CalcBattery Autonomy Calculator
Last updated: 2026-08-10
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| Daily consumption (kWh) (kWh/día) | Days of autonomy (días) | Battery capacity (kWh) (kWh/bat) | |
|---|---|---|---|
| Fin de semana (5 kWh/d, 1 día) | 5 kWh/día | 1 días | 5 kWh/bat |
| Residencia aislada (10 kWh/d, 2 días) | 10 kWh/día | 2 días | 10 kWh/bat |
| Casa rural (15 kWh/d, 3 días) | 15 kWh/día | 3 días | 13.5 kWh/bat |
| Refugio montaña (8 kWh/d, 5 días) | 8 kWh/día | 5 días | 5 kWh/bat |
| Granja aislada (30 kWh/d, 2 días) | 30 kWh/día | 2 días | 13.5 kWh/bat |
The Battery Autonomy Calculator is a free online tool that helps you determine how many storage batteries you need to keep your appliances running during a power outage. Whether you are designing an off-grid solar system or preparing an emergency backup plan, this calculator takes your daily energy consumption, desired days of autonomy, and individual battery capacity to deliver a precise equipment list in seconds.
What the Battery Autonomy Calculator Does and When to Use It
This calculator solves one of the most common questions in energy storage: "How many batteries do I need?" It answers this by converting your daily electricity usage into the total kilowatt-hours (kWh) required to cover a specified number of days without recharging, then divides that by the capacity of the batteries you plan to use. The result includes the exact number of batteries needed, the total installed capacity, and the necessary storage volume.
Use the Battery Autonomy Calculator in these scenarios:
- Off-grid solar system design: When sizing a battery bank for a cabin, RV, or remote home that has no grid connection.
- Emergency backup planning: To ensure your home or small business can run critical loads (lights, fridge, medical devices) during a multi-day blackout.
- Battery replacement or upgrade: When swapping older batteries for newer ones and you need to match or improve your existing storage capacity.
- Comparing battery technologies: Test different battery capacities (for example, 5 kWh, 10 kWh, or 13.5 kWh) to see how they affect the total number of units and total cost.
The calculator automatically accounts for a standard 80% depth of discharge (DoD) limit, which protects most lead-acid and lithium batteries from premature wear. You do not need to be an electrician—just enter three numbers, and the tool does the math.
The Formula Explained Variable by Variable
The calculation performs three simple steps, all based on a single formula. Here is the logic, broken into its core variables:
| Variable | Description | Example Value |
|---|---|---|
| C | Daily energy consumption in kWh (kilowatt-hours). This is the total electricity your devices use in a typical 24-hour period. | 3.5 kWh |
| D | Days of autonomy. How many consecutive days you want the system to run without any recharging source (sun, generator, grid). | 2 days |
| CB | Capacity of a single battery in kWh. This is the usable energy stored in one battery unit, as rated by the manufacturer. | 5 kWh |
| kWh_needed | Total energy the battery bank must store to meet consumption and protect battery health. Calculated as (C × D) ÷ 0.8. The division by 0.8 accounts for the maximum safe depth of discharge (80%). | (3.5 × 2) ÷ 0.8 = 8.75 kWh |
| batteries | Number of battery units, always rounded up to the nearest whole number. Calculated as kWh_needed ÷ CB. | 8.75 ÷ 5 = 1.75, rounds up to 2 batteries |
| kWh_installed | Total installed storage capacity in kWh. This is batteries × CB. It will always be equal to or slightly higher than kWh_needed. | 2 × 5 = 10.0 kWh |
The 0.8 in the formula represents the 80% depth of discharge limit. If you are using batteries that can discharge deeper (for example, some lithium iron phosphate models allow 90% DoD), you would adjust this factor manually, but the default 0.8 is a safe and widely accepted conservative value.
