How long will a solar battery last during a power outage?

Battery backup duration depends on your battery capacity (kWh) and how much power your home uses. A 10 kWh battery powering essential loads (~1,000W) lasts about 8–10 hours. Running your whole home (~3,000W average) cuts that to 3–4 hours. Adding solar production during the day can extend backup indefinitely in sunlight.

What size battery do I need for whole-home backup?

Most whole-home backup systems use 20–40 kWh of storage — typically two or more Tesla Powerwalls or similar units. For critical loads only (refrigerator, lights, some outlets), a single 10–13 kWh battery is usually sufficient. Use a battery backup duration calculator to size based on your actual usage.

What size battery do I need to power my home overnight?

A typical home uses 1–2 kWh per hour overnight. To cover 8 hours with a 10 kWh daily usage home, you need roughly 8–10 kWh of usable battery capacity. Most popular home batteries (Tesla Powerwall, Enphase IQ) are 10–13.5 kWh — one unit covers most overnight needs for average homes.

Can I use a solar battery without solar panels?

Yes. A home battery can be charged from the grid during off-peak hours and discharged during peak rates or outages. However, without solar panels you lose the ability to recharge from the sun, limiting the battery's long-term value and backup duration during extended outages.

Battery Backup Duration Calculator

Calculate how long your home battery will keep critical loads running during a grid outage. Enter your battery specs and power draw to see backup duration at different load levels.

Battery Size (kWh)

Depth of Discharge (%)

Critical Load (kW)

About This Calculator

The Battery Backup Duration Calculator tells you exactly how long your home battery storage system will keep the lights on during a grid outage. Backup duration depends on three factors: how much energy your battery can actually deliver (usable capacity after depth-of-discharge and efficiency losses), and how much power your critical loads are drawing. A Tesla Powerwall 2 holds 13.5 kWh and operates at 90% DoD, giving about 12.15 kWh usable — enough to power a 1.5 kW critical load for roughly 7–8 hours, or a minimal 500W load for over a day.

Understanding the difference between battery capacity and usable energy is essential. A 13.5 kWh battery at 90% DoD delivers 12.15 kWh. After round-trip efficiency losses of about 5%, net delivered energy is closer to 11.5 kWh. Your critical load — the appliances you run during an outage — is the other key variable. A refrigerator draws about 150W, LED lights 50–100W total, a well pump 750W–1,500W, and a central air conditioner 2,000–5,000W. Running the AC during an outage will drain a single Powerwall in 2–4 hours; running just essentials can stretch the same battery to 12–24 hours.

To estimate your critical load accurately, list the appliances you plan to run during an outage and look up their wattage on the nameplate or user manual. Add them up for a total kW figure. Many homeowners aim to back up refrigerator, some lighting, phone charging, and a medical device — a realistic critical load of 500W–1,000W. If you also want to back up a heat pump or well pump, add those watts and recalculate. Multiple battery units can be stacked (e.g., two Powerwalls = 27 kWh) to extend backup duration for whole-home coverage.

Battery backup is increasingly important as grid outages become more frequent due to extreme weather events. Pairing solar with battery storage means your system can keep recharging the battery from the panels during a multi-day outage, dramatically extending effective backup time beyond what the calculator shows for a static charge scenario.

Calculations based on NREL solar modeling data and industry-standard assumptions, built and maintained by the independent SolarToolsOnline research team.

Estimates only — not financial, tax, or legal advice. Verify important results with a licensed solar installer or financial professional before making decisions.