Why estimating inverter runtime accurately matters

Battery datasheets list ampere-hours (Ah) and nominal voltage (V). Those numbers tell you theoretical stored energy but not how long your AC devices will run. To produce a realistic inverter runtime you must account for the portion of battery you’ll actually use (Depth of Discharge, DoD), conversion losses inside the inverter (inverter efficiency), and practical balance-of-system losses (wiring, fuses, BMS). Overestimating runtime is a common and risky mistake: it leads to undersized systems, unexpected outages, shortened battery life, and sometimes warranty disputes.

The basics — energy, usable energy and AC output

Start with three simple, repeatable steps:

1. Calculate stored energy (Wh) from battery Ah and nominal voltage: Wh = Ah × V.
2. Apply usable DoD to get the energy you plan to pull from the battery: Usable Wh = Wh × DoD.
3. Apply inverter & system efficiency to translate DC usable Wh into usable AC Wh: AC usable Wh = Usable Wh × Inverter Efficiency × (1 − other losses).

Finally, divide AC usable Wh by your AC load (in watts) to get runtime in hours.

Key formulas (write them down)

Use these exact formula lines when estimating:

Stored energy (Wh) = Battery Ah × Nominal Voltage (V)

Usable DC energy (Wh) = Stored energy (Wh) × Depth of Discharge (DoD as decimal)

AC usable energy (Wh) = Usable DC energy (Wh) × Inverter efficiency (decimal) × Other system efficiency (decimal)

Runtime (hours) = AC usable energy (Wh) ÷ AC load (W)
      

For convenience you can fold “other system efficiency” into inverter efficiency as a single combined efficiency factor (e.g., 0.88) if you prefer a simpler computation: this factor should reflect wiring losses, BMS/passive balancer losses, and any DC-DC conversions.

Choosing realistic DoD and efficiency values

Picking conservative but realistic values matters much more than extra decimal precision:

• DoD: For LiFePO4 (LFP) packs 80% DoD is a common practical assumption for daily cycling; some systems use 90% but that shortens life. For NMC, many integrators use 70–80% conservative DoD. When systems are grid-tied or critical, consider 50–80% DoD to protect cycle life.
• Inverter efficiency: Typical modern sine inverters run 85–96% depending on load. Inverter efficiency tends to be lower at very light loads. For mid-sized loads assume 88–92% unless you have the inverter datasheet.
• Other losses: Wiring and terminal losses, fuse heating, BMS passive balancing — allow 2–7% additional loss depending on wiring lengths and pack design. Short runs and heavy copper can reduce this; long thin runs increase it.

Example conservative combined efficiency: 0.90 (inverter) × 0.97 (other) ≈ 0.873. But don’t invent extremes — use vendor datasheets when available.

Worked example — step-by-step

We’ll walk through a concrete example so you can follow digits precisely and reproduce it with your numbers.

Given pack

• Nominal voltage = 12.8 V
• Capacity = 200 Ah
• Planned DoD = 80% (0.80)
• Inverter efficiency = 90% (0.90)
• Other system losses = 0% (folded into inverter efficiency for simplicity)
• AC continuous load = 500 W

Step 1 — Stored energy (Wh)

Wh = Ah × V = 200 Ah × 12.8 V = 2560 Wh

Step 2 — Usable DC energy after DoD

Usable Wh = 2560 Wh × 0.80 = 2048 Wh

Step 3 — AC usable energy after inverter efficiency

AC usable Wh = 2048 Wh × 0.90 = 1843.2 Wh

Step 4 — Runtime

Runtime (hours) = 1843.2 Wh ÷ 500 W = 3.6864 hours

Which is roughly 3 hours, 41 minutes, 11 seconds (3 h 41 m 11 s). In many planning contexts you would round down and report ~3.6 hours or ~3 h 40 min to avoid over-promising.

Quick sanity check: 2560 Wh stored → 80% usable = 2048 Wh DC → 90% inverter → ~1843 Wh AC. Divide by 0.5 kW → ~3.7 hours. Good — numbers align with intuition.

Handling multiple and dynamic loads

If you have multiple devices sum their continuous AC power draws to get total AC load. For devices with duty cycles (compressors, pumps, refrigerators), compute average power over time or use the inverter runtime tool to simulate duty cycles and motor start surges (motors draw higher current at startup).

Important: inverter surge capability does not extend runtime — it only covers short high-power starts. Repeated surges increase losses and shorten battery life.

Peaks, BMS cutoffs and safety margins

Runtime is only useful if the system can deliver required peak currents without tripping protections. When sizing runtime consider:

• BMS continuous and peak current ratings — will the pack support sustained load and startup peaks?
• Fuse sizing — fuses protect wiring but must be rated so normal loads don’t blow them repeatedly.
• Voltage sag under load — some inverters will fault if pack voltage drops below a threshold; pack internal resistance and temperature affect sag.

Always include a safety margin (10–20%) on runtime estimates to account for unexpected inefficiencies, colder batteries, or ageing that reduces capacity over time.

Adjusting for ageing and temperature

Battery capacity decreases with age and with adverse temperature. Typical planning adjustments:

• Reduce usable capacity by 5–20% depending on age and expected degradation rate (e.g., after several years of cycling).
• At low temperatures the usable capacity and allowable charge current decline — account for colder months by reducing DoD or allowing heaters to maintain safe charging behavior.

Example policy: when guaranteeing runtime for customers, assume 80% of nominal capacity at 3–5 years depending on chemistry and cycling intensity.

Practical checklist when you estimate runtime

1. Record battery nominal voltage and Ah from the pack label or datasheet.
2. Decide safe DoD based on chemistry, warranty and lifecycle goals (e.g., 80% for LFP daily-cycle systems).
3. Use an honest combined efficiency: inverter efficiency + other losses (wiring, BMS). When in doubt, use 85–90% combined.
4. Sum continuous AC loads and convert to watts if they’re listed in amps (W = V × A for AC only when known; usually better to use watt ratings).
5. Check peak/surge currents vs inverter and BMS capabilities — ensure the pack can deliver peaks without tripping protections.
6. Adjust for temperature and ageing if you’re planning long-term guarantees.
7. Use a runtime tool for complex duty cycles or multiple battery strings — try the Inverter Runtime Estimator or the Battery Runtime Calculator.

Common mistakes (and how to avoid them)

• Using Ah alone — Ah by itself is not energy; always multiply by voltage to get Wh.
• Ignoring inverter efficiency — assuming 100% conversion leads to overestimates.
• Forgetting DoD limits — pulling full capacity repeatedly shortens life and voids many warranties.
• Neglecting ageing — plan for reduced capacity over the warranty period or expected life.

Tools & next steps

Use these site tools to validate and automate calculations:

• Inverter Runtime Estimator — simulate runtime for single and multiple loads, include inverter efficiency and surge behavior.
• Battery Runtime Calculator — quick Wh→runtime conversions and batch checks.

If you’d like, paste your battery Ah and nominal voltage, target DoD, inverter efficiency, and AC load here and I’ll calculate a realistic runtime for you (with the step-by-step numbers shown).