Battery Discharge Calculator

Understanding Battery Discharge and Why Runtime Matters

Battery discharge is simply the process of a battery releasing stored energy to power a connected load. Every time a light, fan, router, or inverter draws current from a battery, the battery is discharging. How long that discharge can continue before the battery is depleted is what most people mean when they ask about "battery runtime" or "battery backup time." Understanding battery discharge time is important for anyone who relies on stored energy — homeowners with an inverter for power cuts, solar users running appliances at night, UPS operators protecting sensitive electronics, and technicians sizing battery banks for a specific job.

A common misconception is that battery capacity alone tells you how long a battery will last. In reality, the amp hour rating printed on a battery label only describes how much charge it can theoretically hold, not how much usable energy is actually available to your equipment. Two identical 100 Ah batteries at different voltages store very different amounts of energy, and the same 100 Ah battery will last a very different amount of time depending on whether it is powering a 50 W LED light or a 1,500 W water pump.

The load connected to a battery has a direct and sometimes non-linear effect on discharge time. A small, steady load draws current gently and allows the battery to deliver closer to its full rated capacity. A large load draws current quickly, generates more internal heat and voltage drop, and can reduce the amount of usable energy the battery is able to deliver — this is part of what is known as the Peukert effect, discussed later on this page.

If an inverter sits between the battery and the load, its efficiency also reduces the energy actually available to run appliances. No inverter is 100% efficient. Typical inverters convert DC battery power to AC power at somewhere between 85% and 95% efficiency, meaning a portion of the battery's stored energy is lost as heat during conversion rather than reaching the connected devices.

Finally, usable battery capacity is different from rated capacity because most battery chemistries should not be discharged all the way to zero. Depth of discharge limits — whether set for safety, longevity, or manufacturer warranty requirements — mean that only a percentage of the rated capacity should be treated as usable in day to day operation. This calculator accounts for all of these factors together so the estimate reflects real system behaviour rather than a simplistic capacity-only number.

What Is a Battery Discharge Calculator?

A battery discharge calculator is a tool that estimates how long a battery can supply a specific electrical load before it reaches a defined discharge limit. It combines several pieces of information — battery voltage, amp hour capacity, load power, efficiency losses, and depth of discharge — into a single runtime estimate expressed in hours and minutes.

At its core, the calculator relies on a simple relationship: energy divided by power equals time. Battery voltage multiplied by amp hour capacity gives total energy in watt hours. That energy figure is then reduced according to depth of discharge and efficiency losses to arrive at usable energy. Dividing usable energy by the load's power draw in watts produces an estimated runtime.

The relationship between the key variables can be summarised as follows:

• Voltage sets the electrical "pressure" of the battery system and, combined with capacity, determines total stored energy.
• Amp hours (Ah) describe how much charge the battery can theoretically deliver over time at a specified discharge rate.
• Watt hours (Wh) describe actual stored energy — voltage multiplied by amp hours — and are a more complete measure than Ah alone.
• Load power in watts determines how quickly that stored energy is consumed.
• Efficiency accounts for losses inside the battery and inverter that reduce how much of the stored energy reaches the load.
• Depth of discharge sets a practical ceiling on how much of the rated capacity should be used before recharging.

As a practical example, a 12V 100Ah battery has 1,200 Wh of stored energy. If it is limited to an 80% depth of discharge, only 960 Wh is considered usable. If that usable energy passes through a 90% efficient inverter to power a 100W load, the calculator works through each step to arrive at an estimated runtime of roughly 8.6 hours — a noticeably different figure from simply dividing 1,200 Wh by 100 W.

How to Use the Battery Discharge Calculator

Using the calculator correctly starts with gathering accurate information about your battery and your load. Here is what each field means and where to find the right value.

Battery Voltage

This is the nominal voltage of your battery or battery bank, commonly 12V, 24V, or 48V for home inverter and solar systems. It is printed on the battery case or in the battery's datasheet. If you have connected multiple batteries in series, use the combined series voltage rather than the voltage of a single unit.

