Battery Life Calculator: Runtime in Hours and Days

Battery life is the number of hours a battery can continuously power a connected load before reaching its usable discharge limit. A 100 Ah battery rated at 12V does not always deliver power for the same length of time. Runtime depends on the current draw of the load, the depth of discharge allowed by the battery chemistry, the ambient temperature, and losses in the power conversion chain.

The base formula for battery runtime is straightforward: divide the battery's usable capacity by the load's average current draw. But a quick division of amp-hours by amps ignores real-world factors that reduce actual runtime by 10 to 40 percent, depending on conditions. A 200 Ah AGM battery feeding a 10 A load looks like 20 hours on paper. In a cold garage at 5 °C with a 50% maximum depth of discharge, usable runtime drops to roughly 8.5 hours. Knowing the formula is half the job. Knowing which derating factors to apply is the other half.

Battery Life Formula: Current-Based Runtime Calculation

This formula works when you know the battery's amp-hour rating and the load's current draw in amps or milliamps. For loads rated in watts rather than amps, use the power-based formula below.

Battery Runtime Formula: Power-Based (Watt-Hour Method)

The power-based method is more accurate when an inverter sits between the battery and the load, because inverter losses are accounted for through the efficiency factor. For a 12V 100 Ah LiFePO4 battery (1,200 Wh) powering a 100 W television through a 90%-efficient inverter at 80% DoD, actual runtime is about 8 hours and 38 minutes. Skipping the efficiency factor would overestimate by nearly an hour.

When the battery's capacity is rated in mAh and the load is rated in watts, convert first: Wh = (mAh × V) / 1000. A 5,000 mAh phone battery at 3.7V stores 18.5 Wh.

How to Use the Battery Life Calculator: Step by Step

1. Enter your battery's amp-hour rating and nominal voltage. If your capacity is in mAh, divide by 1,000 to get Ah; if it is in Wh, divide by the voltage to get Ah.
2. Enter your battery capacity. Check your battery label, datasheet, or product listing. Smartphone batteries are typically 3,000 to 5,500 mAh. Automotive batteries range from 35 to 120 Ah. Solar storage banks run from 100 Ah to several hundred Ah at 12V, 24V, or 48V.
3. Enter your load's power consumption in watts, taken from the device's label or specifications. If your load varies, estimate the average draw across a typical operating cycle. For a load rated in amps, multiply amps by the voltage to get watts.
4. Set depth of discharge. The default is 80% for lithium chemistries and 50% for lead-acid. LiFePO4 batteries can safely run to 90-95% DoD. Flooded lead-acid batteries degrade quickly below 50% DoD. Adjust based on your battery chemistry.
5. Set system efficiency. If your battery powers the load directly (DC to DC at the same voltage), leave this at 100%. If an inverter is involved, set it to 85-95% depending on the inverter quality. Cheap modified sine wave inverters run closer to 85%. Quality pure sine wave units achieve 92-95%.
6. Read the results. The calculator returns runtime in hours and days, usable energy in Wh, and average current draw. Use the hours figure for short-term loads and the days figure for standby or backup applications.

Battery Runtime Worked Examples: 12V, 24V, and 48V Systems

Example 1: 12V Automotive / RV Application (USA Context)

A campervan runs a 12V 60 W refrigerator from a 100 Ah AGM deep-cycle battery. The owner wants to know how many hours the fridge will run before the battery reaches its safe 50% discharge limit.

Current draw: 60 W / 12V = 5 A

Runtime = (100 Ah × 0.50) / 5 A = 10 hours

With no inverter in the circuit (the fridge is a 12V DC unit), system efficiency is effectively 100%. The fridge will run for roughly 10 hours. At 25 °C, this estimate is reliable. At 5 °C in a cold desert night, expect 15-20% less capacity from the AGM battery, cutting runtime to about 8 to 8.5 hours.

Example 2: 48V Solar Battery Bank (Off-Grid Installation)

An off-grid cabin in rural Australia runs on a 48V 200 Ah LiFePO4 battery bank (9,600 Wh). The evening load averages 800 W (lights, router, television, laptop charger) through a 93%-efficient pure sine wave inverter. DoD is set at 90% for LiFePO4.

Runtime = (9,600 Wh × 0.90 × 0.93) / 800 W = 10.04 hours

The cabin gets just over 10 hours of evening power. This aligns with IEC 62619 operational parameters for stationary lithium battery installations. The owner can run from 6 PM to 4 AM without solar input, which covers most off-grid evening cycles in temperate climates.

