Calculate the perfect battery bank size for your solar system. Design off-grid, hybrid, or backup solar systems with accurate battery capacity and solar panel requirements.
☀️ Solar System Design
Configure your off-grid or backup solar system
How to Use the Solar Battery Bank Calculator
Our solar battery bank calculator helps you design the perfect off-grid or hybrid solar system. Calculate exact battery capacity, solar panel requirements, and complete system specifications for your home, RV, cabin, or business.
Step 1: Choose Your System Type
🏕️ Off-Grid Solar System
Completely independent from the utility grid. Ideal for:
- Remote cabins and cottages without grid access
- Tiny homes and eco-villages
- Farm outbuildings and barns
- Remote telecommunications sites
- Locations where grid connection is expensive (£10,000+)
Key Requirement: Must generate and store all energy needed. Requires larger battery bank and solar array with 2-5 days autonomy.
🔌 Hybrid Solar System (Grid-Tied with Battery Backup)
Connected to grid but with battery storage for backup and peak shaving. Benefits:
- Use solar during day, grid at night
- Backup power during grid outages
- Store excess solar for evening use
- Reduce peak-time electricity costs
- Sell excess back to grid (with export tariff)
Best For: Urban homes wanting energy independence with grid as backup. Lower battery capacity needed (1-2 days autonomy).
⚡ Backup/Emergency System
Primarily for power outages, not daily use. Features:
- Smaller battery bank (12-24 hours autonomy)
- Powers essential loads only (fridge, lights, heating controls)
- Lower cost than full off-grid
- Quick ROI in areas with frequent outages
Common Applications: Medical equipment backup, home office continuity, food preservation during storms.
Step 2: Calculate Your Daily Energy Consumption
Simple Mode – Total kWh Method
If you know your daily energy usage (check electricity bill):
- Find monthly kWh usage on bill (example: 450 kWh/month)
- Divide by 30 days: 450 ÷ 30 = 15 kWh/day
- For off-grid, multiply by 1.2 for safety margin: 15 × 1.2 = 18 kWh/day
- Enter 18,000 Wh in calculator
Typical UK Household Daily Usage:
| Home Size | Occupants | Daily Usage |
|---|---|---|
| 1-bed flat | 1-2 people | 5-8 kWh/day |
| 2-bed house | 2-3 people | 10-15 kWh/day |
| 3-bed house | 3-4 people | 15-25 kWh/day |
| 4-bed house | 4-5 people | 25-40 kWh/day |
Detailed Mode – Appliance-by-Appliance
For precise calculations, list every appliance:
Example: Off-Grid Cabin Load Calculation
| Appliance | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| LED Lights (8 bulbs @ 10W) | 80W | 5h | 400 Wh |
| Refrigerator (energy efficient) | 150W | 24h | 3,600 Wh |
| Laptop | 65W | 6h | 390 Wh |
| TV (LED 40″) | 60W | 4h | 240 Wh |
| Water Pump | 200W | 1h | 200 Wh |
| Phone Chargers (2x) | 20W | 3h | 60 Wh |
| TOTAL DAILY ENERGY | 4,890 Wh | ||
Rounded to: 5,000 Wh/day (5 kWh/day)
Step 3: Select Days of Autonomy
Autonomy days = how long system runs without solar input (cloudy/rainy days).
Autonomy Recommendations by System Type:
- 1 Day: Hybrid systems with grid backup, sunny climates, cost-sensitive projects
- 2 Days (Recommended): Standard off-grid, typical UK weather, balanced cost/performance
- 3 Days: Reliable off-grid, winter usage, business-critical applications
- 4-5 Days: Premium off-grid, Scotland/Wales, winter-heavy usage
- 7+ Days: Remote locations, critical medical equipment, extreme reliability requirements
UK Weather Considerations:
| Region | Recommended Autonomy | Reasoning |
|---|---|---|
| South England | 2-3 days | Most sunshine, fewer consecutive cloudy days |
| Midlands | 3 days | Moderate sunshine, typical UK weather |
| North England | 3-4 days | Less sunshine, more overcast days |
| Scotland/Wales | 4-5 days | Lowest sunshine hours, frequent rain |
Step 4: Choose System Voltage
System voltage affects wire sizing, component selection, and efficiency.
