Backup Power 101: What a Home Battery Can (and Can’t) Run During an Outage
When the grid goes down, a home battery can keep your essentials running — and your stress level down. But batteries aren’t magic “whole-home generators.” What they can power depends on two things:
- How much power (kW) your home needs at once
- How much stored energy (kWh) your battery has available
Let’s break it down in plain English, then walk through real examples of what you can (and can’t) run during an outage.
The Two Numbers That Matter: kW vs. kWh
Think of it like a water tank:
kW (kilowatts) = how fast you’re using power (the flow rate)
This is the “instant demand” when appliances are running.
kWh (kilowatt-hours) = how much energy you have stored (the tank size)
This determines how long you can run things.
A battery might handle a big surge for a short moment (like a fridge compressor starting), but your total simultaneous loads must stay under what the battery (and your backup setup) can deliver.
What Batteries Are Great At During Outages
Most homeowners use battery backup to keep critical loads running, such as:
Essentials that typically work well:
- Lights
- Wi-Fi
- Phone charging / laptops
- Medical devices
- TV / small electronics
- Refrigerator
- Garage door opener
- Microwave (short bursts)
- Gas furnace blower (if your furnace is gas, the blower still needs electricity)
- Select outlets (kitchen, living room, bedroom)
If your system is designed for it, you may also be able to run:
Comfort loads (often possible with the right setup)
- Sump pump
- Well pump
- Mini-split or small HVAC zones
- A few rooms of AC (depends heavily on system design and battery size)
What Batteries Usually Can’t (or Shouldn’t) Run
Some loads are simply too power-hungry to run efficiently on most battery setups, especially for long periods.
- High-demand loads that often get excluded
- Central air conditioning (especially older or large systems)
- Electric water heaters
- Electric ovens / ranges
- Washers & Dryers
- EV charging
- Hot tubs / pool heaters
- Whole-house electric heat (heat strips)
Can a battery run some of these? Sometimes — but usually not all at once, and often not for very long without multiple batteries and careful design.
Battery runtime depends on:
- your battery’s usable energy (kWh),
- your average outage load (kW),
- and whether solar can recharge the battery during the day.
Runtime (hours) ≈ Battery usable kWh ÷ Average kW load
The Powerwall 3 Backup Switch is the device that enables whole-home backup during a grid outage. It automatically disconnects your home from the utility for safety, then signals the battery to power your main electrical panel so your home can keep running. When the grid returns, it smoothly reconnects everything back to normal.
Real-World Scenarios: What You Can Run (and For How Long)
Scenario A: “Keep the basics on”
- Typical average load: 0.5–1.0 kW
- What’s on: fridge, Wi-Fi, lights, devices, some outlets
- Typical outcome: long runtimes — especially if you’re mindful
Scenario B: “Comfort mode”
- Typical average load: 1.5–3.0 kW
- What’s on: essentials + more lights + TV + some kitchen use + maybe a small HVAC zone
- Typical outcome: solid runtime, but you’ll want to avoid stacking big loads
Scenario C: “I want to run everything like normal”
- Typical average load: 4–8+ kW (often higher when big appliances kick on)
- What’s on: central AC + electric cooking + laundry + EV charging
- Typical outcome: short runtime and/or the system may hit its power limits
The takeaway: batteries shine when you prioritize what matters most.
How to Make Your Battery Last Longer in an Outage
A few simple habits can double (or triple) your runtime:
- Avoid stacking big loads (microwave + toaster + hair dryer = fast drain)
- Use LED lighting
- Keep the fridge closed
- Skip laundry and electric cooking
- Turn off unused circuits
- Set HVAC to minimal use (or only cool one zone/room if possible)
The Fastest Way to Know What You Can Back Up
Every home is different. The fastest way to get a confident answer is a quick load review:
- What do you need to run?
- What’s the wattage of those loads?
- Do you want hours, overnight, or multi-day coverage?
- Do you have (or want) solar to recharge during outages?
With those answers, we can design a backup plan that matches your priorities—without overspending.
Ready to Build a Backup Plan That Fits Your Home?
If you’re considering battery storage, the goal isn’t to power everything — it’s to power what matters most, reliably.
Clean Solar can help you:
- choose the right battery and configuration,
- build a critical-loads plan,
- and pair it with solar for longer outages and better year-round savings.
Want to know what a battery could run in your home? We’ll map it out in a simple, no-pressure consultation.
Click here and we’ll set one up!