Home batteries are sold for two different jobs: keeping the lights on during outages and lowering electricity bills. A battery sized for one job may not suit the other.
Key Takeaways
- Energy capacity (kWh) determines how long a battery can run loads; power rating (kW) determines how much it can run at once.
- Backing up essential circuits requires far less capacity than whole-home backup.
- Bill savings depend on time-of-use rates, export rules and solar production.
- Check whether the system can start large motors such as well pumps and air conditioners.
Capacity and power
- kWh capacity: how much energy is stored.
- kW power: maximum output at once.
A 13 kWh battery with 5 kW continuous output can run 5 kW of loads for about 2.6 hours, or 1 kW for about 13 hours, before accounting for losses.
Backup goals
List what you need during an outage:
- Refrigerator
- Lights
- Internet
- Medical devices
- Furnace fan or heat pump
- Well pump
Essential-load panels back up selected circuits. Whole-home backup requires more capacity and power.
Bill savings goals
Batteries can save money by:
- Storing solar for evening use when export credits are low
- Shifting usage away from peak time-of-use rates
- Participating in utility programs
Savings depend heavily on local rates.
Motor starting
Some appliances require high surge power to start. Ask installers about surge ratings.
Questions for installers
- Which circuits will be backed up?
- How long will the battery run them?
- How does the battery charge during outages?
- What warranty and capacity guarantees apply?
Estimating backup needs
List essential loads and their approximate power and daily energy use:
- Refrigerator: often around 1 to 2 kWh per day
- Wi-Fi router and modem: small, but always on
- LED lights: small per bulb
- Furnace fan: depends on runtime
- Well pump: high starting power
- Medical devices: check specifications
Add up daily kWh for essential loads. If they total 8 kWh per day, a 13 kWh battery could cover more than a day, and longer with solar recharging.
Whole-home backup
Backing up an entire home, including central air conditioning, electric heat or electric cooking, requires substantial power capacity and often multiple batteries. Some systems can manage large loads by shedding non-essential circuits automatically.
Recharging during outages
With solar panels, batteries can recharge during the day during an outage, if the system is designed to operate off-grid. Without solar, battery backup lasts only as long as stored energy. Some homeowners pair batteries with generators for extended outages.
Bill savings in detail
On time-of-use plans, batteries can charge during low-cost periods or from solar and discharge during peak periods. On net billing tariffs with low export rates, storing midday solar for evening use can increase savings. Utility programs may pay battery owners to discharge during grid emergencies.
Installation considerations
- Indoor vs outdoor location
- Temperature limits
- Fire codes and clearances
- Permits and utility interconnection
Frequently Asked Questions
Do batteries work without solar? Yes, they can charge from the grid.
How long do batteries last? Many warranties cover 10 years.
Can I add batteries later? Some systems are expandable.
Are generators cheaper? Often upfront, but require fuel and maintenance.
The bottom line
Decide whether your priority is backup or savings, then size capacity and power accordingly.