12V Systems
- Power Range: Up to 1,000W (1kW)
- Best For: Small RVs, boats, camping setups, car accessories
- Pros: Abundant 12V appliances, simple, cheaper components
- Cons: High current = thick cables, limited scalability, higher losses
- Example: Campervan with 200Ah battery = 2.4 kWh storage
24V Systems
- Power Range: 1,000W – 3,000W (1-3kW)
- Best For: Large RVs, small homes, workshops
- Pros: Half the current of 12V, thinner cables, better efficiency
- Cons: Fewer 24V appliances, need DC-DC converters for 12V devices
- Example: Small cabin with 400Ah battery = 9.6 kWh storage
48V Systems (Recommended for Homes)
- Power Range: 3,000W+ (3kW+)
- Best For: Off-grid homes, large solar systems, high power applications
- Pros: Lowest current, smallest cables, highest efficiency, scalable
- Cons: More expensive components, requires DC-DC converters for 12V/24V loads
- Example: Off-grid home with 400Ah battery = 19.2 kWh storage
⚡ Current Comparison Example (1,200W Load)
- 12V system: 1,200W ÷ 12V = 100A (requires 25mm² cable)
- 24V system: 1,200W ÷ 24V = 50A (requires 10mm² cable)
- 48V system: 1,200W ÷ 48V = 25A (requires 4mm² cable)
Cable cost savings alone justify 48V for larger systems!
Step 5: Select Battery Chemistry
LiFePO4 (Lithium Iron Phosphate) – Recommended ✅
Why It’s Best:
- Lifespan: 3,000-5,000 cycles (10-15 years daily use)
- Depth of Discharge: 80-100% usable capacity
- Efficiency: 95-98% charge/discharge efficiency
- Weight: 1/3 the weight of lead-acid
- Maintenance: Zero maintenance required
- Temperature Range: -20°C to 60°C (with BMS protection)
- Safety: Most stable lithium chemistry, no fire risk under normal use
Cost: £800-1,200 per 100Ah (12V) battery initially, but lowest cost per cycle.
Best For: Anyone who can afford upfront cost. ROI in 3-5 years vs lead-acid.
Li-ion (Lithium-Ion)
- Lifespan: 1,000-2,000 cycles (5-8 years)
- Depth of Discharge: 80% recommended
- Pros: Higher energy density than LiFePO4, lighter weight
- Cons: Less safe, degrades faster, sensitive to temperature extremes
- Best For: Weight-critical applications (boats, RVs), where space is premium
AGM Lead-Acid
- Lifespan: 500-800 cycles (3-5 years)
- Depth of Discharge: 50% (use only half capacity)
- Pros: Lower upfront cost, sealed/maintenance-free, proven technology
- Cons: Heavy (3x lithium weight), must charge slowly (0.1-0.2C), temperature sensitive
- Cost: £300-500 per 100Ah (12V) battery
- Best For: Budget-conscious projects, backup systems with infrequent use
Flooded Lead-Acid
- Lifespan: 300-500 cycles (2-4 years)
- Depth of Discharge: 50% maximum
- Pros: Cheapest option, recyclable, repairable
- Cons: Requires monthly maintenance (water top-ups), hydrogen gas venting needed, spillable acid
- Cost: £200-350 per 100Ah (12V) battery
- Best For: Extreme budget constraints, systems with ventilated battery room
Battery Chemistry Comparison:
| Feature | LiFePO4 | Li-ion | AGM | Flooded |
|---|---|---|---|---|
| Usable Capacity | 80% | 80% | 50% | 50% |
| Cycle Life | 3,000-5,000 | 1,000-2,000 | 500-800 | 300-500 |
| Maintenance | None | None | None | Monthly |
| Weight (100Ah 12V) | 13 kg | 11 kg | 32 kg | 35 kg |
| Initial Cost | £800-1,200 | £700-1,000 | £300-500 | £200-350 |
| Cost Per Cycle | £0.20-0.30 | £0.50-0.70 | £0.60-0.80 | £0.70-1.00 |
Understanding Your Results
