Solar Battery Sizing Calculator

Off-Grid Battery Sizer

Going off-grid? Calculate exactly how much battery capacity you need to survive cloudy days.

Total energy used in 24h.
Days to survive without sun.
Depth of Discharge capability.

Yes, showing Amp-Hours typically used for sizing battery banks.

Battery Bank Requirements

Total Energy Needed

0 kWh
Usable Capacity

Required Amp-Hours

0 Ah
@ 48V
Recommendation: To get 10 kWh of usable energy, you need a 11.1 kWh bank (due to DoD limits).

How This Tool Works

The Off-Grid Battery Sizing Calculator is a critical planning tool for anyone looking to achieve energy independence. It determines the total storage capacity required to power your loads through the night and across "Days of Autonomy"—the periods when the sun isn't shining.

Our algorithm factors in the Usable vs. Total Capacity dilemma. By selecting your battery chemistry, the tool automatically adjusts for safe discharge limits, ensuring your battery bank has a long and healthy lifespan.

How to Use the Sizer

  • Daily Consumption: Enter your total 24-hour energy use in kWh. A typical energy-efficient home uses 10–15 kWh/day.
  • Days of Autonomy: How many days should the system run without any solar input? 3 days is the standard for reliability.
  • Battery Voltage: Select your system DC voltage. 48V is best for homes; 12V/24V is for mobile/temporary setups.
  • Battery Type: Select your chemistry. Lithium is much more efficient but has a higher upfront cost.

System Design Example

For a 10kWh daily load and 3 days of autonomy:

  • Total Usable Energy Needed: 30 kWh
  • Lithium Bank (90% DoD): ~33.3 kWh (approx. 720 Ah @ 48V)
  • Lead Acid Bank (50% DoD): ~60.0 kWh (approx. 1,250 Ah @ 48V)
  • Verdict: While Lithium looks more expensive, you need nearly 50% less physical battery to do the same job compared to Lead Acid, often making it cheaper in the long run.

Autonomy vs. Battery Lifespan

Sizing your battery correctly isn't just about keeping the lights on; it's about protecting your investment:

  • The 'Daily Cycle' Rule: Ideally, you only want to use 20–30% of your bank's capacity on a normal night. This shallow cycling can double the lifespan of many batteries.
  • Peak Power Draw: Remember to check your inverter's surge capacity. This tool sizes for energy (kWh), but your battery must also be able to handle the surge current (Amps) of starting a well pump or A/C unit.
  • Temperature Factors: Batteries lose capacity in the cold. If your bank is in an unheated garage, consider adding a 20% "Cold Weather Buffer" to your results.

Frequently Asked Questions

How much battery storage do I need for my house?

For most off-grid homes, you need 2 to 3 days of autonomy (storage capacity). If you use 10kWh per day, a 30kWh battery bank is recommended to ensure you don't lose power during a string of cloudy days.

What is Depth of Discharge (DoD) and why does it matter?

DoD represents how much of a battery's total capacity can be used safely. Lithium batteries (LiFePO4) can handle up to 90-95% DoD, while Lead Acid batteries should only be drained to 50% to avoid permanent damage. This means you need a much larger physical bank of Lead Acid batteries to get the same usable energy as Lithium.

Should I use a 24V or 48V battery system?

For whole-home solar, 48V is the industry standard. Higher voltage reduces energy loss through heat and allows for thinner, cheaper wiring. 12V and 24V systems are typically reserved for small RVs, boats, or tiny cabins.
Expert Insight: When sizing for Lithium, look at the manufacturer's C-Rating. This tells you how fast you can discharge the battery without damaging it.