Battery backup keeps the lights, fridge, internet, and outlets running through a blackout, but it does not keep the whole house running. A 13.5 kWh Powerwall holds roughly half of a typical household's daily kWh use, and most installations protect a smaller essential-loads subpanel rather than the full home. Understanding which circuits stay live, how fast the system switches over, and how solar refills the battery during a long outage is the difference between a useful system and a marketing pitch.
The honest math on a 13.5 kWh battery
A single Powerwall holds 13.5 kilowatt-hours of usable energy. The loads people actually need during an outage — refrigeration, lighting, internet, phone charging — are small and easy to sustain. A modern refrigerator averages 1.5 kWh per day. A house full of LEDs running through the evening totals about 0.5 kWh. A WiFi router pulls 15 watts continuously, around 0.4 kWh per day. Charging two phones overnight uses 0.1 kWh. Add a sump pump or a CPAP machine and the essential load profile lands near 3–5 kWh per day.
At that draw, a fully charged Powerwall runs the essential panel for 24 to 36 hours with no solar input. With sun on the panels the next morning, the battery refills while still feeding the house.
Air conditioning rewrites the math. A central AC unit pulls 3 to 5 kilowatts when the compressor runs. One hour of cooling consumes 3 to 5 kWh — meaning a single Powerwall holds 2 to 4 hours of AC before the rest of the essential loads share whatever is left. A heat pump in winter behaves similarly. EV charging is even worse: a Level 2 charger pulls 7 to 11 kilowatts, draining the battery in 60 to 90 minutes if the inverter even permits the load.
This is why battery sizing starts with one question — which loads do you actually want to run during an outage? Anything beyond essential lights, fridge, and outlets pushes the math toward two batteries or more.
How the switchover actually works
When the grid drops, a battery-backed system does not flip a manual switch. The inverter detects loss of grid voltage within 200 milliseconds and disconnects the home from the utility lines automatically. This is called anti-islanding and it is required by code — without it, your solar panels would push power into the lines while utility crews are working on them.
During that brief disconnect window, an older incandescent bulb may dip in brightness for half a second. LEDs are already pulsing thousands of times per second; they do not flicker visibly. A laptop on battery never notices. A desktop computer plugged straight into a wall outlet might reboot. Most residents do not realize the switch happened until they check a clock that lost time.
After the switch, the house is in island mode. The inverter manages voltage and frequency on its own, the battery supplies power, and the backed-up subpanel runs as if nothing happened.
If the system has a hybrid inverter and the sun is up, solar production resumes within seconds of the disconnect. The inverter feeds solar power to the house first, then routes any surplus to the battery. On a clear day, a 7 kW array refills a depleted 13.5 kWh Powerwall in 4 to 6 hours while still powering daytime loads.
Older string inverters installed before 2018 often lack island-mode capability. If you already have solar and plan to add a battery, ask your installer whether the existing inverter supports backup operation or needs replacement.
Essential-loads vs. whole-home backup
Most residential battery installations use an essential-loads subpanel rather than backing the entire home. The installer identifies the breakers worth keeping live — refrigerator, lights, WiFi, outlets, sump pump, garage door, furnace blower or boiler controls — and rewires those circuits onto a new subpanel fed by the battery. The remaining breakers stay on the main panel and lose power during outages along with everything else on the street.
The economics are clean. Essential loads typically draw 200 to 500 watts continuously, which a single Powerwall supports for a day or more. Adding HVAC, the dryer, or an EV charger doubles or quadruples the required battery capacity. A whole-home backup on a 3,000 square foot house with central AC and electric resistance heat needs three to four batteries to deliver one day of autonomy. That hardware runs $40,000 to $60,000 before installation labor.
The middle ground most homes settle on is one or two batteries on an essential-loads subpanel, plus an optional comfort circuit for one window AC or a mini-split in the bedroom. The mini-split is the cheapest way to get cooling during a blackout — a 12,000 BTU unit pulls 800 to 1,200 watts versus 4,000 for central air. Two hours of central AC becomes ten hours of mini-split on the same kWh draw.
Whole-home backup remains the right answer if you live in an outage-prone region and run all-electric heating. For most homes, an essential-loads panel does the actual job at a fraction of the price.
What happens at low battery and beyond 24 hours
A battery does not gradually dim the lights as it discharges. The battery management system maintains full output until state of charge drops to a protected floor — usually 5 to 10 percent — and then disconnects. From the homeowner's perspective, the lights are on and then they are off. No warning, no taper.
If the sun comes up before the battery runs out, the system recharges and the lights come back on. If the grid is still down at sunset on day two, the same cycle repeats. Solar plus battery handles multi-day outages cleanly as long as the weather cooperates.
The failure mode is a long stretch of cloudy weather during a regional outage. A 7 kW array on a heavily overcast day produces 30 to 40 percent of its rated kWh, which may not refill the battery faster than the household uses it. After two or three days of that pattern, the battery cannot recover.
For that scenario the cheaper answer is a natural-gas or propane generator wired to recharge the battery when state of charge falls below a threshold. A 14 kW gas generator costs $4,000 to $6,000 installed and runs indefinitely on utility gas supply. That pairing — one battery plus a small generator — delivers more days of resilience than four batteries alone.
How to size the battery for your house
The right battery size depends on three numbers, not on whatever the salesperson suggests first. First, your essential-load kWh per day. Add up the appliances and circuits you actually want backed up — fridge, freezer, lights, WiFi, microwave, well pump if relevant. Most homes land between 3 and 8 kWh per day for true essentials.
Second, your typical outage length. Pull the last five years of utility reliability reports from your state's public utilities commission. A region averaging four outages a year, mostly under six hours, needs less storage than a coastal area with annual multi-day hurricane outages.
Third, your solar production. A south-facing 7 kW array in Phoenix produces 11,500 kWh annually and refills a Powerwall on most days year-round. The same array in Seattle produces 7,800 kWh and recharges more slowly in winter. The Solar Design Studio runs both numbers using your address and roof orientation.
A useful rule: aim for enough battery to cover one full day of essential loads without solar input, then add solar capacity sufficient to refill the battery on an average day. That formula lands most homes at one or two Powerwalls — not the four-battery installations sometimes pitched as whole-home resilience. The over-sized systems sell margin, not uptime.