Battery Capacity

Battery capacity is expressed in amp hours (Ah) and is also printed on the battery label — for example, "12V 100Ah" or "24V 200Ah." This number represents the charge capacity at a specific discharge rate defined by the manufacturer, usually a 10-hour or 20-hour rate for lead acid batteries. Do not confuse this Ah figure with watt hours; they are different units and require voltage to convert between them.

Load Power

Enter the total power, in watts, of everything the battery needs to run at the same time. This should be based on the running wattage of your appliances, not just their current rating, and should reflect the total connected load rather than a single device if multiple things run together.

Battery Efficiency

Battery efficiency accounts for internal losses during discharge, including internal resistance and heat generation. A well maintained lead acid battery is often modelled at around 85-90% efficiency, while a good lithium battery may be modelled closer to 95-98%. If you are unsure, the default of 90% is a reasonable general estimate.

Inverter Efficiency

If your system uses an inverter to convert battery DC power into AC power for household appliances, enter the inverter's rated efficiency, usually found in its datasheet or user manual, typically between 85% and 95%. If you are calculating a purely DC load with no inverter, you can set this value to 100% to remove inverter losses from the estimate.

Depth of Discharge

This is the maximum percentage of the battery's rated capacity you intend to use before recharging. Conventional flooded lead acid batteries are often limited to 50% depth of discharge for good cycle life, AGM and gel batteries are often used up to around 50-80%, and many lithium iron phosphate batteries can be used up to 80-100% depending on the manufacturer's specification. Always check your specific battery's datasheet rather than relying on general assumptions.

Battery Discharge Calculation Formula

The calculations behind this battery discharge time calculator follow standard electrical relationships. Here is each formula the calculator uses, explained step by step.

Step 1: Battery Energy

This gives the total theoretical energy stored in the battery, in watt hours, before any losses are applied.

Step 2: Usable Battery Energy

This reduces total energy down to the portion you are actually willing to use, based on your chosen depth of discharge limit.

Step 3: Effective Usable Energy After Losses

This accounts for battery efficiency and inverter efficiency together, giving a more realistic estimate of the energy that will actually reach your load.

Step 4: Runtime

Dividing usable energy by the load's power draw gives the estimated number of hours the battery can supply that load.

Step 5: Load Current

This approximates the DC current drawn from the battery to supply the given load, which is useful for checking wiring and fuse sizing.

Below are four worked examples using these formulas.

Example 1: 12V 100Ah battery powering a 100W load

Battery energy = 12 × 100 = 1,200 Wh. With an 80% depth of discharge, usable energy = 960 Wh. With 90% battery efficiency and 90% inverter efficiency, effective usable energy = 1,200 × 0.9 × 0.8 × 0.9 = 777.6 Wh. Runtime = 777.6 ÷ 100 = approximately 7.78 hours. This is a theoretical estimate; real world runtime will vary.

Example 2: 12V 200Ah battery powering a 300W load

Battery energy = 12 × 200 = 2,400 Wh. Effective usable energy at 90% battery efficiency, 80% depth of discharge, and 90% inverter efficiency = 2,400 × 0.9 × 0.8 × 0.9 = 1,555.2 Wh. Runtime = 1,555.2 ÷ 300 = approximately 5.18 hours.

Example 3: 24V 200Ah battery powering a 500W load

Battery energy = 24 × 200 = 4,800 Wh. Effective usable energy = 4,800 × 0.9 × 0.8 × 0.9 = 3,110.4 Wh. Runtime = 3,110.4 ÷ 500 = approximately 6.22 hours.

Example 4: 48V 100Ah battery powering a 1,000W load

Battery energy = 48 × 100 = 4,800 Wh. Effective usable energy = 4,800 × 0.9 × 0.8 × 0.9 = 3,110.4 Wh. Runtime = 3,110.4 ÷ 1,000 = approximately 3.11 hours.

Battery Runtime Examples

The table below compares several common battery and load combinations at 90% battery efficiency, 90% inverter efficiency, and 80% depth of discharge.

These figures are theoretical estimates. Actual results depend on battery condition, temperature, wiring, and real load behaviour.

Amp Hours vs Watt Hours

One of the most common sources of confusion in battery runtime calculations is treating amp hours and watt hours as if they were the same thing. They are not. Amp hours measure electrical charge capacity, while watt hours measure actual stored energy, and the two are only connected through voltage.

