Does this include inverter sizing?

No. It focuses on storage capacity. Inverter sizing depends on peak watts/surge.

What about aging/temperature?

Aging and low temperatures both reduce usable capacity. This calculator does not explicitly model those effects, so it is wise to add extra capacity—often 20–30% or more—above the bare minimum.

Is DoD auto-selected?

No. You choose the usable depth of discharge based on your battery chemistry, manufacturer recommendations, and desired cycle life. Many lithium systems support 70–90% usable, while lead‑acid is often sized more conservatively.

Is this a purchase recommendation?

No. This is a sizing aid, not a product recommendation. Always compare against detailed manufacturer specifications and work with a qualified installer or engineer for final design.

Can I include solar recharge?

Not directly. This calculator assumes no additional charging during the runtime window. For solar‑plus‑storage, you would typically model solar production separately and may reduce the required storage if you expect charging while loads are running.

energy calculator

Battery Storage Sizing Calculator

Estimate battery storage capacity (Wh/kWh) based on load, runtime, depth of discharge, and efficiency.

Results

Required capacity (Wh): 22222.22
Required capacity (kWh): 22.22

Overview

Battery storage sizing is all about matching stored energy to the loads you want to run and how long you want to run them. If you undersize storage, your lights and critical loads may cut out earlier than expected; if you wildly oversize, you can end up overspending on batteries you never fully use.

This battery storage sizing calculator gives you a first‑pass estimate of how many watt‑hours (Wh) or kilowatt‑hours (kWh) of battery capacity you need to support an average load for a set number of hours. It explicitly accounts for usable depth of discharge (DoD) and round‑trip/system efficiency so you can compare different chemistries, system designs, and safety margins.

Use it as a planning tool for backup power, off‑grid cabins, RV and van builds, or solar‑plus‑storage systems before you dive into detailed design with an installer or engineer.

How to use this calculator

List the devices or circuits you want the battery to support and estimate their total average draw in watts over the backup period, then enter that total as Average load (W).
Decide how many hours of runtime you want at that average load (for example, 4 hours of evening peak or 12–24 hours of outage coverage) and enter that in Hours of runtime needed.
Set a usable depth of discharge based on your battery chemistry and desired cycle life; for many lithium‑ion systems, 70–90% is common, while lead‑acid is often lower.
Enter an overall system efficiency that reflects inverter and wiring losses; many modern systems fall in the 85–95% range.
Review the Required capacity (Wh) and Required capacity (kWh) outputs to see the approximate nameplate battery size you would target.
Compare the result with available battery module sizes (for example, 5 kWh, 10 kWh, 13.5 kWh packs) and add safety margin for aging, temperature, and future load growth.

Inputs explained

Average load (W): Your estimated average power draw, in watts, over the period the batteries need to support. You can approximate this by summing the typical wattage of the loads you plan to run and adjusting for duty cycle (how often they are actually on).
Hours of runtime needed: The number of hours you want the system to support the average load without recharging—for example, 4–6 hours for evening peak shaving, or 12–24 hours for outage backup.
Usable depth of discharge (%): The percentage of the battery’s nameplate capacity that you are willing to use on a regular basis. Many lithium‑ion systems are sized around 70–90% usable DoD, while flooded lead‑acid and AGM are often kept closer to 50% to protect cycle life.
System efficiency (%): An overall efficiency factor that accounts for inverter losses, wiring losses, and battery round‑trip efficiency. For many residential systems, 85–95% is a reasonable starting assumption.

Outputs explained

Required capacity (Wh): The estimated nameplate battery capacity in watt‑hours you would target to support the specified average load for the desired runtime, given your chosen depth of discharge and efficiency.
Required capacity (kWh): The same required capacity expressed in kilowatt‑hours, which is how most residential and commercial battery systems are marketed and compared.

How it works

You provide an estimate of your average load in watts over the period you care about (for example, the average power draw of the circuits you want backed up).

You enter how many hours of runtime you want the battery bank to support at that average load. Multiplying load by hours gives the ideal energy requirement in watt‑hours at 100% usable capacity and perfect efficiency.

Because real batteries should not be fully discharged, you enter a usable depth of discharge (DoD) as a percentage; many lithium chemistries are sized at 70–90% usable, while lead‑acid is often closer to 50%.

You also enter an overall system efficiency to account for inverter losses, wiring, and round‑trip battery efficiency. This reduces the effective usable energy you get out of a given nameplate capacity.

The calculator divides the raw energy requirement by (DoD × efficiency) to find the required nameplate battery capacity in Wh, and then converts that to kWh for easier reading.

The result is a ballpark storage requirement that you can compare to common battery module sizes or to spec sheets for specific products.

