Most backup time calculators give you a number that assumes perfect conditions — then your system shuts down two hours early. This calculator models what actually happens: inverter efficiency losses, lead-acid DoD limits, battery aging, and real-world day/night load patterns. Enter your actual values and get a result you can plan around.
How to use this calculator
Step 01
Enter your battery bank
Input capacity in Ah, system voltage, battery type, and number of units. Battery type selection auto-sets safe DoD and efficiency defaults.
Step 02
Set real-world loss factors
Adjust depth of discharge, inverter efficiency, battery age, and wiring losses. These are the factors basic calculators ignore.
Step 03
Define your load profile
Enter daytime and night-time loads separately with hours for each. The calculator derives a weighted 24-hour average for accuracy.
Battery Backup Time Calculator
DoD · Inverter Efficiency · Battery Aging · Load Profile
Eneronix
1Battery Bank
2Real-World Loss Factors
80%
92%
100%
2%
3Load Profile
Daytime hours
16h
Night hours (auto)
8h
Weighted 24-hour average load derived from both values.
4Bank Configuration
—
Real-World Backup Time
Usable Energy
—
Avg Load (Weighted)
—
Nameplate Capacity
—
After DoD
—
After Aging
—
After All Losses
—
Loss Cascade — Step by Step
⚠ Results are engineering estimates. Real backup time varies with temperature, load surge events, BMS protection thresholds, and actual battery state of health. Always design with a minimum 20% safety margin.
Understanding your results
The backup time this calculator returns is not the same as what basic online calculators show. Here’s what each output means and why the differences matter.
Real-World Backup Time
Accounts for all four loss mechanisms: DoD limit, inverter conversion losses, battery aging, and wiring resistance. This is what to expect in operation — not the theoretical maximum your battery label implies.
Usable Energy (kWh)
Your battery’s nameplate kWh figure after all losses are applied. This is the actual AC energy available at your outlets. The gap between nameplate and usable is where most systems underperform.
Weighted Average Load
Since you typically run higher loads during the day and lower loads at night, the calculator blends these into a representative average. Using peak load alone would significantly underestimate runtime.
Loss Cascade Breakdown
Each row shows exactly how much energy is consumed by each loss source. The biggest losses are almost always DoD limit and inverter efficiency — which is why battery type and inverter quality determine your backup window.
Why do basic calculators give different results?
Most backup calculators divide battery Wh by load watts. That gives you theoretical maximum runtime at 100% DoD with a perfect inverter and a brand-new battery. None of those conditions exist in the real world. A 200Ah 24V battery (4.8 kWh nameplate) running a 400W load doesn’t give you 12 hours — it gives you 7–9 hours depending on your actual system.
The most common battery backup sizing mistakes
Using nameplate Wh instead of usable Wh
A 200Ah 24V battery is marketed as 4.8 kWh. But if you’re running AGM at 50% DoD through a 90% efficient inverter on a two-year-old bank at 85% capacity, your actual usable energy is closer to 1.8 kWh. That gap is the difference between planning for 12 hours and getting 4.
Ignoring inverter efficiency at partial load
Most inverters reach peak efficiency at 50–75% of rated load. At 10–15% load — common at night — efficiency can drop to 80–85%. If your night load is very light relative to inverter rating, your effective efficiency is worse than the spec sheet suggests.
Not accounting for battery aging in system design
A system sized correctly for day one may fall short of your backup requirement by year three. Size to your end-of-life requirement, not your day-one requirement. LiFePO4 loses ~2–3% capacity per year. Lead-acid loses 5–15% depending on cycling depth.
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