This is why a 100Ah battery does not always contain the same amount of energy as another 100Ah battery. The amount of energy depends entirely on the battery's voltage:

• 12V × 100Ah = 1,200 Wh
• 24V × 100Ah = 2,400 Wh
• 48V × 100Ah = 4,800 Wh

A 48V 100Ah battery bank stores four times as much energy as a 12V 100Ah battery, even though the Ah rating on the label is identical. When comparing battery systems, always convert to watt hours before drawing conclusions about how long each system can run a given load, because comparing raw Ah numbers across different voltages is misleading.

Battery Discharge Rate

Discharge rate describes how quickly a battery is releasing its stored energy relative to its rated capacity. It is commonly expressed using the "C rate" system, where 1C means discharging the entire rated capacity in one hour, 0.5C means discharging it over two hours, and 0.1C means discharging it over ten hours.

A battery connected to a small load discharges at a low C rate and tends to deliver a runtime close to its rated capacity. A battery connected to a large load discharges at a higher C rate, and many battery chemistries — lead acid in particular — deliver less total usable capacity as discharge rate increases. This means a battery rated for 100Ah at a slow 20-hour discharge rate may deliver noticeably less than 100Ah worth of runtime if discharged in one or two hours at a much higher current.

Actual behaviour at each C rate depends heavily on battery chemistry and the manufacturer's published discharge curves, so these figures should be treated as general guidance rather than exact values for every battery model.

Peukert Effect

The Peukert effect describes how the usable capacity of a lead acid battery decreases as discharge current increases. It was first described by German scientist Wilhelm Peukert and remains one of the most important, and most overlooked, factors in accurate battery runtime calculation.

In simple terms, a lead acid battery rated at 100Ah using a slow, 20-hour discharge test will not deliver the full 100Ah if discharged quickly at a much higher current. Internal resistance and chemical reaction limits inside the battery reduce the effective capacity available at higher discharge rates. This is why a battery may perform well under light, steady loads but underperform its rated capacity under heavy, fast discharge.

The Peukert effect is generally more significant for:

• Flooded lead acid batteries
• AGM (absorbed glass mat) batteries
• Gel batteries

Lithium batteries, including lithium iron phosphate, are generally less affected by discharge rate than lead acid chemistries and tend to deliver a capacity closer to their rated value across a wider range of discharge currents. This does not mean lithium batteries are entirely free from discharge related losses — internal resistance, cell balancing, temperature, and battery management system limits can still affect real world performance, so manufacturer specifications should always be consulted for demanding applications.

Depth of Discharge

Depth of discharge (DoD) describes the percentage of a battery's rated capacity that has been used relative to a full charge. A battery discharged to 20% DoD has used 20% of its capacity and retains 80% charge. A battery discharged to 80% DoD has used 80% of its capacity and retains only 20% charge.

• 20% DoD — light usage, generally very favourable for long battery cycle life across most chemistries.
• 50% DoD — a common conservative target for flooded and AGM lead acid batteries in daily cycling applications.
• 80% DoD — commonly used with lithium batteries designed for regular deep cycling, and sometimes with AGM batteries in backup applications.
• 100% DoD — full discharge, which is not recommended for most battery chemistries on a regular basis and may void certain warranties.

Regularly discharging lead acid batteries very deeply, especially below 50% remaining charge on a frequent basis, tends to shorten their usable cycle life significantly compared to shallower discharge patterns. This is a well documented characteristic of lead acid chemistry, related to sulfation and physical stress on the battery plates during deep discharge cycles.

The acceptable depth of discharge for any battery ultimately depends on its chemistry and the manufacturer's specification sheet. Lithium batteries, particularly lithium iron phosphate models designed for cyclic use, often permit a deeper usable discharge than traditional lead acid batteries while still maintaining a long service life. Even so, it remains good practice to follow the manufacturer's stated DoD limits and any battery management system guidance rather than assuming maximum depth of discharge is always safe.