Example 3: Smartphone Battery (Consumer Electronics, Global)

A smartphone with a 4,500 mAh lithium-ion battery at 3.8V nominal has a typical screen-on power consumption of 3.5 W.

Energy stored: (4,500 × 3.8) / 1000 = 17.1 Wh

DoD for lithium-ion in consumer electronics is managed by the battery management system (BMS), which typically allows 90-95% of rated capacity.

Runtime (screen on) = (17.1 Wh × 0.95) / 3.5 W = 4.64 hours

That's about 4 hours and 38 minutes of continuous screen-on time. Mixed usage with standby periods extends total battery life to 24-48 hours because standby current drops to 10-30 mA. This matches the experience most users have with modern smartphones meeting IEEE 1725 battery safety requirements.

Example 4: UPS Backup System (Commercial, 230V Context)

A small office UPS in London uses a 12V 9 Ah sealed lead-acid battery (108 Wh) to back up a 60 W network switch and a 40 W router through a built-in inverter at 88% efficiency. The IT administrator needs to know how long the network stays up during a power cut.

Total load: 60 + 40 = 100 W

Runtime = (108 Wh × 0.50 × 0.88) / 100 W = 0.475 hours = 28.5 minutes

At 50% DoD for the lead-acid battery, the UPS provides just under 30 minutes of backup. This is tight for anything beyond a brief outage. BS 5266-1 sets minimum duration requirements for emergency lighting and life-safety systems; the administrator should consider whether 28 minutes meets the building's requirements or if a larger battery bank is warranted.

Battery Chemistry Effects on Runtime: DoD, Efficiency, and Self-Discharge

Battery chemistry determines how much of the rated capacity you can actually use. A 100 Ah lead-acid battery and a 100 Ah LiFePO4 battery store the same rated charge, but the usable energy for runtime calculation differs by nearly 50%. The reason sits in depth of discharge limits and discharge efficiency.

Lead-acid batteries lose capacity rapidly when discharged below 50%. Repeated deep discharges below this threshold shorten the battery's cycle life from 500+ cycles to fewer than 200. LiFePO4 cells, by contrast, tolerate 80-95% DoD across 2,000 to 5,000 cycles with minimal degradation. This single difference means a 100 Ah LiFePO4 battery delivers 80-95 Ah of usable capacity per cycle, while a 100 Ah lead-acid battery delivers only 50 Ah.

Table: Battery chemistry runtime parameters. Use these values when adjusting the battery life calculator for your specific battery type.

Self-discharge matters for standby applications. A NiMH battery pack left idle for a month loses 15-30% of its charge without any connected load. For UPS or emergency backup systems, lithium chemistries with 1-3% monthly self-discharge are strongly preferred. IEC 62133 covers safety requirements for portable lithium and nickel battery systems, while IEC 62619 addresses stationary battery installations in solar and backup power contexts.

How C-Rate and Discharge Rate Affect Battery Runtime

C-rate describes how fast a battery is charged or discharged relative to its rated capacity. A 100 Ah battery discharged at 1C delivers 100 A for approximately one hour. At 0.5C, the same battery delivers 50 A for about two hours. At 0.1C, it delivers 10 A for roughly ten hours.

This matters because higher discharge rates reduce effective capacity, especially in lead-acid batteries. A lead-acid battery rated at 100 Ah at the C20 rate (5 A for 20 hours) may deliver only 56-65 Ah when discharged at C2 (50 A for approximately one hour). This non-linear relationship between discharge rate and available capacity is described by Peukert's Law.

Peukert's Law: Why High-Current Loads Reduce Battery Runtime

Peukert's Law quantifies how discharge rate affects lead-acid battery capacity. The Peukert exponent (n) typically ranges from 1.1 to 1.3 for lead-acid batteries. An ideal battery with no rate-dependent losses would have an exponent of 1.0. Lithium batteries have exponents close to 1.02-1.08, which means their capacity is far less affected by discharge rate than lead-acid.

For a lead-acid battery rated 100 Ah at C20 with a Peukert exponent of 1.25, discharging at 20 A (C5 rate) yields roughly 75-80 Ah of usable capacity instead of the rated 100 Ah. At 50 A (C2 rate), usable capacity drops below 60 Ah. The battery life calculator uses the simple capacity / current formula, which assumes capacity is constant across discharge rates. For lead-acid batteries at high discharge rates, apply a 15-25% derating factor to the calculator result. For lithium batteries at rates below 1C, no derating is needed.

