How to Size a Whole Home Battery Backup System
Home battery backup systems like Tesla Powerwall have transformed emergency preparedness. Unlike generators, they provide instant, silent backup power with zero fuel costs. But sizing them correctly is crucial—undersize and you'll run out of power when you need it most.
Understanding Home Battery Capacity
Home batteries are rated in kilowatt-hours (kWh)—the amount of energy they can store. A 13.5 kWh battery can deliver:
- 13.5 kilowatts for 1 hour
- 1.35 kilowatts for 10 hours
- 675 watts for 20 hours
The key is matching your daily energy usage to battery capacity, not just peak power. A home that uses 30 kWh per day needs much more battery than one using 10 kWh.
Essential vs. Whole Home Backup
Essential Circuit Backup
Most homeowners start with essential circuits only:
- Refrigerator and freezer
- Lighting circuits
- WiFi and phone charging
- Sump pump (if applicable)
- Medical equipment
This typically requires 5-15 kWh per day, meaning a single 10-15 kWh battery provides 1-3 days of backup.
Whole Home Backup
Backing up everything including AC, electric cooking, and EV charging requires 30-50+ kWh per day. This means:
- Multiple batteries (2-4+ units)
- Higher upfront cost ($20,000-$50,000+)
- Often paired with solar for daytime recharging
The Solar Factor
Home batteries shine brightest when paired with solar panels. Here's why:
- Without solar: Your battery has fixed capacity. Once drained, you wait for grid power.
- With solar: Batteries recharge daily. A 10 kWh battery with 6 kW of solar can provide indefinite backup for essential loads.
If you're considering home battery backup, adding solar (or planning for future solar) makes the investment much more valuable for extended outages.
Popular Home Battery Systems Compared
| System | Capacity | Power Output | Best For |
|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 11.5 kW | Whole home, solar integration |
| Generac PWRcell | 9-18 kWh | 9 kW | Standby generator alternative |
| Enphase IQ Battery | 5-15 kWh | 3.84-7.68 kW | Microinverter solar systems |
| EcoFlow DELTA Pro Ultra | 6-90 kWh | 7.2-21.6 kW | Portable + whole home hybrid |
| Anker SOLIX F3800 | 3.84-12 kWh | 6 kW | Portable with home integration |
Installed vs. Portable Home Batteries
Installed Systems (Tesla, Generac, Enphase)
- Professionally installed, hardwired to your electrical panel
- Automatic switchover (you won't notice the outage)
- Higher capacity (10-40+ kWh)
- Higher cost ($10,000-$20,000+ per unit installed)
- Eligible for 30% federal tax credit
Portable Systems (EcoFlow, Anker, Bluetti)
- DIY setup, plugs into outlets or transfer switch
- Portable—use for camping, RV, job sites
- Lower capacity (3-10 kWh typically)
- Lower cost ($2,000-$6,000)
- Can be expanded with additional batteries
How Long Will a Home Battery Last?
Runtime depends entirely on your load. Here are realistic examples:
| Battery Size | Essential Load (500W) | Moderate Load (1.5 kW) | Heavy Load (5 kW) |
|---|---|---|---|
| 5 kWh | 10 hours | 3 hours | 1 hour |
| 10 kWh | 20 hours | 6.5 hours | 2 hours |
| 13.5 kWh (Powerwall) | 27 hours | 9 hours | 2.7 hours |
| 27 kWh (2 Powerwalls) | 54 hours | 18 hours | 5.4 hours |
Key insight: Running central AC (3-5 kW) will drain batteries quickly. Most homeowners either skip AC backup or install 3+ batteries for AC support.
Cost Considerations
Home battery costs have decreased but remain significant:
- Installed systems: $10,000-$15,000 per unit (including installation)
- Portable systems: $2,000-$6,000 per unit
- Federal tax credit: 30% credit for installed systems with solar
- Comparison: A whole-house generator costs $5,000-$15,000 installed
Batteries make sense if you value silent operation, have solar panels, or want automatic instant backup. Generators make sense for whole-home power, extended outages, and lower upfront cost.
Next Steps
Compare your battery requirements to a whole house generator, or calculate solar recharge time for your battery system.
Frequently Asked Questions
Most homes need 10-20 kWh for essential circuit backup (refrigerator, lights, WiFi, phone charging) for 12-24 hours. For whole home backup including AC or heating, you'll need 20-40+ kWh. Calculate your daily essential usage and multiply by the number of backup days you want.
One Tesla Powerwall (13.5 kWh) can backup essential circuits for 12-24 hours in most homes. For whole home backup or longer duration, 2-3 Powerwalls are recommended. Homes with electric heating/cooling or high usage may need 4+ units.
Home batteries provide instant, silent backup with no fuel needed. Generators offer unlimited runtime with fuel but require maintenance, make noise, and take 10-30 seconds to start. Batteries are better for short outages; generators excel for extended multi-day outages.
Yes, but capacity matters. A single 10-15 kWh battery can power essentials for 12-24 hours. Running a central AC (3-5 kW) would drain that in 3-5 hours. Whole home backup typically requires 2-4 batteries or pairing with solar panels for daytime recharging.
Runtime depends on your load and battery size. A 13.5 kWh battery running a 500W load lasts about 24 hours. Running 2,000W of loads, it lasts 6 hours. With solar panels, batteries can recharge daily and potentially run indefinitely during extended outages.