Inverter Efficiency

Inverter efficiency measures how much of the DC power drawn from the battery is successfully converted into usable AC power, with the remainder lost as heat during the conversion process. A typical modern inverter operates somewhere between 85% and 95% efficient under normal load conditions, though efficiency can vary depending on how close the load is to the inverter's rated capacity.

Consider a 500W AC load running through a 90% efficient inverter. To deliver 500W of usable AC power, the inverter must draw more than 500W from the battery:

That extra 55.6W is lost as heat inside the inverter rather than reaching the connected appliances. Over several hours of operation, this loss becomes a meaningful reduction in available runtime, which is why inverter efficiency is one of the required inputs in this calculator.

It is also worth noting that inverters draw a small amount of standby power even when no load is connected, and some inverters become less efficient at very low load levels. For the most accurate estimate, use the efficiency value from your inverter's datasheet at a load level similar to your actual usage.

Battery Chemistry and Discharge Characteristics

Different battery chemistries behave differently under discharge, and understanding these differences helps explain why the same calculator inputs can produce different real world results depending on the battery type installed.

Flooded Lead Acid

Flooded lead acid batteries are the traditional, lower cost option for backup power. They are noticeably affected by the Peukert effect, generally recommend a conservative depth of discharge around 50%, and require regular maintenance such as checking electrolyte levels.

AGM (Absorbed Glass Mat)

AGM batteries are sealed, maintenance free lead acid batteries with somewhat better high current performance than flooded batteries, though they still exhibit a meaningful Peukert effect and generally benefit from moderate depth of discharge limits.

Gel

Gel batteries are also sealed lead acid batteries, generally tolerant of moderate discharge rates, but can be more sensitive to overcharging and typically require a charge profile matched to gel chemistry specifically.

Lithium Iron Phosphate (LiFePO4)

LiFePO4 batteries offer a flatter discharge curve, less capacity loss at higher discharge rates, longer cycle life, and typically support a deeper usable depth of discharge than lead acid options, subject to the battery management system's limits and the manufacturer's specification.

Other Lithium Battery Systems

Other lithium chemistries used in backup and portable power applications share many of the general advantages of lithium iron phosphate — lighter weight and less discharge rate sensitivity than lead acid — while specific performance, safety characteristics, and depth of discharge limits vary by manufacturer and cell design. Always consult the specific battery's documentation rather than assuming all lithium batteries behave identically.

How Load Power Affects Battery Runtime

The relationship between load and runtime is straightforward in principle — a larger load consumes stored energy faster and therefore reduces runtime — but the practical impact varies significantly by appliance type. Below are some common examples and what to consider for each.

• LED lights — typically low power draw, often just a few watts each, making them ideal for extending backup runtime.
• Wi-Fi router — modest continuous power draw, but often left running 24/7, which adds up over a full day of backup power.
• Laptop — moderate power draw during use, generally efficient, though charging from empty draws more than steady state use.
• TV — power draw varies significantly by screen size and technology, from around 30W for small LED models to over 150W for large screens.
• Refrigerator — cycles on and off with a compressor, so average power draw is lower than peak draw, but startup current can briefly spike several times higher than running wattage.
• Fan — generally low and steady power draw, well suited to extended battery backup.
• Desktop computer — higher continuous draw than a laptop, especially with a monitor and peripherals included in the total load.
• Small water pump — motor-driven loads like pumps often have a running wattage that is much lower than their startup surge wattage.

Appliances with motors and compressors — refrigerators, pumps, air conditioners, and some fans — typically require a brief surge of power at startup that can be two to six times higher than their steady running wattage. This calculator estimates runtime based on the running wattage you enter; startup surges should be accounted for separately when sizing an inverter's peak power rating, since a system that handles running load fine can still trip or fail if it cannot supply momentary startup surge.

Battery Backup for Home Inverters

Homeowners commonly use battery discharge calculators to estimate how long an inverter battery will keep essential appliances running during a power outage. The key to an accurate estimate is calculating the actual combined load rather than guessing based on appliance names alone.

For example, a household running LED lights (20W total), a ceiling fan (60W), a TV (80W), a Wi-Fi router (10W), and a laptop (50W) simultaneously has a combined load of 220W. Entering this figure into the calculator, rather than entering each appliance separately, gives an accurate combined runtime estimate for the whole set of devices running together.