Formula

Let P = Average load (W)
Let t = Hours of runtime needed
Let DoD = Usable depth of discharge (fraction, e.g., 0.8 for 80%)
Let η = System efficiency (fraction, e.g., 0.9 for 90%)

Ideal energy requirement (no losses) = P × t (Wh)

Required battery capacity (Wh) = (P × t) ÷ (DoD × η)
Required battery capacity (kWh) = Required battery capacity (Wh) ÷ 1000

Example:
P = 2,000 W, t = 8 h, DoD = 0.8, η = 0.9
Required Wh ≈ (2,000 × 8) ÷ (0.8 × 0.9) ≈ 22,222 Wh
Required kWh ≈ 22.22 kWh

When to use it

Sizing a home battery bank to cover essential loads (refrigeration, lighting, networking, small electronics) for a target number of hours during grid outages.
Planning a battery system for an off‑grid cabin, RV, or van conversion by estimating daily energy needs and choosing appropriate storage capacity.
Estimating storage requirements for solar‑plus‑storage projects where you want to shift solar production into evening peak hours.
Evaluating whether a particular commercial or residential battery product line (for example, a 10–15 kWh pack) is likely to be sufficient for your use case.
Providing a quick energy‑based sizing check before engaging a designer, installer, or engineer for detailed system design and protection.

Tips & cautions

Start by overestimating critical loads and then refine with real data (for example, from a smart panel, plug‑in meters, or utility interval data) to dial in a realistic average load.
Add extra margin—often 20–30% or more—for battery aging, cold‑weather performance, and future load growth so the system remains adequate over time.
If you are using lead‑acid batteries, choose a conservative depth of discharge (for example, 40–60%) to preserve battery life; do not size them assuming you will use 100% of nameplate capacity.
Be mindful that large inductive loads (such as well pumps or compressors) may have high startup currents; capacity sizing here is energy‑based and does not replace inverter or conductor sizing for surge.
Consider how frequently you expect to cycle the system; heavier cycling may justify more conservative DoD and higher capacity to reduce stress on the batteries.
Uses a single average load value and does not model time‑varying load profiles, peaks, or diversity of loads throughout the day.
Does not account for battery aging, temperature derating, or manufacturer‑specific charge/discharge limits beyond the DoD and efficiency inputs you provide.
Focuses on required energy capacity only; it does not size inverters, charge controllers, conductors, overcurrent protection, or other balance‑of‑system components.
Assumes that the battery can be fully charged before the backup period and that no additional charging source (like solar or a generator) is contributing during the runtime window.
Not a code‑compliant design tool or a substitute for system engineering; always validate with product specifications and local electrical codes.

Worked examples

Essential home loads for an 8‑hour outage

Identify essential loads (fridge, some lights, Wi‑Fi, a few outlets) and estimate an average draw of 2,000 W.
Choose 8 hours of runtime for overnight coverage during an outage.
Assume usable DoD of 80% (0.8) for a lithium‑ion battery and system efficiency of 90% (0.9).
Required Wh ≈ (2,000 × 8) ÷ (0.8 × 0.9) ≈ 22,222 Wh.
Required kWh ≈ 22.22 kWh, so you might target a battery bank around 22–28 kWh to include margin.

Off‑grid cabin daily storage for 5 hours of evening use

Average evening load is estimated at 1,200 W (lights, small appliances, electronics).
You want 5 hours of runtime in the evening from batteries, with solar recharging during the day.
Assume usable DoD of 70% (0.7) and system efficiency of 85% (0.85).
Required Wh ≈ (1,200 × 5) ÷ (0.7 × 0.85) ≈ 10,084 Wh.
Required kWh ≈ 10.08 kWh, suggesting a roughly 10–13 kWh battery bank with appropriate margin.

RV or van build with 800 W average draw for 6 hours

An RV or van conversion is expected to draw an average of 800 W (fridge, fans, lights, and electronics) for about 6 hours in the evening and night.
You use a lithium battery bank with 85% usable DoD and a system efficiency of 90%.
Required Wh ≈ (800 × 6) ÷ (0.85 × 0.9) ≈ 6,275 Wh.
Required kWh ≈ 6.28 kWh, indicating you might want a battery bank in the 6–8 kWh range depending on how conservative you want to be.

Deep dive

Estimate battery storage size by entering average load, desired runtime hours, usable depth of discharge, and system efficiency to get required Wh and kWh.

Use this battery sizing calculator to plan home backup, off‑grid, RV, or solar‑plus‑storage systems before talking to an installer.

FAQs

Does this include inverter sizing?: No. It focuses on storage capacity. Inverter sizing depends on peak watts/surge.
What about aging/temperature?: Aging and low temperatures both reduce usable capacity. This calculator does not explicitly model those effects, so it is wise to add extra capacity—often 20–30% or more—above the bare minimum.
Is DoD auto-selected?: No. You choose the usable depth of discharge based on your battery chemistry, manufacturer recommendations, and desired cycle life. Many lithium systems support 70–90% usable, while lead‑acid is often sized more conservatively.
Is this a purchase recommendation?: No. This is a sizing aid, not a product recommendation. Always compare against detailed manufacturer specifications and work with a qualified installer or engineer for final design.
Can I include solar recharge?: Not directly. This calculator assumes no additional charging during the runtime window. For solar‑plus‑storage, you would typically model solar production separately and may reduce the required storage if you expect charging while loads are running.

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This battery storage sizing calculator provides a simplified energy‑based estimate only. It does not perform a full electrical design, account for all safety factors, or ensure compliance with electrical codes and manufacturer requirements. Actual systems must consider surge currents, charge/discharge limits, wiring, protection, environmental conditions, and integration with other equipment. Always review your design and equipment choices with product documentation and qualified professionals before purchasing or installing any battery storage system.