Battery Bank Sizing
The calculator determines battery capacity using this formula:
Example Calculation:
- Daily Energy: 5,000 Wh (5 kWh)
- Autonomy: 2 days
- System Voltage: 48V
- Battery Type: LiFePO4 (80% DoD)
- Capacity = (5,000 × 2) / (48 × 0.80) = 10,000 / 38.4 = 260 Ah
- Rounded up to standard size: 300 Ah battery bank
Solar Panel Sizing
Solar array must generate daily energy needs plus account for system losses:
UK Peak Sun Hours by Season:
- Summer (June-Aug): 4-5 hours/day
- Spring/Autumn (Mar-May, Sep-Nov): 3-4 hours/day
- Winter (Dec-Feb): 1-2 hours/day
- Annual Average: 2.5-3 hours/day (UK average)
System Efficiency Factors:
- MPPT Charge Controller: 96-98% efficient
- Battery Charging: 95% efficient
- Cable Losses: 2-3%
- Dust/Dirt on Panels: 5%
- Temperature Derating: 5-10%
- Combined Efficiency: ~75%
Example Solar Calculation:
- Daily Energy: 5,000 Wh
- Peak Sun Hours: 3 hours (UK average)
- System Efficiency: 75%
- Required Solar: 5,000 / (3 × 0.75) = 5,000 / 2.25 = 2,222W
- Using 300W panels: 2,222 / 300 = 7.4 panels → 8 panels
- Total Array: 2,400W (2.4kW)
Inverter Sizing
Inverter must handle peak load plus surge current for motor-driven appliances:
- Continuous Rating: 1.2-1.5× your highest simultaneous load
- Surge Rating: 2-3× continuous (for motor starts)
Common Appliance Surge Requirements:
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150W | 600W (4x) |
| Washing Machine | 500W | 2,000W (4x) |
| Water Pump | 200W | 800W (4x) |
| Air Conditioner | 1,500W | 4,500W (3x) |
| LED TV | 60W | 60W (no surge) |
Charge Controller Selection
MPPT vs PWM Controllers
MPPT (Maximum Power Point Tracking) – Recommended
- Efficiency: 96-98%
- Advantages: 20-30% more power harvest, works in low light, tolerates temperature variations, can handle higher voltage panels
- Cost: £200-600
- Best For: All permanent installations, systems over 500W
PWM (Pulse Width Modulation)
- Efficiency: 75-80%
- Advantages: Cheaper (£50-150), simpler, reliable
- Disadvantages: 20-25% less efficient, panel voltage must match battery voltage
- Best For: Small systems (<500W), budget builds, temporary setups
Complete System Design Examples
Example 1: Small Off-Grid Cabin
Scenario: Weekend retreat cabin, used 2 days/week
Daily Load:
- LED Lights: 50W × 5h = 250 Wh
- 12V Fridge: 40W × 24h = 960 Wh
- Laptop: 65W × 4h = 260 Wh
- Water Pump: 200W × 0.5h = 100 Wh
- Total: 1,570 Wh/day (1.6 kWh/day)
System Design:
- Batteries: 200Ah LiFePO4 @ 12V = 2.4 kWh (1.5 days autonomy)
- Solar Panels: 600W (2× 300W panels)
- Inverter: 1,000W pure sine wave
- Charge Controller: 40A MPPT
- Total Cost: ~£2,500
Example 2: Full Off-Grid Home
Scenario: 3-bedroom house, family of 4, year-round living
Daily Load:
- Lighting (LED): 200W × 6h = 1,200 Wh
- Refrigerator: 150W × 24h = 3,600 Wh
- Freezer: 100W × 24h = 2,400 Wh
- Washing Machine: 500W × 1h = 500 Wh
- Laptop × 2: 130W × 6h = 780 Wh
- TV: 80W × 5h = 400 Wh
- Kitchen Appliances: 1,500 Wh
- Water Pump: 200W × 2h = 400 Wh
- Heating Controls: 50W × 8h = 400 Wh
- Total: 11,180 Wh/day (11.2 kWh/day)
System Design:
- Batteries: 600Ah LiFePO4 @ 48V = 28.8 kWh (2 days autonomy)
- Solar Panels: 5,000W (17× 300W panels)
- Inverter: 5,000W (5kW) hybrid inverter
- Charge Controller: 100A MPPT (48V)
- Total Cost: ~£18,000-22,000
- Payback: 8-12 years vs grid connection costs
