Battery planning

Battery Runtime Calculator

Estimate how long your 12V, 24V, or 48V battery bank will power connected appliances in hours and minutes, factoring in DOD reserves, battery health, and inverter losses.

Calculate estimated runtime

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⚡ 1-Click Autofill: Top 5 Battery Setups

Estimated runtime

7 h 12 min

≈ 7.2 hours

A planning estimate based on your battery energy and average device load.

Technical References & Model Basis:

🛡️IEEE 485 🛡️IEC 62619 🛡️UL 1973

Usable Energy Window80% Usable

Reserve 20%

0.80 kWh

Usable (80%)

Reserve (20%)

⏱️ Outage Survival Clock & Timeline7h 12m Backup

Usable Power (80%)

20% Reserve

100% Full

50% usable-energy point

20% Configured cutoff

📈 Estimated Runtime vs. Continuous Load12V • 83.33333333333333Ah • 80% DoD

Battery capacity: 1,000 Wh
Usable battery energy: 800 Wh
Device load: 100 W average
Battery-side load: 111 W

Energy Conversion Flow

Nominal Capacity1,000 Wh

Rated pack capacity

Usable After Reserve800 Wh

80% usable DOD window

Effective Delivered Energy720 Wh

After 90% efficiency

Runtime at different loads

75 W: 9 h 36 min
100 W Your load: 7 h 12 min
125 W: 5 h 46 min

Assumptions used

Battery type: LiFePO4 / LFP
Starting charge: 100%
Reserve: 20%
State of Health (SOH): 100%
AC inverter efficiency: 90%

If your actual load is 10% higher, runtime is about 9% shorter.

Want a more accurate result?

Use your battery's actual Wh/Ah rating, your appliance's measured or nameplate watts, inverter efficiency if known, current charge, and battery health.

📊 Battery Backup Runtime Quick Lookup Matrix (12.8V LiFePO4 / 90% DoD)

Estimated runtimes across standard battery capacities under continuous AC appliance loads (90% inverter efficiency).

Continuous AC Load	50 Ah (640 Wh)	100 Ah (1.28 kWh)	200 Ah (2.56 kWh)	300 Ah (3.84 kWh)
50 W (Wi-Fi, LED lights, CPAP)	10.4 hrs	20.7 hrs	41.5 hrs	62.2 hrs
100 W (Laptop + Workstation)Popular	5.2 hrs	10.4 hrs	20.7 hrs	31.1 hrs
250 W (Desktop PC + Monitor)	2.1 hrs	4.1 hrs	8.3 hrs	12.4 hrs
500 W (Sump pump / Refrigerator)	1.0 hrs	2.1 hrs	4.1 hrs	6.2 hrs
1,000 W (Microwave / Power tools)	0.5 hrs	1.0 hrs	2.1 hrs	3.1 hrs
1,500 W (Space heater / Kettle)	0.3 hrs	0.7 hrs	1.4 hrs	2.1 hrs

Assumes 12.8V nominal LiFePO4 chemistry with 90% usable DoD (10% reserve) and 90% AC inverter efficiency without inverter tare. Real-world runtime varies with ambient temperature, inverter standby losses, and cell health.Source: IEEE Std 485 / IEC 62619 Planning Reference

Engineering WalkthroughGoverned by IEEE Std 485 / IEC 62619

How to Calculate Battery Backup Runtime (Step-by-Step)

How to estimate battery backup runtime step-by-step using usable stored watt-hours, depth-of-discharge limits, and inverter conversion efficiency.

Calculate Effective Battery-Side Power Demand

Divide the AC appliance load wattage by inverter conversion efficiency to find the total power drawn from the battery bank.

P_{battery}=\frac{P_{load}}{η_{inverter}}

💡 Standard Example: 100W AC load with 90% inverter efficiency draws: 100 / 0.90 = 111.1 Watts from the battery

Determine Usable Stored Energy

Multiply rated battery watt-hour capacity by the usable Depth of Discharge window (e.g., 80%–90% for LiFePO4; 50% for Lead-Acid) and State of Health (SOH).

E_{usable}=E_{nominal} × DoD_{usable} × SOH

💡 Standard Example: 12V 100Ah LiFePO4 battery (1,200 Wh nominal) at 80% usable DoD and 100% SOH provides: 1,200 × 0.80 × 1.0 = 960 Usable Wh

Solve for Estimated Backup Runtime

Divide net usable battery watt-hours by the battery-side power demand to find the total operating duration.

t=\frac{E_{usable}}{P_{battery}} = \frac{E_{nominal} × DoD × SOH × η_{inverter}}{P_{load}}

💡 Standard Example: 960 usable Wh / 111.1W battery load = ~8.64 hours of continuous backup

How to Calculate Battery Backup Runtime

Enter Battery Capacity (Ah or Wh): Choose nominal system voltage (12V, 24V, 48V) and Amp-hour capacity.
Select or Enter Appliance Load (Watts): Enter continuous average running watts or use the appliance load builder.
Set Depth of Discharge (DOD) Reserve: Lithium LiFePO4 batteries allow 80% to 90% usable capacity; Lead-Acid/AGM allows 50%.
Review Operating Duration: View estimated hours and minutes of backup power available for planning.

⚡

Battery Discharge & Backup Flow Topology

Multi-source charging, chemical storage management, and pure sine wave inverted backup delivery.

