A 13.5 kWh battery (the size of a Tesla Powerwall 3) runs a fridge, lights, and wifi for roughly 24 to 36 hours, depending on what else you power. The math is simple once you list your loads in watts. This guide works through realistic combinations so you know exactly what to expect.
- Fridge + lights + wifi + phones draws about 300 to 400 watts, giving a 13.5 kWh battery roughly 30+ hours of runtime.
- Add a TV, laptops, and fans and runtime drops to about 18 to 24 hours. Add air conditioning and it drops to 4 to 8 hours.
- The formula is simple: hours = 13,500 / your watts, minus about 10 percent for inverter losses.
- Daytime solar recharge extends all of these numbers dramatically; the battery only needs to cover the night.
The short answer
On a lean essentials load (fridge, a few LED lights, wifi router, phone chargers: ~350 watts), a 13.5 kWh battery lasts 30 to 35 hours. On a comfortable essentials load (add TV, laptops, fans: ~650 watts), expect 18 to 22 hours. Run a window AC unit (~1,200 watts) on top and you are down to 8 to 10 hours. Central air conditioning (~3,500 watts) drains it in 3 to 4 hours alone. For background on how usable capacity and depth of discharge shape real backup time, the Department of Energy’s homeowner guide to solar is a solid non-vendor starting point, and if you are sizing for storm outages rather than bill savings, Ready.gov’s power outage guide covers what to prioritize when the grid goes down.
These are the numbers that should drive your battery decision, not marketing. If your outage plan is “keep food cold and stay connected,” one 13.5 kWh unit is plenty. If it is “live normally,” you need more storage or a different plan. Our battery comparison puts these runtimes in the context of choosing between systems.
The runtime formula
Battery runtime is division: hours = usable kWh x 1,000 / watts of load, then subtract ~10 percent for inverter and wiring losses. For a 13.5 kWh battery: 13,500 / 350W = 38.6 hours theoretical, ~35 hours real. At 1,000W: 13.5 hours theoretical, ~12 hours real.
Two subtleties. First, loads cycle. A fridge rated 150W average does not draw 150W constantly; the compressor cycles on and off. “Average watts” already accounts for this, which is why we use averages, not nameplate ratings. Second, surge does not consume much energy. A fridge’s starting surge (1,000W+ for a second) matters for whether the battery’s power rating can handle it, not for runtime. Energy is about sustained watts over time.
Usable capacity deserves a note: 13.5 kWh is the usable figure for a Powerwall 3 (manufacturers quote usable, not total, for home batteries). You can use essentially all of it, unlike old lead-acid batteries that hated deep discharge. LFP chemistry handles full cycling gracefully, which is one reason modern batteries can be sized tighter than the old ones.
Realistic load scenarios
| Scenario | Load (watts) | Runtime on 13.5 kWh |
|---|---|---|
| Ultra-lean: fridge, 3 LEDs, wifi, 2 phones | ~300 | 35 to 40 hours |
| Comfortable: above + TV, laptop, 2 fans | ~650 | 18 to 22 hours |
| Working from home: above + monitor, printer use | ~850 | 14 to 16 hours |
| Plus window AC (8,000 BTU) | ~1,800 | 6 to 8 hours |
| Plus central AC (3-ton, cycling) | ~4,000 | 3 to 4 hours |
| Plus electric water heater | ~5,000 | 2 to 3 hours |
The pattern: everything up to “working from home” fits comfortably in a day. Cooling is the cliff. This is why installers ask about air conditioning before anything else: it single-handedly decides whether one battery suffices. Planning for longer outages? Our 3-day outage guide scales this math up.
What drains the battery fastest
Ranked by damage to your runtime: electric resistance heat (space heaters, electric water heaters: 1,500 to 4,500W each) is the worst; a single space heater can halve your runtime. Central AC is next, especially in extreme heat when it barely cycles off. Electric dryers and ovens (3,000 to 5,000W) are brutal but brief; one dryer cycle costs about 3 kWh, or 20+ percent of the battery.
Well pumps deserve special mention for rural homes: 1,500W running with big starting surges, cycling unpredictably. A well pump plus a fridge can unexpectedly dominate consumption. EV charging is in a class of its own: even slow Level 1 charging (1,400W) drains the battery in under 10 hours; Level 2 (7,000W+) empties it in under two. Do not charge the car from backup power unless it is an emergency.
The common thread: anything that makes heat or cold with electricity is expensive. Lights, electronics, and communications are cheap. Design your outage plan around that divide.
How to stretch runtime
Unplug the vampires. Cable boxes, game consoles on standby, and old plasma TVs draw 20 to 100W each doing nothing. During an outage, walk the house once and unplug everything non-essential. Consolidate refrigeration: move essentials into one fridge and leave the garage freezer closed (a closed freezer holds temperature for 24 to 48 hours unopened). Time-shift big loads to solar hours: run the microwave, charge devices, and do laundry while the sun shines.
Use the app’s reserve setting. Both Tesla and Enphase let you set a backup reserve (say 30 percent) that daily cycling will not touch. During an outage, you can lower the reserve to access the full battery. Pre-cool the house if the outage is forecast (storms, PSPS events): run the AC hard while the grid is up, then coast on the thermal mass with fans during the outage.
These habits routinely stretch a 13.5 kWh battery from 18 hours to 30+ on the same hardware. Behavior is free storage, and it is the difference between a battery that feels small and one that feels generous. Make the walkthrough a household routine at the start of storm season so everyone knows the plan.
The solar multiplier
Everything above assumes no recharge. With solar panels, the battery only bridges the night. A 6 to 8 kW array produces 20 to 35 kWh on a decent day: enough to run the house by day and refill the battery by evening. In this setup, a single 13.5 kWh battery carries essentials indefinitely through multi-day outages in fair weather, because each morning resets the tank.
This is the strongest argument for solar-plus-storage over storage alone: the panels are a fuel supply that never runs out. It is also why outage planning should consider the array and battery as one system. If you have panels already and are adding storage, read adding a battery to an existing system. Comparing battery brands? See our Powerwall vs FranklinWH guide.
Can a 13.5 kWh battery run a sump pump?
Yes, easily. A sump pump draws 800 to 1,500W while running but cycles briefly, using well under 1 kWh per day in most cases. It is a light load for a battery this size. The one caution is the starting surge, which a Powerwall 3 or similar handles without trouble.
How long does solar take to recharge the battery?
A 6 kW array in decent sun refills 13.5 kWh in about 3 to 4 hours of good production. Smaller arrays or cloudy days take longer. This is why solar-plus-storage works for multi-day outages: each day resets the battery by afternoon.
Does the battery drain when nothing is running?
Barely. The inverter’s standby draw is 20 to 50 watts, or about 1 kWh per day. The bigger “drain” is usually forgotten loads, not the battery itself.
Can I run my fridge and freezer on one battery?
Easily. Together they average under 300W, giving 35+ hours. Keep the doors closed and they cycle less, stretching it further.
Will the battery power my home automatically in an outage?
Yes. The gateway disconnects from the grid in milliseconds and the battery takes over. You will notice the lights flicker; the fridge will not notice at all.
Does cold weather reduce battery runtime?
LFP batteries lose some usable capacity in freezing temperatures, roughly 10 to 20 percent below freezing. Indoor or garage installations in most US climates are unaffected. Outdoor installs in northern winters should account for it.
How do I know my real-time consumption?
Both Tesla and Enphase apps show live home consumption in watts. Spend a normal evening watching it; you will quickly learn your household’s baseline and what spikes it. That awareness alone usually cuts waste before you ever buy a battery.
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