Example 3: Hybrid Backup System
Scenario: Grid-tied home with solar + battery backup for power cuts
Essential Load (Backup Only):
- Refrigerator: 150W × 24h = 3,600 Wh
- Freezer: 100W × 24h = 2,400 Wh
- LED Lights: 50W × 6h = 300 Wh
- Internet/Router: 20W × 24h = 480 Wh
- Boiler Controls: 30W × 8h = 240 Wh
- Total: 7,020 Wh/day (7 kWh/day)
System Design:
- Batteries: 300Ah LiFePO4 @ 48V = 14.4 kWh (2 days backup)
- Solar Panels: 3,600W (12× 300W panels)
- Inverter: 5kW hybrid inverter with grid-tie
- Charge Controller: Built into hybrid inverter
- Total Cost: ~£12,000-15,000
- Benefits: Backup power + reduced electricity bills + export income
Cost Optimization Strategies
How to Reduce System Costs
- Start Smaller, Expand Later
- Begin with 50% of calculated capacity
- Add batteries and panels as budget allows
- Ensure inverter and charge controller can handle future expansion
- Prioritize Energy Efficiency First
- LED lights save 80% vs incandescent (£150 investment saves £1,500+ in solar)
- Energy-efficient fridge: A+++ uses 50% less than A+ rated
- Insulation reduces heating/cooling load by 30-50%
- Every 1kWh/day saved = £700-1,000 less solar system cost
- Use DC Appliances Where Possible
- DC fridge: 40W vs AC fridge: 150W (after inverter losses)
- 12V LED lights (no inverter needed)
- USB device charging (efficient DC-DC conversion)
- Saves inverter size and energy losses
- Consider Hybrid System Initially
- Stay grid-connected for backup
- Smaller battery bank needed (1 day vs 3-5 days)
- Can go fully off-grid later
- Saves 30-40% on initial cost
- DIY Installation (If Competent)
- Professional install: £2,000-5,000
- DIY saves labor but requires electrical knowledge
- Still hire certified electrician for final AC connections
- Must notify local Building Control
Maintenance & Lifespan
LiFePO4 Battery Maintenance
- Daily: None required
- Monthly: Check voltage and charge level via BMS
- Quarterly: Visual inspection for swelling, corrosion on terminals
- Annually: Full capacity test, clean terminals, check mounting
- Expected Lifespan: 10-15 years (3,000-5,000 cycles)
Solar Panel Maintenance
- Monthly: Visual inspection for cracks, damage
- Quarterly: Clean panels (rain usually sufficient in UK)
- Annually: Check mounting bolts, electrical connections, output voltage
- Expected Lifespan: 25-30 years (warranties typically 25 years at 80% output)
System Monitoring
Install monitoring system to track:
- Daily solar production (kWh)
- Battery state of charge (%)
- Daily consumption (kWh)
- System voltage and current
- Alerts for faults or low battery
Popular Monitoring Systems: Victron VRM, Renogy BT, SolarEdge monitoring
UK Regulations & Permissions
Planning Permission
- Roof-mounted panels: Usually permitted development (no planning needed) if:
- Protrusion ≤200mm from roof surface
- Not in conservation area or listed building
- Not visible from highway (if historic building)
- Ground-mounted panels: May require planning if:
- Within 5m of boundary
- Over 4m² in area
- In designated areas (National Parks, AONB)
Building Regulations
- Notify Building Control before installation
- Electrical work must comply with Part P (or use registered installer)
- Structural calculations may be needed for roof mounting
- Fire safety regulations for battery storage rooms
