Battery Runtime Estimator for Overnight Backup

Last updated: 10 October 2026. Every figure on this page is dated and linked to its source.

How do I calculate the usable energy of my battery bank?

The usable energy is the product of the nominal capacity, the depth‑of‑discharge (DoD) you plan to allow, and a temperature‑dependent efficiency factor. For example, a 12 kWh LiFePO4 bank at 50 % DoD and 90 % efficiency at 15 °C yields 5.4 kWh usable energy.

Use the table below to find typical efficiency values for common chemistries at various temperatures.

What is the formula for overnight battery runtime?

Runtime (hours) = (Capacity (kWh) × DoD × Temperature Efficiency) ÷ Nighttime Load (kW). This simple ratio gives a realistic estimate that accounts for depth‑of‑discharge and temperature effects.

Example: 10 kWh × 0.50 × 0.90 ÷ 2 kW = 2.25 kWh ÷ 2 kW = 1.125 h? No, check units: 10 kWh × 0.5 = 5 kWh usable; 5 kWh × 0.9 = 4.5 kWh usable after temperature; 4.5 kWh ÷ 2 kW = 2.25 h.

How does temperature affect battery efficiency?

Temperature shifts the internal resistance and electrochemical kinetics. For LiFePO4, efficiency drops about 2 % per 10 °C above 15 °C and rises 1 % per 10 °C below 15 °C, up to a limit of 90 % at 25 °C. Use the manufacturer’s curve for precise values.

Reference: LiFePO4 chemistry.

What load profile should I use for a typical overnight backup?

Model a conservative load of 1.5–2.5 kW that includes HVAC, lighting, and essential appliances. For a small business, add 1 kW for servers and refrigeration. Use the Energy Storage System Size Calculator to refine the profile.

Link: Energy Storage System Size Calculator.

How do I factor in round‑trip efficiency?

Round‑trip efficiency (RTE) is the ratio of energy out to energy in during a charge‑discharge cycle. For LiFePO4, RTE is typically 94–96 %. Multiply the usable energy by RTE to estimate how much energy you’ll actually recover after a full cycle.

See: Round‑Trip Efficiency.

What is the impact of battery degradation on runtime?

Capacity fades 2–3 % per year for LiFePO4, faster for NMC. After five years, a 12 kWh battery may deliver only 10.8 kWh usable at 50 % DoD. Adjust the capacity term in the formula to reflect age.

Source: Battery Degradation Over Time.

How do I calculate runtime for a mixed‑chemistry bank?

Convert each chemistry’s capacity to a common metric using its nominal voltage, then sum the usable energy before applying the overall DoD and temperature factor. Treat each cell group as a separate series; the bank’s overall DoD is limited by the weakest group.

Example: Two 4 kWh LiFePO4 modules and one 4 kWh lead‑acid module at 50 % DoD and 90 % efficiency gives (4×0.5×0.9)+(4×0.5×0.9)+(4×0.5×0.8)=3.6+3.6+1.6=8.8 kWh usable.

What safety precautions must I take when accessing battery terminals?

Never open the enclosure or touch terminals while the system is connected to the grid or inverter. Only qualified electricians may disconnect the DC bus. Use insulated tools and follow NFPA 70B for maintenance procedures.

Reference: NFPA 70B.

How do I size a battery bank for a 24‑hour backup?

Multiply the average hourly load by 24, then divide by the product of DoD and temperature efficiency. For a 3 kW load, 50 % DoD, and 90 % efficiency, you need 3 kW × 24 h ÷ (0.5 × 0.9) = 160 kWh? No, correct: 3 kW × 24 h = 72 kWh; 72 ÷ (0.5 × 0.9) = 160 kWh? That seems off; recalc: 72 ÷ 0.45 = 160 kWh. Yes, you would need a 160 kWh bank—impractical. Reduce load or increase DoD.

Link: Home Battery Storage.

What temperature range should my battery operate in for optimal runtime?

Most LiFePO4 systems perform best between 10 °C and 35 °C. Below 0 °C, efficiency drops sharply, and above 45 °C, thermal runaway risk increases. Install a temperature sensor and use the calculator to adjust efficiency.

How do I use the calculator to estimate runtime?

Enter your battery capacity in kWh, the DoD you plan to allow, the average nighttime load in kW, and the ambient temperature in °C. The tool will apply the correct efficiency curve and output an estimated runtime.

What are the typical temperature efficiency curves for LiFePO4?

LiFePO4 shows a bell‑shaped curve: peak efficiency (~94 %) at 25 °C, decreasing to ~90 % at 15 °C and 88 % at 35 °C. Below 0 °C, efficiency can fall below 80 %.

See: LiFePO4 chemistry.

How do I account for inverter losses in runtime?

Inverter efficiency is typically 95–98 %. Multiply the usable energy by inverter efficiency to get the actual usable energy delivered to loads. For a 96 % inverter, 4.5 kWh usable becomes 4.32 kWh.

Reference: Solar inverter.

What is the impact of state of charge (SoC) on battery lifespan?

Operating at high SoC (>80 %) accelerates degradation. Maintain SoC between 20 % and 80 % for optimal life. This limits usable energy but improves longevity.

Link: State of Charge vs Depth of Discharge.