Temperature Effects on Battery Life Estimation

Cold temperatures reduce battery capacity. The chemical reactions inside a battery slow down as temperature drops, which limits the rate at which ions can move between electrodes. At 0 °C (32 °F), expect 10-20% capacity loss for lithium batteries and 20-30% loss for lead-acid. At -20 °C (-4 °F), lead-acid capacity can drop by 40-50%.

Heat has the opposite short-term effect: capacity increases slightly above 25 °C. But sustained operation above 40 °C accelerates degradation and shortens cycle life. IEEE 1725 specifies safe operating temperature ranges for portable lithium-ion batteries used in consumer electronics. For stationary systems, IEC 62619 requires thermal management provisions to keep cells within their rated temperature range.

Table: Temperature derating factors for battery runtime estimation. Multiply the calculator result by the percentage shown to get temperature-adjusted runtime.

Battery Runtime Standards and Regional Electrical Practices

Battery runtime calculations exist within a framework of international safety and performance standards. The standards that apply depend on the battery chemistry, the application, and the region of installation.

For lithium batteries shipped internationally, UN 38.3 testing is required regardless of the origin or destination country. This standard verifies that lithium cells can withstand altitude, temperature cycling, vibration, shock, and external short circuit without posing a fire or explosion risk.

NEC Article 480 governs battery installations in the USA and requires overcurrent protection, disconnecting means, and proper ventilation for battery rooms. For residential solar storage systems, NEC 706 provides specific requirements for energy storage systems, including runtime-related considerations like maximum discharge depth and thermal management.

Battery Runtime Applications: Solar, UPS, EV, IoT, and Consumer Electronics

Battery life calculations apply across six major application categories. The approach and the critical variables change with each context.

Solar Energy Storage Runtime

Off-grid and hybrid solar systems size their battery banks for overnight autonomy. A typical residential solar storage system in Arizona might use a 48V 200 Ah LiFePO4 bank (9,600 Wh) with an evening load of 500-800 W. At 90% DoD and 93% inverter efficiency, runtime covers 10-16 hours. System designers calculate runtime to confirm the battery bank carries the household from sunset to sunrise with margin for cloudy mornings. AS 5139 in Australia and NEC 706 in the USA provide safety requirements for these installations.

UPS and Emergency Backup Runtime

Uninterruptible power supplies protect critical equipment during grid outages. Runtime ranges from 5-15 minutes for desktop UPS units (enough for a safe shutdown) to 4-8 hours for server room battery banks. A 3 kVA online UPS with a 192V 9 Ah battery string (1,728 Wh) at 50% DoD and 90% inverter efficiency provides roughly 15 minutes at full load (less at the actual 15-minute discharge rate; see the C-rate section). Underpowered UPS runtime is a common failure point in commercial IT installations, and IEEE 1184 provides guidance on battery sizing for UPS applications.

Electric Vehicle Range Estimation

EV batteries are rated in kWh. A 75 kWh battery pack driving a vehicle that consumes 15 kWh per 100 km at highway speed has a theoretical range of 500 km. At 80% usable DoD (the BMS reserves capacity at both ends), practical range drops to about 400 km. Cold weather, aggressive driving, and cabin heating can reduce that by another 20-30%. The calculator can estimate EV runtime by entering the battery's kWh rating and the vehicle's average power consumption in kW.

IoT and Embedded Device Battery Life

Internet of Things devices spend most of their time in sleep mode, waking periodically to read sensors and transmit data. A LoRa sensor node might draw 80 mA for 2 seconds while transmitting, then sleep at 10 µA for 298 seconds. The average current is (80 × 2 + 0.01 × 298) / 300 = 0.543 mA. A CR2477 coin cell (1,000 mAh) at this average current lasts approximately 1,842 hours, or about 77 days. This type of duty-cycle calculation is what the Duty cycle mode of the battery life calculator handles. ESP32 and Raspberry Pi battery life calculations follow the same pattern with different current values.