Grid Connection (Hybrid Systems)
- Must notify DNO (Distribution Network Operator)
- G98 application for systems ≤3.68kW (microgenerators)
- G99 application for systems >3.68kW
- Smart Export Guarantee (SEG) for selling excess back to grid
Return on Investment (ROI)
Off-Grid System ROI Calculation
Scenario: Property 500m from nearest grid connection
Grid Connection Cost:
- Utility company quote: £15,000-25,000 (typical for 500m)
- Plus ongoing electricity bills: £1,200/year average
Off-Grid System Cost:
- Complete 5kW solar + 15kWh battery system: £18,000
- Maintenance: £200/year
- Battery replacement (year 12): £5,000
25-Year Comparison:
- Grid: £15,000 connection + (£1,200 × 25 years) = £45,000
- Solar: £18,000 + (£200 × 25) + £5,000 battery = £28,000
- Savings: £17,000 over 25 years
- ROI: Immediate for remote properties!
Hybrid System ROI
Assumptions:
- System Cost: £12,000 (3kW solar + 10kWh battery)
- Annual Electricity Bill Reduction: £900/year (75% reduction)
- SEG Export Income: £150/year
- Annual Savings: £1,050
Payback Period: £12,000 ÷ £1,050 = 11.4 years
25-Year Savings: (£1,050 × 25) – £12,000 – £5,000 (battery replacement) = £9,250
Related Calculators
- Battery Runtime Calculator – How long will your battery bank last?
- Battery Charge Time Calculator – Solar charging duration
- Solar Panel Output Calculator – Expected daily kWh generation
- Wire Gauge Calculator – Proper cable sizing for solar/battery
- Inverter Sizing Calculator – Match inverter to load requirements
Frequently Asked Questions
🔋 How many batteries do I need for off-grid solar?
Battery bank size depends on:
1) Daily energy usage (kWh/day)
2) Days of autonomy needed (2-5 days typical)
3) Battery type (LiFePO4 allows 80% DoD, lead-acid only 50%)
Example: 10 kWh/day usage with 2 days autonomy and LiFePO4 batteries at 48V requires 520Ah capacity.
This equals 5-6 × 100Ah batteries or 2-3 × 200Ah batteries.
Use our calculator above for exact requirements based on your specific situation!
☀️ What size solar panel array do I need for off-grid living?
Formula: Solar array size = Daily energy (kWh) ÷ (Peak sun hours × 0.75 efficiency)
UK averages 2.5-3 peak sun hours daily.
Example: 10 kWh/day usage requires:
10 ÷ (3 × 0.75) = 4,444W solar
Using 300W panels: 15 panels minimum
Size up by 20-30% for winter reliability. South England needs less oversizing than Scotland due to better sun exposure.
⚡ Is 12V, 24V or 48V better for solar systems?
12V: Best for small systems (<1kW), RVs, boats. High current requires thick expensive cables.
24V: Suitable for 1-3kW systems, half the current of 12V.
48V (Recommended for Homes): 3kW+, lowest current, thinnest/cheapest cables, highest efficiency, most scalable.
Example: 2,400W load on 12V = 200A (needs 50mm² cable)
Same load on 48V = 50A (needs 10mm² cable)
48V saves hundreds in cable costs!
💰 How much does a complete off-grid solar system cost UK?
UK off-grid solar costs 2025:
• Small cabin (2kWh/day): £2,500-4,000
• Medium home (10kWh/day): £15,000-20,000
• Large home (20kWh/day): £25,000-35,000
Costs include:
• LiFePO4 batteries: £800-1,200 per 100Ah
• Solar panels: £150-200 per 300W
• Inverter: £400-2,000
• MPPT controller: £300-600
• Professional installation: £2,000-5,000
DIY installation saves 20-30%
🏠 Can I go completely off-grid in the UK?