🔌InputGrid / Solar InputPrimary Energy Input

⚡ProtectionSmart BMS ChargerMulti-Stage CC/CV Charging

🔋ReservesBattery Bank12V / 24V / 48V Storage

🔄InverterPure Sine InverterDC to 120V/240V AC (88–92%)

💡ProtectedCritical SubpanelRefrigeration, Medical, Wi-Fi

Engineering Principle: Continuous AC backup runtime is determined by usable Depth of Discharge (DOD) and inverter conversion efficiency.

Common Battery Runtime Scenarios (100Ah vs 200Ah LiFePO4)

Estimated continuous operating hours for popular appliances powered by a 12V lithium battery (80% usable capacity, 90% inverter efficiency):

Estimated runtime on 12V 100Ah (864Wh delivered AC) vs 12V 200Ah (1,728Wh delivered AC)
Device / Load	Average Power	100Ah 12V Runtime	200Ah 12V Runtime
Wi-Fi Router + Modem	15 W	~57.6 hours (2.4 days)	~115.2 hours (4.8 days)
CPAP Machine (illustrative load, no heater)	35 W	~24.7 hours (~3 nights)	~49.4 hours (~6 nights)
Starlink Satellite Terminal	50 W	~17.3 hours	~34.6 hours
12V Portable Camping Fridge	30 W avg (cycling)	~28.8 hours (1.2 days)	~57.6 hours (2.4 days)
Desktop PC + Monitor	200 W	~4.3 hours	~8.6 hours
Full-Size Refrigerator (cycling at 35% duty)	52.5 W avg (150W peak)	~16.5 hours	~32.9 hours

Battery Runtime Calculation Formula

Estimates battery backup duration for planning purposes by determining net usable stored energy after Depth-of-Discharge (DOD) limits, battery State of Health (SOH), and inverter conversion losses.

Runtime (hours)=CapacityWh × UsableSOC × BatteryHealth × InverterEfficiencyLoadWatts

Variable Definitions

Capacity_WhNominal Battery Energy(Wh): Rated battery watt-hours (or Volts × Amp-Hours).
Usable_SOCUsable State of Charge Window(fraction): Fraction of capacity available above minimum reserve (e.g., 80% for LiFePO4, 50% for Lead-Acid).
Battery_HealthState of Health (SOH)(fraction): Available capacity factor relative to original factory rating (default 100%).
Inverter_EfficiencyConversion Efficiency (η)(fraction): Inverter efficiency for AC loads (85%–93%) or DC-DC step efficiency.
Load_WattsContinuous Power Demand(W): Average real-time appliance consumption (Running Watts × Duty Cycle).

Calculation Notes

For cycling loads like refrigerators, average continuous demand = running wattage × duty cycle (e.g., 150 W × 35% = 52.5 W).
Real-world runtime varies with ambient temperature, cell aging/SOH, discharge rate, BMS voltage cutoff thresholds, standby inverter tare losses, and dynamic load cycling.

Technical References & Model Basis

The mathematical models in this calculator reflect industry planning guidelines and test standards for stationary and portable energy storage systems:

IEEE Std 485 / IEC 62619

Industry engineering recommendations for battery sizing, defining depth-of-discharge reserve thresholds, and state-of-health capacity retention in lithium and lead-acid battery banks.

UL 1973 & NFPA 70 / NEC 706

Safety and installation standards governing energy storage systems (ESS), inverter integration boundaries, and electrical protection requirements.

Frequently Asked Questions (FAQ)

How long will a 100Ah 12V battery run a refrigerator?

A household refrigerator with a 150W compressor cycling at a 35% duty cycle averages approximately 52.5W. On a 12V 100Ah LiFePO4 battery (1,200Wh nominal, 80% usable DoD = 960Wh, 90% inverter efficiency yielding 864Wh delivered AC), it will run for approximately 16.5 hours (864 Wh ÷ 52.5 W). On a 12V 200Ah battery, it will run for about 32.9 hours. If the refrigerator runs continuously at 150W without cycling, runtime is approximately 5.8 hours on 100Ah and 11.5 hours on 200Ah.

How long will a 100Ah battery run a CPAP machine?

An illustrative CPAP machine consuming ~35W (without a heated humidifier or heated tube) will run for approximately 24.7 hours on a 12V 100Ah LiFePO4 battery (864Wh delivered AC), or around 3 full 8-hour nights of sleep. Actual runtime varies depending on machine model, therapy pressure settings, humidifier heating (which can increase draw to 70W–100W+), mask seal quality, and whether powered via an AC inverter or a high-efficiency native DC-DC adapter.

Why does a 12V 100Ah battery not deliver the full 1,200 watt-hours to AC appliances?

Nominal stored energy is 12V × 100Ah = 1,200Wh (or 1,280Wh for a 12.8V 4S LiFePO4 pack). Delivered AC energy is lower due to: (1) Safe Depth of Discharge reserve limits (typically 20% reserve / 80% usable for LiFePO4, 50% for Lead-Acid) to protect battery lifespan, leaving 960Wh usable DC; and (2) Inverter conversion losses (typically 85%–92% efficiency), delivering approximately 864Wh of AC power to connected appliances.

How do I calculate battery runtime for AC appliances?

Divide usable battery watt-hours by the battery-side load. For AC loads: Usable Wh = Nominal Wh × Usable Fraction × SOH. Battery-Side Load = Appliance Watts ÷ Inverter Efficiency. Runtime (Hours) = Usable Wh ÷ Battery-Side Load.