Solar Battery Sizing 2025: Complete Guide with Dynamic Tariffs

The sizing of a solar battery is now a major challenge in optimising your photovoltaic installation. With the arrival of dynamic tariffs in Belgium in early 2026, the traditional rules are changing radically.

While traditional sizing was based on your average night-time consumption, the new tariff system transforms your battery into a real financial optimisation tool. With batteries such as the Delong 15kWh at less than £2,500 at Wattuneed, or £0.04 per kWh returned, the economic equation changes completely.

"Since I resized my Delong 15kWh battery in anticipation of dynamic tariffs, I'm already saving £120 per month on my bill, even in winter!" - Marc D., Wattuneed customer in Liège

☀️ Traditional Sizing vs. New Dynamic Pricing Approach

The traditional approach: night-time consumption

Historically, battery capacity calculations followed this simple logic:

• Average night-time consumption: 4-6 kWh for a typical Belgian family
• Safety margin: +20% for days without sunshine
• Recommended battery capacity: 5-7 kWh

This method works perfectly... when your photovoltaic installation produces enough surplus! But what happens in winter?

🎯 Revolution 2026: The tariff arbitrage strategy

Dynamic battery tariffs introduce five time slots in Wallonia:

Result: With this new logic, a 12-15 kWh battery becomes more cost-effective than a small 5 kWh battery!

🔋 Which Battery Capacity Should You Choose Based on Your Profile?

Analysis of your consumption 24 hours a day

To optimise your solar battery size, first analyse your complete profile:

• Night-time consumption (10 p.m. to 7 a.m.): 30-35% of your daily total
• Morning peak (7 a.m.-11 a.m.): 25-30% - household appliances, food preparation
• Midday trough (11 a.m.-5 p.m.): 15-20% - ideal recharging period
• Evening peak (5 p.m. to 10 p.m.): 30-35% - cooking, lighting, TV

Our recommendations by consumption profile

"Our Victron system with 3 Pylontech units automatically manages cycles according to real-time tariffs. ROI achieved in just 24 months!" - Sophie M., architect in Namur

💡 Smart Charging Strategy to Maximise Your Savings

The optimal 4-phase cycle

To maximise your energy autonomy with dynamic tariffs:

1. 🌙 Phase 1 - Night-time charging (10 p.m. to 7 a.m.)
   → Battery recharged to 100% from the grid at the lowest rate
   → Cost: ~€0.10/kWh
2. 🌅 Phase 2 - Morning discharge (7 a.m. to 11 a.m.)
   → Battery used during the first peak tariff
   → Savings: £0.15-0.20/kWh
3. ☀️ Phase 3 - Solar Recharging (11 a.m.-5 p.m.)
   → Combination of solar production + moderate grid tariff
   → Preparation for the evening peak
4. 🌆 Phase 4 - Maximum discharge (5pm-10pm)
   → Battery use during peak tariff period
   → Savings: £0.25-0.30/kWh

📊 Economic Analysis: ROI and Battery Profitability

Comparison of conventional sizing vs. dynamic pricing

🧮 Detailed calculation for a Delong 15kWh

Let's take the specific example of a Delong 15kWh battery (€2,950):

• Night-time arbitrage: 12 kWh × £0.12 differential × 365 days = £525/year
• Evening arbitrage: 10 kWh × £0.20 differential × 365 days = £730/year
• Optimised self-consumption: Additional gain = £400/year
• 🎯 Total savings: £1,655/year
• 📈 ROI: 1.8 years

⚙️ Installation and Compatibility: Our Turnkey Solutions

Compatible hybrid inverters

Our lithium batteries integrate perfectly with:

• 🔋 Victron Energy: MultiPlus-II, EasySolar-II GX - smart tariff management
• ⚡ Deye: 5-12kVA hybrid inverters - excellent value for money
• 🌟 Solax: X-Hybrid series - advanced monitoring included
• 🏆 Fronius: GEN24 - premium Austrian technology

🔧 Simplified installation with Wattuneed

A Delong battery can be installed in less than 5 minutes:

1. 🎲 Stackable system: Plug-and-play connection
2. 🛞 Mobility: Integrated castors for easy movement
3. 📱 Auto communication: RS485/CAN detected automatically
4. ⚡ Commissioning: Quick configuration via application

Discover our complete selection of lithium batteries recommended by our experts.

