The Math Behind Battery Backup Runtime
A fully charged 13.5 kWh home battery like the Tesla Powerwall will run essential circuits in an average home for 12-24 hours — but that’s if you’re smart about what you keep on. Try running your whole house like normal and you’ll burn through it in 4-6 hours, maybe less if you’ve got central AC cranking.
After 20 years as an electrician and helping over 200 homeowners with solar and battery systems since 2019, I’ve learned that the real answer isn’t about the battery size — it’s about your load. I’ve watched families stretch a 10 kWh battery for two days during an outage, and I’ve seen others drain the same battery by dinnertime because they forgot to turn off the pool pump.
How to Calculate Your Actual Runtime
Here’s the formula I give every homeowner: Battery capacity (kWh) ÷ Your average hourly load (kW) = Runtime (hours)
The catch? Most people have no idea what their hourly load actually is. Let me walk you through it.
Step 1: Figure Out Your Essential Load
Open your breaker panel and make a list of what you must keep running during an outage:
- Refrigerator: 150-200W continuous (spikes to 800W on startup)
- Chest freezer: 100-150W continuous
- LED lights (10 bulbs): 100W
- WiFi router + modem: 20-30W
- Phone chargers: 10-15W
- Well pump (if you have one): 500-1,200W when running
- Gas furnace blower: 400-800W
- Sump pump: 800-1,200W when running
Add those up and you get your continuous essential load. For most homes, that’s 800-1,500W (0.8-1.5 kW) without heating or cooling.
Step 2: Account for Peak Loads
Battery systems need headroom for when multiple things turn on at once. If your fridge compressor kicks in while the sump pump is running and someone’s using the microwave, you could hit 3-4 kW instantly. Most home battery backup systems can handle 5-7 kW continuous and 10+ kW peak, but older batteries may struggle with simultaneous heavy loads.
Real-World Runtime Examples
Let me show you what I’ve seen with actual homeowners:
| Battery Size | Load Profile | Actual Runtime |
|---|---|---|
| 13.5 kWh (Tesla Powerwall) | Essentials only (1 kW avg) | 12-13 hours |
| 13.5 kWh | Essentials + window AC (1.8 kW avg) | 7-8 hours |
| 13.5 kWh | Whole house, normal use (3 kW avg) | 4-5 hours |
| 10 kWh (Enphase IQ 10) | Essentials only (0.8 kW avg) | 11-12 hours |
| 20 kWh (2× Powerwall) | Essentials + mini-split AC (2 kW avg) | 9-10 hours |
| 27 kWh (Franklin Whole Home) | Whole house with central AC (5 kW avg) | 5-6 hours |
Notice the pattern? It’s not about battery size — it’s about discipline. The family with the 10 kWh battery outlasted the one with 27 kWh because they weren’t trying to run everything.
What Kills Your Battery Runtime Fastest
After troubleshooting hundreds of “my battery doesn’t last” complaints, here are the usual suspects:
1. Central Air Conditioning
A central AC unit pulls 3-5 kW continuous. That’s more than your entire essential load combined. Run it for 3 hours and you’ve drained a 13.5 kWh battery to 20%. If you’re in a hot climate and need cooling during outages, you either need multiple batteries or a portable air conditioner for one room (1.2 kW vs 4 kW).
2. Electric Water Heaters
These pull 4-5 kW when heating. I tell every client: put your electric water heater on a separate circuit that’s NOT backed up by the battery, or get a heat pump water heater that uses 80% less power. You can survive an outage with cold showers. Your battery can’t survive an electric water heater.
3. Electric Ovens and Cooktops
Even on medium heat, these draw 2-3 kW. Use a portable butane stove or grill instead during outages.
4. Space Heaters
1,500W each, and people tend to use multiple. If you’re heating with electricity during a winter outage, your battery won’t make it past morning. Gas furnaces with electric blowers are fine (400-800W), but pure resistance heating is a battery killer.
How to Extend Your Battery Runtime
These are the tactics that actually work:
Set up a critical loads panel. This is a sub-panel with only essential circuits. Your battery powers this panel, not your whole house. Costs $800-1,500 for an electrician to install, but it forces you to be selective about what runs during an outage. I’ve seen this double runtime overnight.
