v2 external relay backup)
Disadvantages
- Requires functional electric water heater (if not present, purchase separately)
- Higher upfront cost than plain electric (but payback is fast)
Comparison Matrix
| Criterion | Electric | Solar Thermal | Heat Pump | SolBox |
|---|---|---|---|---|
| Renewable energy use | None | Sun (thermal) | Indirect (air) | Sun (PV) |
| Operational cost | High | Low (summer) | Medium | Very low |
| Roof space | Not required | Panel + tank (heavy) | Not required | Panels only |
| Paperwork / permit | None | None | None | None (off-grid) |
| Works at night? | Yes | Yes (grid backup) | Yes | Yes (grid normally) |
| Works in cloudy weather? | Yes | Partially (grid backup) | Yes | Yes (grid normally, or v2 alt. source) |
| Uses existing water heater | Is the same | No (new tank) | No (new unit) | Yes |
| Lifespan | 8-12 years | 15-20 years | 10-15 years | Panels 20+ years |
| Noise | None | None | 40-55 dB | None |
| Fits in old building? | Yes | Weight/waterproofing issue | Yes | Yes (panels only) |
Which System Should I Choose?
In the vast majority of Greek homes, SolBox is the optimal choice. See typical scenarios:
The Verdict
Comparing the 4 technologies objectively across 10 criteria, SolBox emerges as the best choice for the vast majority of Greek homes, from detached houses to multi-story apartments (via Multi-Mount bases for balconies). Heat pump remains an option only in very specific cases without any access to a sunny outdoor surface.
SolBox combines:
- Zero paperwork, off-grid, no inverter, no declaration
- Uses existing equipment, does not replace the water heater
- Lower weight, panels only on the roof
- Longer lifespan, panels 20+ years
- Modular expansion, add panels whenever you want
- Backup path (v2), automatic transition to alternative source
Frequently Asked Questions
Find the right SolBox for your home
Axinar produces SolBox in 4 versions based on your needs, from the basic edition to Pro with auto backup.
