Solar Battery Calculator
Size a battery bank for solar backup from your daily energy use, days of autonomy needed, and each battery's usable capacity.
How this calculator works
A solar battery bank needs to hold enough usable energy to carry your loads through the days you’re planning for, with no sun to recharge it. Because batteries shouldn’t be drained all the way down, the bank has to be built bigger than your raw energy need to leave that reserve untouched.
Formula: Needed storage = (daily kWh use × days of autonomy) ÷ depth of discharge. Number of batteries = needed storage ÷ each battery’s capacity, rounded up.
Worked example
10 kWh of daily use, 2 days of autonomy, 80% usable depth of discharge, and 5 kWh batteries:
- Needed storage: 10 × 2 ÷ 0.80 = 25 kWh
- Batteries required: 25 ÷ 5 = 5 batteries
Notes
Depth of discharge matters because most batteries shouldn’t be fully drained. An 80% usable figure is typical for lithium battery banks, while older lead-acid batteries often use a more conservative 50%, meaning a lead-acid bank needs roughly twice the rated capacity to store the same usable energy as a lithium bank.
How to use
Enter your average daily energy use in kWh, how many days of autonomy you want the bank to cover without sun, the usable depth of discharge for your battery type, and the capacity of a single battery. The calculator returns the total storage you need and how many of that battery to buy.
Electrical work is code-governed and safety-critical. Confirm circuit, conductor, and equipment sizing with a licensed electrician and your local code.
Frequently asked questions
What is depth of discharge and why does it matter?
Depth of discharge is the share of a battery's rated capacity you're actually allowed to use. Draining a battery fully shortens its life, so batteries are rated for a safe usable percentage instead. A battery bank sized only on its rated capacity, without accounting for depth of discharge, will run out of usable power sooner than expected.
What is days of autonomy?
Days of autonomy is how many days your battery bank should be able to power your loads with zero solar input, such as during a stretch of cloudy weather. More days of autonomy means a bigger, more expensive battery bank, so it's a tradeoff between backup security and cost.
Why do lithium and lead-acid batteries size so differently?
Lithium batteries can safely be drained further per cycle than lead-acid batteries, so a lithium bank needs less rated capacity to deliver the same usable energy. A lead-acid bank sized the same way as a lithium bank will come up short, because more of its rated capacity has to stay unused to protect the battery.
How do I find my daily kWh use?
Add up the wattage of everything you want backed up, multiply by the hours each item runs per day, and convert to kWh (divide by 1,000). Your utility bill also lists average daily or monthly kWh use, which is a good starting estimate for whole-home backup sizing.
Estimates only. Verify quantities with your supplier before purchasing.