Two Worked Examples with Concrete Numbers
Example 1: Small Cabin with Weekend Autonomy
Situation: You have a weekend cabin that consumes 2.0 kWh per day (a few LED lights, a small fridge, and phone charging). You want the battery system to run for 3 days without solar input. You are looking at 5 kWh lithium batteries.
- Calculate the needed energy: 2.0 kWh/day × 3 days = 6.0 kWh. Divide by 0.8: 6.0 ÷ 0.8 = 7.5 kWh needed.
- Count the batteries: 7.5 kWh ÷ 5 kWh per battery = 1.5. Round up to 2 batteries.
- Result: You need 2 batteries, providing a total installed capacity of 10.0 kWh (versus the required 7.5 kWh). The extra 2.5 kWh gives you a small safety margin.
Example 2: Home Backup with Extended Outage
Situation: Your home uses 8.5 kWh per day (running the fridge, well pump, internet router, and a few lights). You want protection for 4 days during a major storm. You already own 3.5 kWh deep-cycle lead-acid batteries.
- Calculate the needed energy: 8.5 kWh/day × 4 days = 34.0 kWh. Divide by 0.8: 34.0 ÷ 0.8 = 42.5 kWh needed.
- Count the batteries: 42.5 kWh ÷ 3.5 kWh per battery = 12.14. Round up to 13 batteries.
- Result: You need 13 batteries, giving you an installed capacity of 45.5 kWh. This is 3.0 kWh more than the calculated need, which helps offset any efficiency losses in the inverter or wiring.
Common Mistakes When Using the Battery Autonomy Calculator
Avoid these frequent errors to get accurate results:
- Entering watts instead of kilowatt-hours: The calculator expects kWh, not watts. If your appliance uses 100 watts and runs 10 hours per day, that is 100 W × 10 h = 1,000 Wh, which is 1.0 kWh. Do not enter 100 or 1000.
- Ignoring the depth of discharge (DoD): The formula already includes an 80% DoD. If you manually add another 20% safety factor or assume batteries can discharge to 100%, the result will be wrong. Let the calculator handle the 0.8 factor.
- Mixing battery types in the same bank: The calculator assumes all batteries have the same capacity (CB). Mixing old and new batteries, or different brands, can cause uneven charging and reduce the bank's actual usable capacity.
- Forgetting that days of autonomy means NO recharging: If you plan to run a generator or have solar panels that produce some power every day, you can often reduce the "days of autonomy" number to 1 or 2. The tool is designed for full off-grid scenarios where the battery must carry all loads alone.
- Rounding down instead of up: The calculator always rounds the number of batteries up. If you manually round down (for example, from 1.2 to 1 battery), you will fall short of the required kWh, risking a blackout on day 2.
Frequently Asked Questions
Can I use this calculator for both lead-acid and lithium batteries?
Yes, the Battery Autonomy Calculator works for any battery chemistry as long as you know the usable capacity in kWh. For lead-acid batteries, note that the 80% DoD is standard. For lithium ion, some models permit 90% or 95% DoD. In that case, you can adjust the math manually: instead of dividing by 0.8, divide by 0.9. The calculator's default 0.8 is conservative and safe for all types.
How do I find my daily consumption in kWh if I don't have a monitor?
Add up the wattage of every device you will power and multiply by the hours used per day. For example: a 60 W light bulb used 5 hours equals 300 Wh (0.3 kWh). A refrigerator using 150 W running 8 hours per day equals 1,200 Wh (1.2 kWh). Sum all devices. Alternatively, if you have a grid-tied electric bill, look for your daily average: divide your monthly kWh by 30 days.
What does it mean if the calculator tells me I need 9.5 batteries?
Because you cannot buy a fraction of a battery, the result auto-rounds up to the next whole number. In this example, 9.5 becomes 10 batteries. The total installed capacity will then be 10 × CB, which is slightly higher than the minimum requirement. This extra capacity is beneficial—it reduces the DoD on each cycle and extends battery life.