Solar Battery Discharge

This calculator is also useful for solar power system planning, particularly for estimating how long a solar battery bank can supply nighttime loads when there is no solar charging input. Key considerations for solar battery discharge include:

• Solar batteries — typically deep cycle lead acid or lithium batteries designed for daily charge and discharge cycles.
• Nighttime loads — the total load that must be supplied entirely from stored battery energy between sunset and sunrise, or during periods of low solar production.
• Battery bank capacity — the combined voltage and amp hour rating of all batteries in the system, calculated using the series and parallel principles described later on this page.
• Inverter efficiency — the same conversion losses apply whether the source is grid backup or solar storage.
• Depth of discharge — solar battery banks are cycled daily, so respecting manufacturer DoD limits is particularly important for long term battery health.
• Daily energy consumption — total household energy use per day, in watt hours, helps determine whether the battery bank and solar array are appropriately sized together.

It is important to understand that this battery discharge calculator estimates runtime for a given battery bank and load — it does not replace a complete solar system design, which also needs to account for solar panel output, charge controller sizing, seasonal sunlight variation, and daily charging capacity relative to daily consumption.

Series and Parallel Battery Banks

When batteries are combined into a bank, how they are wired together determines whether voltage or capacity increases. Understanding this distinction is essential before entering values into any battery discharge calculator for a multi-battery system.

Series Connection

Connecting batteries in series increases total voltage while capacity in amp hours stays the same as a single battery. Two 12V 100Ah batteries wired in series produce a 24V 100Ah bank.

Parallel Connection

Connecting batteries in parallel increases total amp hour capacity while voltage stays the same as a single battery. Two 12V 100Ah batteries wired in parallel produce a 12V 200Ah bank.

In both cases, the total stored energy of the combined bank is approximately the same when identical batteries are used — 2,400 Wh in this example — because energy is the product of voltage and capacity regardless of how the batteries are arranged. What changes is the voltage and current characteristics of the resulting bank, which affects wiring requirements, fusing, and compatibility with a given inverter's input voltage.

Common Battery Discharge Calculation Mistakes

Theoretical vs Real World Battery Runtime

Every battery discharge calculator, including this one, produces a theoretical estimate based on the values entered. Real world battery runtime can differ from this estimate for several reasons, and understanding them helps set realistic expectations rather than treating the calculator output as an exact prediction.

• Battery age — capacity gradually declines with charge cycles and calendar age, even with good maintenance.
• Temperature — both very cold and very hot temperatures can reduce usable battery capacity compared to the manufacturer's rated conditions, typically specified at 25°C.
• Internal resistance — increases as batteries age or degrade, generating more heat and reducing usable output during high current discharge.
• Discharge current — higher discharge rates reduce usable capacity for many chemistries, particularly lead acid, as discussed under the Peukert effect.
• Battery chemistry — different chemistries respond differently to depth of discharge, discharge rate, and temperature.
• Battery condition — a battery with a partially failed cell or existing damage will underperform its rated specifications.
• Inverter efficiency — actual efficiency can vary from datasheet figures depending on load level and inverter condition.
• Wiring resistance — long or undersized cables introduce voltage drop and additional losses not captured in a simple calculator.
• Connections — corroded or loose terminals increase resistance and reduce delivered power.
• Load variation — real loads often vary over time rather than staying perfectly constant, unlike the fixed wattage used in a calculation.
• Battery manufacturer specifications — actual rated capacity, recommended DoD, and discharge curves vary between manufacturers and models.

As a practical example, two identical calculator inputs might produce an estimated 6-hour runtime, but a new, well maintained battery at moderate temperature might deliver close to that figure, while a five year old battery in a cold garage might deliver noticeably less. This is not a flaw in the calculation — it reflects the genuine difference between theoretical energy math and real electrochemical behaviour.

