12V 100Ah LiFePO4 Deep Cycle Battery
Lightweight, 4000+ deep cycles, and built-in BMS for reliable emergency backup.
View on AmazonKnow your backup time.
Before the next outage.
Before the next outage.
Calculate how long your batteries will power your home loads, or size an inverter and battery bank for power outages — completely local & private.
- Local Math Only
- LiFePO4 & Lead-Acid Support
- Inverter Efficiency & Surge Included
Private · Local · No account
- LiFePO4 & lead-acid support
- Surge & duty cycle aware
- Private, in your browser
Battery backup & inverter sizing
Local math onlyStep 1 of 4: What do you need?
Your results appear here, directly below the calculator, once you press the final button.
Planning estimate only. Actual runtime varies with battery age, temperature and load swings. Everything runs locally in your browser.
Gear that matches this calculator
Turn the readout into a real setup
The three pieces that decide whether a backup system actually works: a chemistry-matched battery, an inverter that covers your surge, and a monitor so you always know the remaining runtime.
2000W / 3000W Pure Sine Wave Inverter
Clean AC power safe for sensitive electronics and capable of handling high motor starting surges.
View on AmazonBluetooth Smart Battery Monitor / Shunt
Real-time tracking of state of charge, voltage, and remaining runtime directly from your phone.
View on AmazonDisclosure: as an Amazon Associate we earn from qualifying purchases. These are planning suggestions, not endorsements of specific performance — check the device ratings against the numbers from this calculator before buying.
A clear answer, before the long explanation.
Set your load
Add appliances with quantity, running watts, starting surge and duty cycle — or enter a total wattage directly. Totals update live.
Choose battery & voltage
Pick LiFePO4, AGM/gel or flooded lead-acid (or set custom DoD and efficiency sliders), the system voltage, capacity in Ah or Wh, and inverter efficiency.
Read the result
See estimated backup time with an energy-flow bar, or the recommended battery bank and inverter continuous/surge ratings — with the formula shown.
The math behind this tool
Every number in the readout comes from the formulas below. Open any block to see how duty cycle, surge, depth of discharge and inverter efficiency change the answer.
Continuous running power
W_total = Σ (W_i × Q_i × DutyCycle_i ÷ 100)Each appliance contributes its running watts times quantity, scaled by the share of time it is actually on. A refrigerator rated 150 W with a 35% duty cycle adds 52.5 W to your continuous load, not 150 W.
Peak starting surge
W_surge = W_total + max(SurgeWatts_i − RunningWatts_i)Motors in compressors and pumps draw several times their running wattage for a moment at startup. Only the single largest spike is added on top of the running load, because motors rarely all start at the same instant. Inverters must be able to cover this peak or they will shut down.
Effective usable battery energy
Wh_usable = V_system × Ah_capacity × DoD% × BatteryEff%Depth of discharge (DoD) is the share of capacity you may safely use: LiFePO4 typically 80–90%, lead-acid about 50%. Battery efficiency accounts for energy lost inside the chemistry during discharge (about 95% for LiFePO4, 80–85% for lead-acid).
Estimated backup runtime
Hours = (Wh_usable × InverterEff%) ÷ W_totalInverters lose roughly 10–15% of energy converting DC battery power to AC; the default here is 85% efficiency (15% loss). Note this model assumes a constant effective load — the Peukert effect (capacity sag under heavy discharge in lead-acid) and inverter idle draw make real-world lead-acid runtime slightly shorter than calculated.
Required capacity for a backup goal
Ah_required = (W_total × Hours_target) ÷ (V × DoD% × BatteryEff% × InverterEff%)Mode B inverts the runtime formula, then adds a 20% safety buffer so the bank is not routinely discharged to 100% DoD or run in cold conditions.
Inverter sizing
W_inverter = W_total × 1.25 (surge rating ≥ peak surge)A 25% continuous overhead covers load drift and motor re-starts; the surge rating must at least match the peak surge demand, otherwise the inverter trips the first time the fridge or sump pump kicks in.
Definitions
- Depth of discharge (DoD): the percentage of rated capacity you may use. Deeper discharges shorten battery life, especially for lead-acid.
- Peukert effect: lead-acid batteries deliver less usable capacity at high discharge rates than at low ones. This calculator uses a flat efficiency factor, so treat lead-acid estimates at heavy loads as optimistic.
- Inverter idle draw: real inverters consume a small constant power just being on (roughly 10–40 W). Include it in your load list if you want maximum precision.
Worked example
A 100 Ah, 12 V LiFePO4 bank (85% DoD, 95% battery efficiency) with an 85%-efficient inverter powering 65 W of continuous load stores 1,200 Wh, uses 969 Wh after DoD and chemistry, delivers 824 Wh to the load, and runs for about 12 h 41 min. The same bank as flooded lead-acid (50% DoD, 80% efficiency) would run about 7 h 5 min.
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