Power outages are inconvenient at best and disastrous at worst. Many homeowners invest in battery systems to keep lights on and refrigerators running when the grid fails. Understanding how long that stored energy will last helps you decide which appliances to power and when to conserve. This calculator evaluates the relationship between battery capacity, conversion efficiency, and the electrical load you expect to support.
Your battery bank's total capacity is typically measured in kilowatt-hours (kWh). It represents how much energy the batteries can store when fully charged. System efficiency accounts for inverter losses and the fact that you rarely drain batteries to zero. The average load is the combined power draw of the devices you plan to run, measured in kilowatts (kW). Once you know these values, you can estimate backup time.
The core calculation is straightforward:
Here, represents the number of hours your battery backup can supply. For example, a 10 kWh bank running a 1 kW load at 90% efficiency would last about 9 hours. If the average load increases, the available hours decrease proportionally.
To make realistic estimates, think about which circuits you truly need during an outage. Refrigeration, some lighting, and basic communications are typical priorities. Heating and cooling equipment often consume the most power, so you may decide to leave them off to extend backup time. Modern battery systems allow you to program smart circuits so that non-essential devices shut off automatically as the charge level drops.
Suppose you plan to power a small refrigerator (0.15 kW), a few LED lights (0.05 kW), and a Wi‑Fi router (0.01 kW). The total load is 0.21 kW. With a battery capacity of 13.5 kWh (similar to a single Powerwall) and an overall efficiency of 0.9, the calculator shows the system could run these devices for about 58 hours. That’s more than two days without grid power—long enough for many short-term outages.
If you face frequent extended outages or want to power larger appliances, you may need multiple battery modules. Simply add their capacities together in the input field. Because efficiency losses grow as you discharge deeper, keep the efficiency value slightly below 1 to stay realistic. As renewable energy systems drop in cost, more homeowners choose to pair solar panels with batteries. This combination offers daily energy savings and extended emergency power.
Monitor your usage carefully during an outage. Turn off devices when not needed and switch to energy-efficient bulbs and appliances whenever possible. Some systems let you view real-time discharge rates on a smartphone app, making it easier to adapt. If the forecast predicts days of bad weather, ration your stored energy for essential needs until grid service is restored.
Even when you aren't using your backup system, it's wise to check charge levels regularly and keep firmware up to date. Manufacturers often recommend cycling the batteries at least once a month to prevent capacity loss. Avoid storing them in extremely hot or cold conditions, which can shorten lifespan.
If you notice performance dropping over time, monitor the charge-discharge cycles and consult the battery documentation. Replacing worn-out modules promptly ensures your system performs as expected when the grid goes down.
A battery backup system provides peace of mind and a sense of independence from the power company. By entering a few simple values in this calculator, you can predict how long your battery bank will keep critical devices running. Use the result to plan for emergencies, decide whether to add more capacity, or determine which appliances are worth keeping online. With thoughtful preparation, you’ll be ready the next time the lights go out.
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