🌡️ Seasonal adaptation of your sizing

Optimisation according to the seasons

Your solar energy storage adapts to conditions:

☀️ Summer (May-August)

• 📈 Abundant production: 25-35 kWh/day depending on the installation
• 🎯 Strategy: Maximum direct self-consumption
• 🔋 Battery use: Surplus storage for evening peaks only

❄️ Winter (November-February)

• 📉 Limited production: 3-8 kWh/day
• ⚡ Strategy: Systematic night-time grid charging
• 💰 Focus: Pure tariff arbitrage - maximum ROI!

🍂 Off-season (March-April, September-October)

• ⚖️ Variable production: 12-20 kWh/day
• 🎨 Hybrid strategy: Mix of solar production + grid arbitrage

💰 Solar Battery Price: 2025-2026 Buying Guide

Price and technology trends

Solar battery prices have fallen by 60% since 2020:

🚀 Developments in 2026 and Future Anticipations

Trends to watch

• 🤖 Predictive AI: Automatic optimisation based on weather and tariffs
• 🔄 V2H (Vehicle-to-Home): Your electric car as a backup battery
• 🏘️ Energy communities: Energy sharing between neighbours
• 📱 Smart apps: Total control from your smartphone

"In 2027, our community battery will power the entire neighbourhood during peak hours. The future is smart energy sharing!" - Pierre D., smart grid pioneer in Ghent

📋 FAQ: Your questions about solar battery sizing

What is the lifespan of a properly sized solar battery?

A properly sized LiFePO4 lithium battery lasts 15-20 years (6,000+ cycles). Oversizing by 20% compared to your actual needs significantly extends this lifespan by avoiding repeated deep discharges.

Are batteries cost-effective without solar panels?

Yes! With the 2026 dynamic tariffs, pure arbitrage is sufficient. By charging at night at £0.10/kWh and feeding back at peak times at £0.35/kWh, you save £0.25 per kWh. That's £1,100/year for a 15kWh battery - ROI of 2.7 years without any panels!

Can I add extra batteries later?

Absolutely! Our Pylontech (up to 16 units in parallel) and Delong (up to 4 stackable units) systems are 100% modular. Start small and expand as your needs or tariffs change.

How can I protect my battery in the event of a power cut?

A hybrid inverter with backup function (Victron MultiPlus-II, Deye) automatically switches to battery power. Size it to keep your essential equipment running for at least 6-12 hours, depending on your profile.

What is the difference between kW and kWh for a battery?

kWh = storage capacity (how much energy), kW = power (charge/discharge speed). Optimal ratio: 1:3 (e.g. 5kW for 15kWh). The Delong 15kWh offers 5kW of power - perfect for a family home.

🎯 Your Personalised Sizing with Wattuneed

Every project is unique! Our team of experts will help you determine the optimal solution based on:

• 📊 Comprehensive energy audit: Detailed analysis of your consumption profile
• 🏠 Technical constraints: Available space, existing installation
• 💰 Economic objectives: Target ROI, available budget
• 🌱 Long-term vision: Future expansion, electric vehicle

Use our smart simulator for a preliminary assessment or contact our experts directly for a free personalised study.

"The Wattuneed team helped me perfectly size my Pylontech. With the new tariffs, I have already recouped 70% of the investment in 18 months!" - Jean-Pierre L., entrepreneur in Brussels

Ready to maximise your savings with the 2026 dynamic tariffs? Request your free, personalised sizing now.