Reversible Heat Pump + Photovoltaic: Should the Power Be Increased?

Reversible Heat Pump and Photovoltaic: The Winning Equation

Are you considering installing photovoltaic panels on your home equipped (or to be equipped) with a reversible heat pump with underfloor heating? This question frequently comes up among our Belgian customers: "My installer suggests 3 kWp, but with my heat pump, shouldn't I increase the capacity?"

The short answer: YES, absolutely. A heat pump, even a very efficient one, represents significant electrical consumption that fully justifies higher photovoltaic dimensioning than standard configurations. Here's why and how to optimize your installation.

Understanding the Real Consumption of a Reversible Heat Pump

Concrete Figures in Belgium

Contrary to popular belief, a heat pump consumes electricity to operate, even if its efficiency (COP) is excellent. In Belgium, field data shows:

Sources: Buderus Belgium, Viessmann BE, Guide-Pompe-Chaleur.be - 2024-2025 data

"A heat pump with a COP of 4 consumes 1 kWh of electricity to produce 4 kWh of heat. But over a year, this still represents 2000 to 5000 kWh depending on area and insulation." - Belgian Heat Pump Association

Consumption Profile: Why It Matters

What makes the heat pump particularly interesting for photovoltaics is its consumption profile:

• Heating mode (winter): consumption mainly during daytime and evening
• Air conditioning mode (summer): consumption during the day when the sun produces most
• Domestic hot water: consumption spread throughout the day
• Underfloor heating: thermal inertia allowing heat storage produced during the day

This compatibility between solar production and heat pump consumption is the key to optimal self-consumption.

Photovoltaic Dimensioning: Mistakes to Avoid

The Under-Dimensioning Trap

Many installers still propose "classic" installations of 3 to 4 kWp (10-13 panels) based on an average consumption of 3500-4000 kWh/year. But with a heat pump, this calculation is obsolete:

The Correct Calculation: Wattuneed Method

To correctly dimension a photovoltaic installation with heat pump, follow this methodology:

1. Assess your total consumption (house + HP): typically 6000-9000 kWh/year
2. Aim for 80-100% coverage to maximize self-consumption
3. Include an evolution margin: electric mobility, future equipment
4. Add storage if your budget allows (lithium batteries)

What Photovoltaic Capacity to Install with a Heat Pump?

Recommendations by Area (Belgium)

Calculations based on average yield in Belgium of 1000 kWh/kWp/year (center of the country).

Solar Batteries: The Strategic Asset with a Heat Pump

Why Storage Changes Everything

With a heat pump, adding lithium batteries to your photovoltaic installation becomes particularly profitable:

• Self-consumption increased to 70-85% (vs 30-40% without battery)
• Use of solar surplus for delayed heating/cooling
• Smoothing of heat pump consumption peaks in the evening
• Backup during grid outage (with hybrid inverter)
• Preparation for capacity tariff (Wallonia 2026)

Storage Dimensioning

"With our air-water heat pump and 18 panels + 10 kWh batteries, we cover 82% of our annual needs. The electricity bill went from €2100 to €380/year." - Wattuneed customer testimonial, Wavre

Financial Analysis: Profitability of an Increased Installation

Comparative Scenario: 4 kWp vs 9 kWp

Let's take the example of a 150 m² house in Belgium with reversible heat pump (consumption: 7000 kWh/year):

Calculations based on electricity rate of €0.30/kWh and injection rate of €0.03/kWh (Wallonia).

Impact of Capacity Tariff (Wallonia 2026)

With the arrival of the capacity tariff in Wallonia, correctly dimensioning your installation becomes even more critical:

• Capacity tariff: billing based on highest consumption peak
• Heat pump = significant peaks at startup (2-3x nominal power)
• Batteries = peak smoothing and reduction of grid bill
• Additional savings: €200-400/year according to CREG

For more information on the Wallonia 2026 incentive tariff, consult our detailed guide.

Advanced Optimization Strategies

1. Intelligent Heat Pump + PV Control

Modern hybrid inverters allow automatic control of your heat pump to maximize self-consumption:

• Programmable relays: heat pump activation according to solar production
• Daytime overheating: +2°C during the day to benefit from free sun
• Priority DHW heating: hot water produced during solar hours
• Air conditioning boost mode: maximum cooling during production hours

2. Photovoltaic Oversizing

With a heat pump, oversizing (inverter/panels oversizing) becomes very profitable:

Belgian roofs often allow installing more panels than a "classic" inverter can handle. 🔗 See example

• Optimal ratio: 1.2 to 1.5 (e.g. 12 kWp panels on 10 kVA inverter)
• Optimized production in cloudy weather or winter
• Better heat pump utilization year-round
• Improved ROI: low marginal cost of panels

3. Integration with Thermodynamic Boiler

For maximum efficiency, combine heat pump + PV + thermodynamic boiler:

• Optimized DHW production: COP of 3-4 for hot water
• Thermal storage: 200-300L water = "thermal battery"
• Solar control: heating only when PV produces
• Additional savings: €300-500/year on DHW

Wattuneed offers complete heat pump + thermodynamic boiler kits for turnkey installation.

Practical and Regulatory Aspects (Belgium)

Green Certificates and Grants

In Belgium, the regulatory framework varies by region:

Important: Grants are cumulative, which significantly improves the profitability of the complete heat pump + PV project.

Electrical Dimensioning

With an increased PV installation and a heat pump, check:

• Electricity meter: switch to three-phase often necessary beyond 10 kVA
• Main circuit breaker: 40A minimum (63A recommended)
• Wiring: adapted sections (minimum 6 mm² heat pump, 4-6 mm² PV)
• Protection: mandatory Type 2 surge protector (PV + heat pump)

Use our voltage drop calculator to correctly dimension your cables.

Conclusion: Increasing is an Investment, Not a Cost

When facing a reversible heat pump, under-dimensioning your photovoltaic installation is an economic mistake. The figures speak for themselves:

• Heat pump = +2000-5000 kWh/year of additional consumption
• Adapted installation = 70-85% self-consumption with battery
• Return on investment: 7-9 years (vs 12-15 years without heat pump)
• Gain over 25 years: €30,000-40,000 depending on configuration

Our Wattuneed recommendation: For any house with reversible heat pump, aim for an installation of 7-11 kWp minimum (16-24 panels) with 10-15 kWh lithium storage. This is the economic sweet spot that guarantees minimal energy bill and optimal preparation for future tar