Use load shedding. Smarter battery systems like the Tesla Powerwall and Enphase IQ can automatically disconnect heavy loads when the battery gets low. You set thresholds — say, cut the AC when you hit 30% battery — and the system handles it.
Get a generator as backup for your backup. A portable generator (3-4 kW) costs $500-800. Run it for 2-3 hours midday to recharge your battery, then shut it off. You get the quiet, clean power of the battery most of the time, with the generator as a range extender for multi-day outages.
Add solar panels. If you’ve got solar paired with your battery, runtime becomes almost irrelevant for daytime outages. Even a small 5 kW array will produce 2-4 kW during peak sun, enough to run your essentials and recharge the battery simultaneously. Multi-day outages turn into minor inconveniences instead of crisis management.
Battery Depth of Discharge Matters
Here’s something most homeowners miss: you can’t use 100% of your battery capacity. Lithium batteries need to hold back 5-10% to protect the cells, so that “13.5 kWh” battery really gives you 12-12.5 kWh of usable power.
Plus, manufacturers recommend avoiding full discharge cycles when possible. Repeatedly draining to 0% shortens battery life. I tell clients to plan around using 80-90% capacity per outage if you want your battery to last its full 10-year warranty.
What About Sizing for Multi-Day Outages?
If you need to survive 2-3 days without grid or solar, here’s the math: 1.5 kW essential load × 48 hours = 72 kWh. That’s five or six Powerwalls. The cost? $60,000-75,000 installed. For most people, that’s insane.
The smarter play: size your battery for 12-24 hours of backup, then add a generator or solar panels. A $5,000 battery + $800 generator handles 99% of outages better than a $70,000 battery fortress.
Should You Oversize Your Battery?
I get this question constantly: “Should I buy two batteries instead of one?” Here’s my take:
If you live somewhere with frequent multi-hour outages (California wildfire shutoffs, Texas winter storms, hurricane zones), yes. Two 10-13 kWh batteries give you flexibility — you can run more circuits or stretch runtime to 24+ hours on essentials.
If your power is reliable and you’re buying the battery “just in case,” start with one. You can always add a second later. Most modern systems like Enphase battery backup systems are modular — stack up to four batteries on the same inverter.
One mistake I see: people buying huge battery banks but not installing solar. Without solar, you’re just buying time until the battery’s dead. With solar, you’re buying independence.
Frequently Asked Questions
Can I run my whole house on a single home battery?
Yes, but not for long. A single 13.5 kWh battery can physically power your entire house, but if you’re using 3-5 kW continuously (normal household load), you’ll drain it in 3-5 hours. For whole-home backup that lasts 12+ hours, you need either multiple batteries or aggressive load management — turning off AC, water heater, dryer, etc.
How long will a 10 kWh battery power my refrigerator?
A typical refrigerator uses 150-200W continuously, or about 0.15-0.2 kW. A 10 kWh battery could theoretically run it for 50-65 hours straight (10 kWh ÷ 0.15-0.2 kW). In practice, you’ll get 40-50 hours because of inverter efficiency losses and keeping some reserve capacity. That’s nearly two full days just for the fridge alone.
Do home batteries recharge themselves when the power comes back on?
Yes. Once grid power is restored, the battery automatically recharges from the grid (or from solar panels if you have them). Most batteries recharge fully in 4-8 hours depending on size and your home’s electrical service. You don’t need to do anything — the system handles it.
Will a battery backup run my sump pump during an outage?
Absolutely. Sump pumps draw 800-1,200W when running, which is well within any home battery’s capability. The challenge is runtime — if you’re in a flooding situation where the pump runs every 10 minutes, it’ll consume 200-400Wh per hour. A 10 kWh battery can handle 24+ hours of frequent pump cycles, but pair it with a generator or solar for longer events.
How does temperature affect battery backup runtime?
Cold temperatures reduce battery capacity by 10-20%. If your battery is installed in an unheated garage and it’s 20°F outside, expect 15-20% less runtime than rated capacity. Heat above 95°F also degrades performance and shortens battery lifespan. Most batteries have thermal management systems, but extreme temperatures still matter. Install in a climate-controlled space if possible.
About Mike Reeves
Home Energy Consultant · Former Licensed Electrician
20 years as a licensed electrician before going solar myself in 2019. Made every mistake in the book. Now I help homeowners size systems correctly and avoid costly mistakes — no installer referral fees, no skin in the game. Read more →