LiFePo4 vs AGM batteries
How to Size a Solar Battery Bank: Complete UK Guide for 2026
Choosing the correct battery bank size is one of the most important decisions when designing an off-grid solar system. A battery bank that is too small will leave you without power during periods of poor weather, while an oversized system can add unnecessary cost.
This guide explains how to calculate your battery requirements accurately, understand battery capacity ratings, and select the right battery bank for your solar installation.
To size a solar battery bank, calculate your daily energy usage in watt-hours, multiply by the number of days of autonomy required, divide by the battery depth of discharge, and convert the result into amp-hours based on system voltage
What Is a Solar Battery Bank
A solar battery bank stores excess electricity generated by your solar panels for use when the sun is not shining.
Battery storage allows you to:
- Use solar energy during the evening
- Maintain power during cloudy weather
- Improve energy independence
- Power off-grid properties and mobile applications
Battery banks are commonly used in:
- Off-grid homes
- Remote cabins
- Garden offices
- Workshops
- Campervans
- Boats and marine systems
Why Battery Sizing Matters
An incorrectly sized battery bank can cause several problems.
Battery Bank Too Small
- Frequent deep discharges
- Reduced battery lifespan
- Power shortages
- Inability to cope with poor weather
Battery Bank Too Large
- Higher upfront costs
- Longer charging times
- Unnecessary investment
The goal is to balance storage capacity with daily energy requirements.
Step 1: Calculate Daily Energy Usage
Before selecting a battery bank, you must calculate how much electricity you use each day.
List every appliance you intend to power.
Example Daily Energy Consumption
| Appliance | Power (W) | Hours Used | Daily Consumption |
|---|---|---|---|
| LED Lighting | 20W | 5 Hours | 100Wh |
| Laptop | 60W | 5 Hours | 300Wh |
| Fridge | 50W Average | 12 Hours | 600Wh |
| Phone Charging | 10W | 5 Hours | 50Wh |
Total Daily Consumption
100Wh + 300Wh + 600Wh + 50Wh
Total = 1,050Wh per day
This figure is the foundation for sizing your battery bank.
Step 2: Decide How Many Days of Autonomy You Need
Autonomy refers to the number of days your battery bank can supply power without receiving solar input.
Typical recommendations:
| Application | Days of Autonomy |
| Summer Cabin | 1 Day |
| Garden Office | 1–2 Days |
| Campervan | 1–2 Days |
| Year-Round Off-Grid Property | 2–3 Days |
For this example, we will use:
2 Days Autonomy
Calculation:
1,050Wh × 2 = 2,100Wh
Your battery bank must store at least 2,100Wh.
Step 3: Consider Battery Depth of Discharge
Not all battery capacity is usable.
AGM Batteries
Recommended maximum discharge:
50%
Example:
100Ah AGM Battery
- Total Capacity = 1,200Wh
- Usable Capacity ≈ 600Wh
LiFePO4 Lithium Batteries
Recommended maximum discharge:
80–90%
Example:
100Ah LiFePO4 Battery
- Total Capacity = 1,280Wh
- Usable Capacity ≈ 1,150Wh
This is one reason lithium batteries have become the preferred choice for modern solar systems.
Step 4: Calculate Required Battery Capacity
Formula:
Battery Capacity (Wh) = Daily Consumption × Days of Autonomy
Using our example:
1,050Wh × 2 = 2,100Wh
For lithium batteries:
2,100Wh ÷ 0.9 = 2,333Wh
Required storage:
Approximately 2.3kWh
Step 5: Convert Watt-Hours to Amp-Hours
Most batteries are sold in Amp-hours (Ah).
Formula:
Ah = Wh ÷ System Voltage
Example: 12V System
2,333Wh ÷ 12V
= 194Ah
Recommended battery bank:
200Ah LiFePO4 Battery
Common Battery Bank Sizes
Small Solar System
Suitable for:
- Lighting
- Phone charging
- Small electronics
Typical battery:
- 100Ah Lithium
Medium Solar System
Suitable for:
- Lighting
- Laptops
- Refrigeration
Typical battery:
- 200Ah Lithium
Large Off-Grid System
Suitable for:
- Multiple appliances
- Continuous daily use
Typical battery:
- 400Ah–600Ah Lithium
Example Solar Battery Sizing Scenarios
Garden Office
Daily Usage:
800Wh
Recommended Battery:
100–150Ah Lithium
Campervan
Daily Usage:
1,200Wh
Recommended Battery:
200Ah Lithium
Remote Cabin
Daily Usage:
2,000Wh
Recommended Battery:
300–400Ah Lithium
Full-Time Off-Grid Living
Daily Usage:
4,000Wh+
Recommended Battery:
600Ah+ Lithium
Lithium vs AGM Battery Sizing
Because lithium batteries provide significantly more usable capacity, a smaller lithium bank often replaces a much larger AGM installation.
Example:
| Battery Type | Rated Capacity | Usable Capacity |
| AGM | 200Ah | 100Ah |
| LiFePO4 | 120Ah | 108Ah |
This makes lithium batteries more space-efficient and cost-effective over their lifespan.
Common Solar Battery Sizing Mistakes
Underestimating Consumption
Many users forget:
- Inverters consume power
- Refrigerators cycle continuously
- Seasonal usage increases
Always add a safety margin.
Ignoring Winter Conditions
Solar production in winter can be significantly lower than summer output.
Systems designed for year-round use should include additional battery capacity.
Choosing the Cheapest Battery
Battery replacement costs can exceed initial savings.
Quality lithium batteries often provide the lowest long-term cost per cycle.
Frequently Asked Questions
What size battery do I need for a 1kW solar system?
This depends on your energy usage rather than panel size. Most users require between 100Ah and 300Ah of lithium storage.
Is 100Ah enough for an off-grid solar system?
For basic lighting and charging applications, yes. For refrigeration or extended autonomy, larger batteries are usually required.
Are lithium batteries worth the extra cost?
For most solar applications, yes. They offer longer life, greater usable capacity, and faster charging.
How many batteries do I need for an off-grid cabin?
This depends on your daily energy usage and desired autonomy. Most cabin systems use between 200Ah and 600Ah of lithium storage.
Final Thoughts
Sizing a solar battery bank correctly is essential for reliable off-grid power.
The process can be summarized in five simple steps:
- Calculate daily energy consumption.
- Decide how many days of autonomy you require.
- Account for battery depth of discharge.
- Calculate required storage in watt-hours.
- Convert watt-hours into amp-hours based on system voltage.
By following these steps, you can build a battery bank that delivers reliable performance, maximizes battery life, and provides the energy storage needed for your application.
For best results, choose high-quality lithium batteries, allow a margin for future expansion, and design around your worst-case winter usage rather than summer conditions.
