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The number of solar batteries needed depends on the size of your solar panel system and your average daily energy use. Perform an energy audit to determine your average daily energy consumption. Size your solar panel system to meet 100-130% of your average daily consumption. Use a solar battery sizing calculator or rule of thumb of 1 battery per 1,000 watts of solar panels.
Transitioning to solar energy for your home is an excellent way to lower your energy bills and minimize your environmental impact. However, relying solely on solar panels means you can only harness sunlight during daytime hours. Adding a solar battery storage system allows you to store excess solar energy for use at night or on overcast days. But determining the right size solar battery bank for your needs takes some careful calculations. Here is what you need to know to properly size your solar battery bank.
Calculate Your Average Daily Energy Usage
The starting point for solar battery sizing is calculating your average daily energy usage, also known as your load. Your average daily load determines how many solar panels you need, which then dictates the minimum battery capacity required. To determine your load, you will need to conduct an energy audit.
An energy audit involves recording your hourly or daily kilowatt-hours (kWh) of usage over the period of a month. To get your kWh usage, check the usage section on your electricity bills. Alternatively, you can use an electricity monitor that clamps around your main electrical feed and tracks real-time usage. Be sure to record usage during weekends and weekdays, as well as seasonal variations.
Once you have at least a month’s worth of energy usage data, you can calculate your average daily load. Just divide your total kWh usage over the period by the number of days. This gives you a baseline for how much energy you need your solar battery bank to provide on a daily basis.
Factors That Impact Energy Usage
When evaluating your average energy consumption, keep in mind that certain factors can cause your usage to fluctuate:
- Seasonal changes – Usage often spikes in summer and winter months when heating or cooling needs increase.
- Family size and schedules – More people at home during the day increases usage.
- Large appliances – Washing machines, electric dryers and electric stove/ovens can use a lot of energy.
- Electric vehicles – Charging electric cars requires substantial energy.
Be sure to account for these types of factors when extrapolating your long-term average daily usage from monthly data. It’s better to slightly overestimate your average daily load than underestimate it.
Size Your Solar Panel System
Once you know your average daily load, the next step is sizing your solar panel system. As a rule of thumb, your solar panels should be able to generate 100-130% of your average daily energy usage. Oversizing slightly allows your panels to recharge batteries fully even on partly cloudy days.
To determine how many solar panels you need, divide your average daily load by the wattage of the panels you plan to install. For example:
Average daily load: 30 kWh
Panel wattage: 300W
30,000 Wh / 300 W per panel = 100 panels
In this example, you would need a 30 kW system with 100 panels to cover your average daily energy usage. Remember to round up slightly for battery recharging needs.
Solar Panel Sizing Tips
- Factor in your regional sun hours. Sunnier locations need fewer panels than cloudier ones.
- Account for future energy usage if you anticipate adding appliances or electric vehicles.
- Split panels between rooftop and ground arrays to maximize production.
- Choose high-efficiency panel models to reduce the number of panels required.
Determine Battery Capacity
Once your solar panel array is sized, you can determine the minimum battery capacity required. As a general rule of thumb, you need 1 kWh of battery storage capacity for every 1,000 watts of solar panels. So for a 10,000 W solar array, you would need at least 10 kWh of batteries. Using this simplified method provides a good starting point.
To fine tune your battery sizing, you can use one of the many online solar battery calculators. These tools take into account additional variables like your location, battery type, and backup needs. They also often recommend oversizing your battery bank to increase energy reserves.
Some key factors to consider for battery sizing include:
- Days of autonomy – The number of days the battery bank must supply energy without solar recharging. 3-5 days is recommended.
- Battery efficiency – Lithium-ion batteries are 90-95% efficient versus 85% for lead-acid.
- Depth of discharge – Limiting discharge to 50-80% extends battery life.
- Future expansion – Size for 20-30% more capacity than currently needed.
Using a detailed calculator provides a more accurate battery bank size for your specific needs. Always stick to the sizing recommendations from reputable solar professionals or battery manufacturers.
Battery Sizing Calculators
Here are some free online solar battery sizing tools to help fine tune your battery bank calculations:
Wholesale Solar Battery Calculator
The Wholesale Solar battery calculator recommends battery capacity based on energy usage, location, autonomy days and other variables. It also estimates charging needs and recommends inverter size.