How to Get a More Accurate Battery Runtime Estimate

1. Measure actual load — use a plug-in power meter or clamp meter to measure real running wattage rather than relying on nameplate figures alone.
2. Check battery specifications — review the manufacturer's datasheet for rated capacity, recommended discharge rate, and temperature performance.
3. Use realistic efficiency values — enter efficiency figures based on your actual inverter and battery type rather than optimistic assumptions.
4. Use manufacturer recommended DoD — apply the depth of discharge limit specified for your exact battery chemistry and model.
5. Consider discharge rate — for lead acid batteries under heavy load, reduce the expected capacity to account for the Peukert effect.
6. Account for inverter consumption — include the inverter's own standby draw if it will remain powered on during the backup period.
7. Check battery condition — test older batteries periodically, since capacity naturally declines with age and use.
8. Consider temperature — adjust expectations for batteries operating outside typical room temperature conditions.
9. Test the system under real load — where practical, run an actual timed discharge test to compare against the calculator's theoretical estimate.

Battery Discharge Rate Formula Explained

The discharge current drawn from a battery can be calculated directly from load power and battery voltage:

Once current is known, it can be expressed as a C rate relative to the battery's rated capacity:

Some reference points for a 100Ah battery:

• 100Ah battery with a 10A load = 10 ÷ 100 = 0.1C, approximately a 10-hour discharge
• 100Ah battery with a 50A load = 50 ÷ 100 = 0.5C, approximately a 2-hour discharge
• 100Ah battery with a 100A load = 100 ÷ 100 = 1C, approximately a 1-hour discharge

These figures describe the discharge rate relative to rated capacity, not necessarily the exact runtime that will be achieved, since actual battery behaviour at each C rate depends on chemistry and the manufacturer's specific discharge curve data.

Battery Runtime Formula Examples

Worked Example: 12V, 150Ah battery, 250W load, 85% battery efficiency, 90% inverter efficiency, 70% DoD

Step 1: Battery energy = 12 × 150 = 1,800 Wh.

Step 2: Usable energy at 70% DoD = 1,800 × 0.70 = 1,260 Wh.

Step 3: After 85% battery efficiency and 90% inverter efficiency, effective usable energy = 1,800 × 0.85 × 0.70 × 0.90 = 963.9 Wh.

Step 4: Runtime = 963.9 ÷ 250 = approximately 3.86 hours.

Step 5: This is a theoretical result. A well maintained battery under moderate temperature should come reasonably close to this figure, while an older or heavily used battery may deliver less.

Worked Example: 24V, 100Ah battery, 200W load, 90% battery efficiency, 95% inverter efficiency, 90% DoD

Step 1: Battery energy = 24 × 100 = 2,400 Wh.

Step 2: Usable energy at 90% DoD = 2,400 × 0.90 = 2,160 Wh.

Step 3: Effective usable energy = 2,400 × 0.90 × 0.90 × 0.95 = 1,846.8 Wh.

Step 4: Runtime = 1,846.8 ÷ 200 = approximately 9.23 hours.

Step 5: A 90% DoD figure suggests a lithium battery designed for deeper cycling; confirm this is within the specific battery's manufacturer specification before relying on it regularly.

Lead Acid vs Lithium: Discharge Considerations

Battery Discharge Calculator: Understanding Your Battery Runtime

Estimating battery runtime accurately requires more than looking at a single capacity number on a label. Voltage, amp hour capacity, connected load, battery efficiency, inverter efficiency, and depth of discharge all combine to determine how long a battery can actually supply power. This battery discharge calculator brings all of these variables together, converting amp hours into watt hours, applying realistic efficiency and depth of discharge limits, and dividing the result by your load to produce a transparent, step by step runtime estimate.

It is worth repeating that every figure produced here is a theoretical estimate. Actual battery runtime depends on battery chemistry, discharge rate, load behaviour, temperature, battery age, and overall system condition — factors that a calculator alone cannot fully measure. Using realistic, manufacturer-based inputs for efficiency and depth of discharge, and validating important calculations with real world testing where possible, will always produce the most reliable results.

Whether you are sizing a home inverter battery bank, planning a solar backup system, checking a UPS runtime, or simply trying to understand how amp hours translate into hours of usable power, working through the calculation with accurate, chemistry-appropriate values will give you a far more dependable estimate than relying on capacity numbers alone.