Consumer Electronics Battery Life

Smartphones, tablets, laptops, and wireless earbuds all use lithium-ion batteries rated in mAh or Wh. A laptop battery rated at 56 Wh powering a 15 W workload lasts about 3.6 hours (accounting for BMS overhead and 95% DoD). Battery life varies with screen brightness, processor load, Wi-Fi activity, and ambient temperature. Manufacturers test battery life under controlled conditions per IEEE 1625 (laptop) and IEEE 1725 (mobile phone) standards, but real-world results vary.

Automotive and Marine Battery Runtime

12V automotive batteries power accessories when the engine is off. A 65 Ah starting battery running a 10 A load (lights, radio, phone charger) will last approximately 3.25 hours at 50% DoD. This is not the same calculation as cranking capacity. CCA (cold cranking amps) measures the battery's ability to start the engine at -18 °C. Runtime and CCA are different specifications that serve different purposes, and both differ from lifespan, the number of years a battery lasts before it needs replacing (see how long car batteries last). For marine trolling motor applications, a 100 Ah deep-cycle battery at 25 A draw provides about 2 hours of continuous operation at 50% DoD. For cranking-capacity conversions, use the Ah to CCA converter; for capacity itself, the battery capacity calculator.

Common Battery Runtime Reference: mAh to Hours at Various Loads

These are approximate runtimes at 25 °C with 100% DoD (theoretical maximum). For real-world estimates, multiply by your DoD factor (0.5 for lead-acid, 0.8-0.9 for lithium).

Table: Theoretical battery runtime at various current draws. Real-world runtime is lower due to DoD limits, efficiency losses, and temperature effects.

Common Battery Life Calculation Mistakes and How to Avoid Them

Incorrect runtime estimates cause real problems. An undersized UPS shuts down a server mid-backup. A solar battery bank runs out of power at 3 AM. Both are avoidable.

• Ignoring depth of discharge. The most common mistake. Rated capacity is not usable capacity. Because a lead-acid battery should not discharge past its 50% ceiling, plugging the full nameplate figure into your calculation overestimates runtime by a factor of two.
• Forgetting inverter efficiency. Any system with a battery-to-AC inverter loses 5-15% of energy in the conversion. A 1,000 Wh battery through a 90%-efficient inverter delivers only 900 Wh to the AC load. Missing this factor means your runtime estimate is 10-15% too high.
• Confusing Ah with Wh. A 100 Ah battery at 12V stores 1,200 Wh. A 100 Ah battery at 48V stores 4,800 Wh. The Ah number alone does not tell you how much energy is stored. When comparing batteries at different voltages, convert to Wh first. For Ah-to-Wh conversions, use the Ah to Wh converter. Runtime answers how long a charge lasts; to size the charger for the refill, the battery charge time calculator works the same battery the other way.
• Using rated capacity instead of actual capacity for old batteries. A three-year-old lead-acid battery may retain only 70-80% of its original rated capacity. Age, cycle count, and storage conditions degrade real capacity. For critical applications, measure actual capacity with a load tester rather than trusting the label.
• Assuming constant current draw. Most real loads vary. A refrigerator compressor cycles on and off. A laptop's power consumption changes with processor load. Use the average current draw over a full operating cycle, not the peak draw or the idle draw.
• Ignoring self-discharge for standby applications. A NiMH battery pack sitting idle for three months may lose half or more of its charge to self-discharge. For emergency backup applications, lithium batteries with 1-3% monthly self-discharge are the right choice.

Battery Discharge Safety Warnings

• Never discharge lead-acid batteries below 50% DoD unless they are specifically rated for deep-cycle use. Starter batteries can be permanently damaged by a single deep discharge.
• Lithium batteries must not be discharged below the BMS cutoff voltage. Overdischarge can cause cell reversal, internal copper dendrite formation, and permanent capacity loss. In severe cases, subsequent charging of an over-discharged lithium cell can cause thermal runaway and fire.
• High-current discharge generates heat. Monitor battery temperature during sustained high-rate discharge. If the battery case is hot to the touch, reduce the load or allow cooling time.
• Battery rooms and enclosures require ventilation per NEC Article 480.10(A) (USA) and BS EN 50272-2 (UK/EU) to prevent hydrogen accumulation from lead-acid batteries and to manage thermal conditions for lithium installations.

Disclaimer: This calculator provides estimates based on the values you enter. Actual battery runtime varies with temperature, battery age, discharge rate, and other environmental factors. Always verify calculations against manufacturer specifications and consult a licensed electrician or engineer for battery installation and system sizing.