EnergySage Solar Calculator
The EnergySage calculator sizes your solar array and battery storage based on your average electric bill, location, net metering policies and other factors. It also estimates the return on investment.
AltE Solar Storage Sizing Assistant
The AltE Calculator recommends the solar array size, battery capacity, inverter size and generator needed based on your electrical load and location details.
Sunrun Solar Battery Size Calculator
The Sunrun calculator estimates the ideal battery capacity for your system based on household energy bills and backup needs during power outages.
Battery Bank Sizing Example
Here is an example applying both the rule of thumb and online solar calculator to size a battery bank.
System Details
- Location: Phoenix, AZ
- Average daily load: 45 kWh
- Panel rating: 320W
- Desired backup: 3 days
Sizing Steps
- Use rule of thumb: 45,000 Wh / 320W per panel = 140 panels x 1 kWh per 1,000W = 140 kWh minimum battery capacity
- Use online calculator and input details
- Recommended battery capacity: 168 kWh
The solar calculator provides a more precise battery capacity of 168 kWh after factoring in the local solar potential and desired backup days. This capacity would likely be provided by wiring twelve to sixteen 12-volt lithium batteries together.
Battery Bank Configuration
Once you’ve determined the total battery capacity needed, the next step is configuring your battery bank. Battery banks wire multiple batteries together to create a single large bank that meets your voltage and capacity requirements.
Battery Bank Voltage
Most home solar systems operate on a 48-volt DC battery bank. To create a 48V bank:
- Wire 4 x 12V batteries in series to achieve 48V with 12V capacity.
- Wire additional 4-battery series strings in parallel to increase capacity.
For example, 16 batteries configured as 4 parallel strings of 4 batteries in series (4S4P) creates a 48V bank with 4x the 12V capacity.
Wiring Considerations
- Use equal size and type (e.g. lead-acid or lithium-ion) batteries in each series string.
- Limit series strings to 4-8 batteries to avoid excessive voltage.
- Use the same wire gauge and length for each parallel connection.
- Include a fuse or circuit breaker on each series string.
Avoid arranging batteries only in series as it results in dangerously high DC voltages. Always include parallel connections to increase capacity as needed.
Battery Management System
A battery management system (BMS) is essential for monitoring battery health and performance. The BMS protects against overcharging/discharging, balances the cells, and provides status data like remaining capacity. Choose a sufficiently robust BMS for the size of your battery bank.
Sample Battery Banks
Here are some example 48V battery bank configurations for different capacities:
150 kWh Battery Bank
- 75 kWh usable capacity
- 16x 12V 200Ah lithium batteries
- 4 parallel strings of 4 batteries in series (4S4P)
250 kWh Battery Bank
- 125 kWh usable capacity
- 20x 12V 200Ah lithium batteries
- 5 parallel strings of 4 batteries in series (4S5P)
400 kWh Battery Bank
- 200 kWh usable capacity
- 32x 12V 200Ah lithium batteries
- 8 parallel strings of 4 batteries in series (4S8P)
Larger systems may also incorporate multiple 48V battery banks. Always consult a qualified solar installer when designing your battery bank.
Battery Bank Maintenance
To maximize your battery bank’s performance and lifespan, follow these maintenance best practices:
Monthly
- Inspect batteries for damage or leaks
- Check and tighten connections
- Clean battery terminals
- Take voltage readings across terminals
Every 6 months
- Recalibrate BMS
- Equalize charge flooded lead-acid batteries
- Check specific gravity of lead-acid electrolyte
Annually
- Professional deep cycle capacity test
- Check and retighten all connections
- Wipe down exterior of batteries
Following the manufacturer’s maintenance guidelines is also essential. With proper care, your battery bank can provide many years of reliable energy storage.
Key Takeaways
- Determine average daily load via energy audit.
- Size solar array at 100-130% of average load.
- Use 1 kWh batteries per 1,000W solar as rule of thumb.
- Refine battery sizing with online solar calculators.
- Create 48V banks using 4-8 batteries in series and paralleled as needed.
- Include battery management system and maintenance.
Conclusion
Right-sizing your solar battery bank involves careful analysis of your energy consumption and solar system capabilities. While the process may seem complicated, the large energy bill savings and grid independence provided by an adequate battery bank make the effort well worthwhile. Use the guidelines and tools provided in this article to determine the optimal battery capacity, configuration and maintenance plan for your solar energy needs.
