Power Beyond Backup
How long will your inverter keep your lights, fans and Wi-Fi working? The answer depends on two things: how much usable energy your battery can supply and how much power your appliances consume.
Here's a simple way to estimate backup time, with examples for lithium and tubular batteries. The figures are planning estimates, not guaranteed runtimes.
The simple backup-time formula
Estimated backup time (hours) = battery energy (Wh) × usable battery fraction × inverter efficiency ÷ average load (W).
If your battery is rated in volts and ampere-hours, first calculate nominal energy:
Battery energy (Wh) = nominal voltage (V) × battery capacity (Ah).
Use each factor only once. If you already know the usable AC energy delivered to appliances, divide that figure by the load; do not subtract inverter losses again. For a bank with multiple batteries, use the bank's total nominal voltage and Ah, without counting the number of batteries twice.
Step 1: Add up the appliances you want to run
Check the power rating on each appliance. The following figures are examples—your appliances may draw more or less.
| Appliance | Quantity | Example rating | Running load |
|---|---|---|---|
| Ceiling fan | 2 | 60W each | 120W |
| LED light | 4 | 9W each | 36W |
| Wi-Fi router | 1 | 12W | 12W |
| Television | 1 | 80W | 80W |
| Total | 248W |
If appliances cycle on and off, their average consumption over the outage may differ from their nameplate rating. To check whether an inverter can run everything at once, also consider simultaneous running demand and starting surges.
Step 2: Find the battery's nominal energy
- 12.8V × 100Ah = 1280Wh: the nominal battery energy listed for the Li Spark ESS 1200.
- 25.6V × 100Ah = 2560Wh: the nominal battery energy listed for the Li Spark ESS 2500.
- 12V × 150Ah = 1800Wh: an example of nominal energy for a 12V-class 150Ah lead-acid battery.
These are nominal DC battery-energy figures. They are not the energy guaranteed at your appliance sockets.
Step 3: Allow for usable energy and inverter losses
Not all nominal battery energy is available to your appliances. Battery operating limits leave some energy unused, and the inverter consumes energy while converting DC to AC. Battery condition, temperature and load affect the result too.
For the examples below, we assume 80% of nominal energy is usable for the lithium examples and 85% inverter efficiency. For the tubular example, we assume 50% usable battery fraction and 85% efficiency. These are illustrative assumptions only—not measured specifications, recommended discharge settings or promises for any APHOSYS product. Use your equipment manufacturer's figures when available.
Example 1: Li Spark ESS 1200 at a 200W load
Nominal energy is 12.8V × 100Ah = 1280Wh. At an assumed 80% usable fraction and 85% efficiency:
1280 × 0.80 × 0.85 ÷ 200 = approximately 4.35 hours, or about 4 hours 21 minutes.
This example assumes a constant 200W load and a fully charged battery. The ESS 1200 has a listed 700W load rating; verify both continuous and starting demand before connecting appliances.
Example 2: Li Spark ESS 2500 at a 400W load
Nominal energy is 25.6V × 100Ah = 2560Wh. With the same illustrative assumptions:
2560 × 0.80 × 0.85 ÷ 400 = approximately 4.35 hours, or about 4 hours 21 minutes.
The larger battery and larger load both double in this example, so the calculated time is the same. The ESS 2500 has a listed 1400W load rating. Always check appliance starting demand and the system's limits.
Example 3: 12V 150Ah tubular battery at a 300W load
Nominal energy is 12V × 150Ah = 1800Wh. Assuming 50% usable energy and 85% inverter efficiency:
1800 × 0.50 × 0.85 ÷ 300 = approximately 2.55 hours, or about 2 hours 33 minutes.
This is an illustrative calculation, not a claim for a particular tubular battery. Lead-acid capacity can also change with discharge rate; manufacturer ratings and real-world conditions matter. See Victron Energy's explanation of battery capacity and discharge rate.
Why actual backup time can differ
- Battery state of charge: a partially charged battery has less energy available at the start.
- Changing load: fans, refrigerators and other appliances may draw different power over time.
- Inverter and wiring losses: efficiency varies with equipment and load.
- Battery age and temperature: available capacity can change with use and conditions.
- Discharge rate and cut-offs: particularly for lead-acid batteries, usable capacity and system cut-off behaviour vary with demand.
- Battery management settings: the permitted operating range depends on the specific battery and system.
Check power capacity as well as backup time
A runtime estimate does not establish that an inverter can start or safely operate a load. Compare the appliances' total running watts with the inverter's stated continuous watt rating, and check starting surges, battery discharge limits and installation requirements. kVA and watts are different ratings; use the model's specified watt rating instead of assuming every 1 kVA inverter supplies 1000W.
Use a qualified professional to verify compatibility, mounting, wiring, protection and earthing. See Victron Energy's explanation of battery-bank voltage and capacity for more on series and parallel configurations.
Choose a backup system for your needs
List the appliances you need, their actual wattage and your desired backup duration. Then compare the inverter's load rating and battery energy together.
Explore APHOSYS Li Spark ESS systems or Li Reserve lithium batteries. For help, email info@aphosys.com with your appliance list, inverter model and desired duration.