Quick Answer: Batteries Needed for a 20kW Solar System
A 20kW solar system typically requires 5 to 21 LiFePO4 batteries (51.2V), depending on battery size and local peak sun hours. Common estimates based on 4 to 6 peak sun hours per day:
- 51.2V 100Ah batteries: 13 to 21 units
- 51.2V 200Ah batteries: 7 to 12 units
- 51.2V 300Ah batteries: 5 to 8 units
The exact count depends on daily solar generation, depth of discharge (DoD), system efficiency, and whether you are running off-grid or hybrid.
How Much Energy Does a 20kW Solar System Actually Generate?
The "20kW" figure refers to your solar panel array's peak output capacity, not the amount of electricity it produces per day. Actual daily generation depends on peak sun hours (PSH): the average number of hours per day when sunlight intensity is strong enough for panels to operate at maximum output.
Formula: Daily Generation (kWh) = System Power (kW) x Peak Sun Hours x System Efficiency (80%)
The same 20kW array produces very different outputs depending on where it is installed:
| Region | Avg. Peak Sun Hours | Daily Generation (kWh) | Notes |
|---|---|---|---|
| Australia (most regions) | 4.5 to 6 hrs | 72 to 96 kWh | Higher in inland/northern areas |
| South Africa | 5 to 6.5 hrs | 80 to 104 kWh | Among the highest globally |
| Southeast Asia | 4 to 5.5 hrs | 64 to 88 kWh | Varies with monsoon season |
| Latin America (tropical) | 4.5 to 6 hrs | 72 to 96 kWh | Strong solar resource year-round |
| Europe (southern) | 4 to 5 hrs | 64 to 80 kWh | Lower in winter months |
Always look up your specific location's PSH data rather than using a regional average. A site in coastal Queensland and one in inland New South Wales can differ by a full hour.
Quick Battery Count Reference for a 20kW Solar System
Estimated battery counts for three common 51.2V LiFePO4 specifications across three peak sun hour scenarios. Figures assume 80% system efficiency, 90% DoD, and 90% battery round-trip efficiency.
| Battery Spec | Capacity per Unit | 4 PSH (64 kWh/day) | 5 PSH (80 kWh/day) | 6 PSH (96 kWh/day) |
|---|---|---|---|---|
| 51.2V 100Ah | 5.12 kWh | 13 to 15 units | 16 to 18 units | 19 to 21 units |
| 51.2V 200Ah | 10.24 kWh | 7 to 9 units | 8 to 10 units | 10 to 12 units |
| 51.2V 300Ah | 15.36 kWh | 5 to 6 units | 6 to 7 units | 7 to 8 units |
These figures cover fully off-grid systems sized for one full day of solar generation. Use the step-by-step calculation below for a precise number.
Key Parameters You Need Before Calculating
These four inputs go directly into the battery sizing formula. Gather all four before you run the calculation.
1. Peak Sunlight Hours (PSH)
Hours per day when your panels run at full output. Look up your location on the Australian Bureau of Meteorology solar radiation map or the Global Solar Atlas.
2. Battery Voltage and Capacity (Ah)
Multiply voltage by Ah, then divide by 1,000 to get kWh per unit. A 51.2V 200Ah battery stores 10.24 kWh. For 20kW systems, 51.2V LiFePO4 batteries are the standard choice.
3. Depth of Discharge (DoD)
LiFePO4 supports 90% DoD; lead-acid is limited to 50%. At 50% DoD you need roughly twice as many batteries for the same usable capacity.
4. System and Battery Round-Trip Efficiency
Two losses feed into the formula. System efficiency (80%) covers wiring and conversion losses from panels to battery. Battery round-trip efficiency (90%) covers energy lost in each charge-discharge cycle. Both are applied in Steps 1 and 2 below.
Step-by-Step Calculation
We show two worked examples: 4 PSH for moderate-sun regions and 6 PSH for high-sun regions.
Step 1: Calculate Daily Actual Power Generation (kWh)
Formula: Daily Generation = System Power (kW) x Peak Sun Hours x System Efficiency (0.8)
Example A (4 PSH): 20 kW x 4 hrs x 0.8 = 64 kWh/day
Example B (6 PSH): 20 kW x 6 hrs x 0.8 = 96 kWh/day
Step 2: Calculate Required Total Battery Capacity (kWh)
Formula: Required Capacity = Daily Generation / DoD / Battery Efficiency
*For LiFePO4: DoD = 0.9, Battery Efficiency = 0.9.
Example A: 64 kWh / 0.9 / 0.9 = 79 kWh required total capacity
Example B: 96 kWh / 0.9 / 0.9 = 119 kWh required total capacity
Step 3: Calculate Energy Storage of a Single Battery (kWh)
Formula: Single Battery Capacity = Voltage (V) x Capacity (Ah) / 1,000
Example (51.2V 200Ah): 51.2V x 200Ah / 1,000 = 10.24 kWh per unit
Step 4: Calculate Number of Batteries (Round Up)
Formula: Number of Batteries = Required Total Capacity / Single Battery Capacity
*Round up to the nearest whole number and add one extra unit as margin.