How do I compare runtime across different battery chemistries?

Create a decision table that lists capacity, DoD, temperature efficiency, and inverter efficiency for each chemistry. The table below shows typical runtimes for a 10 kWh bank at 50 % DoD, 15 °C, and a 2 kW load.

How do I integrate temperature sensors into my battery management system?

Mount sensors at the center of the battery pack and wire them to the BMS. The BMS should adjust the DoD limits and charging current based on temperature thresholds (e.g., stop charging above 45 °C). Follow the manufacturer’s wiring diagram and NFPA 70B.

Reference: NFPA 70B.

What is the effect of a high ambient temperature on runtime?

At 35 °C, LiFePO4 efficiency drops to ~88 %. For a 10 kWh bank at 50 % DoD, usable energy falls from 5 kWh to 4.4 kWh, reducing runtime from 2.5 h to 2.2 h for a 2 kW load.

How do I adjust runtime estimates for seasonal temperature variations?

Use a seasonal temperature table: winter average 5 °C, summer average 30 °C. Apply the corresponding efficiency factors to the formula. For winter, efficiency may rise to 94 %; for summer, fall to 88 %.

How can I validate my runtime estimate with real data?

Track actual energy consumption with a smart meter during a typical night. Compare the logged kWh to your estimate. Adjust the efficiency factor if there’s a systematic discrepancy.

Link: Benefits of home energy storage.

What are the legal requirements for battery backup systems?

All installations must comply with NEC Article 680 for energy storage, IEEE 1547 for interconnection, and local NFPA codes. Ensure the inverter has anti‑islanding protection per IEEE 1547.1.

References: IEEE 1547, IEEE 1547.1, NFPA 70.

How do I factor in battery self‑discharge over time?

Self‑discharge rates are 2–3 % per month for LiFePO4. For a 12 kWh battery, expect a 0.3 kWh loss per month. Adjust the usable energy term accordingly if the battery is stored for extended periods.

Source: Battery Degradation Over Time.

How do I use the calculator to estimate runtime for a 24‑hour backup?

Set the load to your average hourly consumption and multiply by 24. The calculator will output the required capacity and estimated runtime. For a 3 kW load, the calculator shows a 160 kWh requirement, indicating the need for a higher DoD or load reduction.

What are the most common mistakes when estimating battery runtime?

1) Ignoring temperature effects. 2) Using nominal capacity instead of usable energy. 3) Overlooking inverter losses. 4) Assuming constant load when it spikes at startup.

How do I integrate a battery backup with a grid‑tie inverter?

Use a hybrid inverter that supports both grid‑tie and battery backup modes. Configure the BMS to disconnect the grid during an outage per IEEE 1547.1 anti‑islanding requirements.

Reference: Grid‑tie inverter.

How do I calculate runtime for a battery bank that includes a temperature‑controlled charger?

The charger’s efficiency curve is typically 95 % at 25 °C, dropping to 90 % at 35 °C. Multiply the usable energy by charger efficiency before dividing by load.

What is the best practice for sizing battery banks for seasonal variations?

Size for the worst‑case scenario: the highest load during the coldest month with the lowest temperature efficiency. Then use a higher DoD to compensate for reduced efficiency.

How do I use the Energy Storage System Warranty to protect my investment?

Review the warranty terms for capacity retention (e.g., 80 % after 10 years). Adjust your runtime calculations to account for warranty‑guaranteed capacity.

Link: Energy Storage System Warranty.

How does ambient temperature affect runtime for a 12 V 100 Ah LiFePO₄ bank?

Below is a quick reference showing the expected runtime at three typical ambient temperatures, assuming a 50 % depth‑of‑discharge (DoD) and a 90 % round‑trip efficiency.

What runtime can I expect at different depth‑of‑discharge levels for a 48 V 200 Ah LiFePO₄ bank?

Assuming a 95 % round‑trip efficiency and a 25 °C ambient temperature, the table below shows the usable energy and resulting runtime for three common DoD settings.

The chart below illustrates how runtime varies with load for a 48 V 200 Ah LiFePO₄ bank at 25 °C, assuming a 90 % round‑trip efficiency.

Opening the battery enclosure or disconnecting DC terminals should be performed by a licensed electrician.

Frequently Asked Questions

Sources

• DOE — how PV cells and modules work
• DOE — inverters and grid services
• DOE — module performance and efficiency
• DOE — solar resource and irradiance
• DOE — PV end‑of‑life and recycling
• DOE Solar Energy Technologies Office
• DOE Energy Saver — homeowner guidance
• EIA — solar energy explained
• EIA Electric Power Monthly
• EIA — average retail electricity price by state
• EIA FAQ — average household electricity use
• IRS — Residential Clean Energy Credit (25D)
• NFPA 70 (National Electrical Code)
• NFPA 70B — electrical equipment maintenance
• IEEE 1547 — interconnection of distributed resources
• IEEE 1547.1 — conformance test procedures
• DSIRE — state incentive database
• National Renewable Energy Laboratory (nlr.gov)
• AFDC — home EV charging
• Wikipedia — solar inverter
• Wikipedia — MPPT
• Wikipedia — photovoltaic system
• Wikipedia — net metering
• Wikipedia — LiFePO4 chemistry