Yes, completely feasible with proper system sizing.
Challenges:
• Winter solar production is 1/4 of summer
• December-January average 1-1.5 peak sun hours/day
• Requires 3-5x oversized solar array or generator backup
Solutions:
✅ Oversize solar by 300%
✅ Use wood burner/gas for heating (not electric)
✅ Maintain 5 days battery autonomy
✅ Consider small backup generator for extended cloudy periods
Scotland/Wales harder than South England due to less sunshine.
⏱️ How long do solar batteries last?
Battery Lifespan by Type:
LiFePO4: 10-15 years (3,000-5,000 cycles at 80% DoD) ✅ Best
Li-ion: 5-8 years (1,000-2,000 cycles)
AGM Lead-Acid: 3-5 years (500-800 cycles at 50% DoD)
Flooded Lead-Acid: 2-4 years (300-500 cycles)
Lifespan depends on:
• Depth of discharge (shallower = longer life)
• Temperature (cool is better)
• Charging method (proper voltage)
• Maintenance
LiFePO4 offers best longevity and lowest cost per cycle despite higher upfront cost.
📋 Do I need planning permission for solar panels UK?
Roof-mounted solar: Usually NO planning permission needed if:
✅ Panels protrude <200mm from roof
✅ Not in conservation area/listed building
✅ Not visible from highway (historic buildings only)
Ground-mounted solar: Planning MAY be needed if:
• Within 5m of property boundary
• Over 4m² total area
• In AONB/National Park/conservation area
Always notify Building Control. Battery storage indoors: usually no permission.
Contact local planning authority if uncertain. Most residential solar is permitted development.
🔄 What happens to solar panels when battery is full?
When battery reaches 100% charge, solar charge controller automatically:
1️⃣ Reduces charging current to float/maintenance level (trickle charge)
2️⃣ Diverts excess power to AC loads if hybrid inverter
3️⃣ Exports to grid if connected
4️⃣ Curtails solar production if no loads and battery full (panels safely idle)
Modern MPPT controllers and BMS prevent overcharging. Never a safety issue - system designed for this scenario.
Hybrid systems can use excess for electric heating or export for income under SEG scheme.
🔧 Can I add batteries to existing solar panels?
Yes, can retrofit batteries to existing grid-tied solar.
Requirements:
• Hybrid inverter (replaces existing grid-tie inverter) or add AC-coupled battery system
• Compatible charge controller
• DNO notification (G98/G99 forms)
• Electrical work by qualified installer
Costs: £4,000-8,000 for 10kWh battery + hybrid inverter installed
Benefits: Use solar at night, backup during outages, reduce grid reliance
Most cost-effective with LiFePO4 batteries. ROI: 8-12 years.
🔢 How many solar panels for 10kWh per day?
UK calculation:
10 kWh/day ÷ (3 peak sun hours × 0.75 efficiency) = 4,444W solar needed
Using standard 300W panels: 15 panels (4,500W total)
Using 400W panels: 12 panels (4,800W)
This provides reliable year-round generation.
For winter-only reliability, increase by 30% (20 × 300W panels).
South-facing roof at 30-40° tilt optimal. Always round up to next whole panel.
💡 Is off-grid solar worth it financially?
Financially worthwhile if:
1️⃣ Grid connection costs >£10,000 (remote property)
2️⃣ Frequent power outages costing business income
3️⃣ Off-grid lifestyle preference (value beyond money)
Typical ROI:
• Remote properties: Immediate savings vs grid connection
• Grid-tied homes: 10-15 year payback
• Hybrid systems: 8-12 year payback with battery
Not purely financial decision - factors include: energy independence, environmental impact, resilience, rising electricity prices.
Government incentives: SEG payments for export, 0% VAT on solar (until 2027)
Best ROI for self-install, energy-efficient homes, and areas with high electricity rates.