Example A (79 kWh / 10.24 kWh): 7.7 -> Round up to 8, recommend 9 units
Example B (119 kWh / 10.24 kWh): 11.6 -> Round up to 12 units
Off-Grid vs. Hybrid: Does System Type Change the Battery Count?
Yes, significantly. The calculation above assumes a fully off-grid system. Grid-connected systems follow different sizing logic:
- Off-grid systems must store a full day of solar generation. The battery bank is the only power source when the sun is not shining.
- Grid-tied with battery backup systems only need to cover the hours of outage you want to protect against. Four hours of backup at 5 kW average load requires just 20 kWh of usable capacity, far fewer batteries than a full off-grid setup.
- Hybrid systems target self-consumption: store surplus daytime generation and use it during evening peak. The battery bank is sized against evening load, not total daily generation.
For hybrid or backup systems, define your backup hours and critical loads before choosing battery quantity.
5 Real-World Factors That Adjust Your Calculated Battery Count
The four-step formula gives you a baseline. These five factors are not in the formula but regularly cause real installations to differ from that number.
1. Days of Autonomy
Multiply your daily battery count by the number of cloudy days you need to cover. A 20kW system requiring 3-day autonomy with 51.2V 200Ah batteries may need 24 to 36 units.
2. Actual Load Profile vs. Solar Generation
If your consumption is lower than daily generation, size to consumption instead. A facility using 30 kWh/day from an 80 kWh/day system needs only 3 to 4 units of 51.2V 200Ah, not the full calculated bank.
3. High Ambient Temperature
Above 45 degrees C, LiFePO4 effective capacity drops 10 to 15%. Add 10 to 15% extra units when installing in hot climates such as inland Australia, the Middle East, or Sub-Saharan Africa.
4. Long-Term Capacity Degradation
LiFePO4 retains about 80% capacity at end of rated cycle life. Size the battery bank at 110 to 120% of your calculated requirement to maintain performance through year 10 and beyond.
5. Seasonal Solar Variation
Summer-to-winter generation can vary by 30 to 40%. If your location has a pronounced low-sun season, use your worst-case monthly PSH in Step 1, not the annual average.
BSLBATT LiFePO4 Battery Recommendations for 20kW Systems
BSLBATT manufactures three 51.2V LiFePO4 rack batteries suited to 20kW solar systems:
| Model | Voltage | Capacity | Energy | Best For |
|---|---|---|---|---|
| B-LFP48-100E | 51.2V | 100Ah | 5.12 kWh | Budget-conscious builds; more units required |
| B-LFP48-200E | 51.2V | 200Ah | 10.24 kWh | Most popular for 20kW residential systems |
| B-LFP48-300E | 51.2V | 300Ah | 15.36 kWh | Fewer units, less wiring complexity; ideal for large installs |
For most 20kW off-grid residential installations, the 200Ah or 300Ah models offer the best balance of unit count, installation simplicity, and cost per kWh.
All three models from BSLBATT are certified to UN38.3, CE, and IEC 62619 standards, meeting installation requirements across major international markets. The B-LFP48-100E additionally holds CEC approval for Australian installations. All support parallel stacking for flexible capacity expansion.
Frequently Asked Questions about Batteries for 20kW Solar System
Q: How many kWh does a 20kW solar system produce per day?
64 to 120 kWh per day, depending on local peak sun hours. Most Australian locations average 72 to 96 kWh per day.
Q: Is 20kW solar enough to power a house off-grid?
Yes. A typical Australian home uses 18 to 25 kWh per day. A 20kW system in a 5 PSH location generates around 80 kWh per day, well over three times the average household need.
Q: Can I use lead-acid batteries with a 20kW solar system?
You can, but you will need roughly twice as many units due to lead-acid's 50% DoD limit. With a lifespan of 500 to 1,200 cycles versus 3,500 to 6,000+ for LiFePO4, the lifetime cost is significantly higher. LiFePO4 is the standard choice for systems at this scale.
Q: How long will LiFePO4 batteries last in a 20kW off-grid system?
10 to 15 years with daily cycling. BSLBATT LiFePO4 batteries retain over 80% capacity at end of rated cycle life.
Q: Can I expand my battery bank after installation?
Yes, by adding batteries in parallel. Use the same model and batch for best results. If you plan to expand, size your inverter and BMS for the higher capacity from the start.
Q: What if I want to cover extra days of cloudy weather?
Multiply your single-day battery count by the number of backup days needed. Most installations plan for 1 to 2 days of autonomy; remote systems typically use 3 days.
When the Formula Is Not Enough
Every 20kW installation is different. The calculation in this guide gives you a solid baseline, but the right final count depends on site-specific variables: actual load data, local climate, and planned autonomy days. BSLBATT's engineering team has sized and deployed 20kW systems across Australia, South Africa, Southeast Asia, and Latin America. If you want a verified recommendation based on your specific location and load profile, contact us for a consultation.
Marketing Director| Focused on ESS · BSLBATT
Aydan is a Marketing Director and energy storage specialist at BSLBATT, focusing on residential, commercial, and off-grid battery solutions. He works closely with solar distributors, installers, and EPC companies across global markets, supporting the design and deployment of reliable energy storage systems.
Post time: Jul-09-2026