Reviewed and updated August 31, 2026. Product specifications, utility programs and local code requirements can change.
Home battery sizing becomes much easier when you separate two questions: how much power must the system deliver at one time? and how much energy must it store for the outage duration?
Power is measured in kilowatts (kW). Stored energy is measured in kilowatt-hours (kWh). A battery can have plenty of energy but still be unable to start a well pump or air conditioner. Another system can deliver high power but run out of energy after a few hours.
Step 1: Define essential loads
List only what needs to work during an outage. A common essential-load plan includes a refrigerator, freezer, internet equipment, selected lights, medical devices, garage door, gas-furnace blower and a few outlets. Add a well pump, sump pump or home office only when it is truly required.
Whole-home backup means something different: the system must manage larger coincident loads such as electric cooking, central air, heat pumps, water heating, laundry and EV charging. Many “whole-home” systems still use controls that shed or delay loads.
Step 2: Estimate daily energy
For each load, multiply watts by expected hours of use. Convert watt-hours to kilowatt-hours by dividing by 1,000.
If you want a worksheet-style starting point, use our battery backup planner to organize critical loads and runtime.
| Example load | Illustrative use | Energy |
|---|---|---|
| Refrigerator | Average 70W × 24h | 1.68kWh |
| Internet | 20W × 24h | 0.48kWh |
| Lights | 60W × 5h | 0.30kWh |
| Furnace blower | 500W × 5h | 2.50kWh |
| Office devices | 150W × 8h | 1.20kWh |
This illustrative list totals 6.16kWh per day before conversion losses and reserve. At 85% usable system efficiency, it requires roughly 7.25kWh of nominal stored energy for one day. Real appliances cycle, and a plug-in meter or circuit monitor produces a better estimate than a label.
Step 3: Check peak and startup power
Add the loads that could run at the same time. Then identify motors and compressors with startup surges. A pump may draw several times its normal power for a short period. Battery and inverter data sheets use different time windows for peak output, so compare the actual starting requirement with the applicable specification.
A load-management system can keep the battery from trying to start multiple large appliances at once. This is often more cost-effective than installing enough battery and inverter capacity for every theoretical combination.
How common battery sizes compare
Enphase lists 5.0kWh of energy capacity and a 3.84kW power rating for the IQ Battery 5P. Tesla lists 13.5kWh of nominal energy for Powerwall 3. These published figures show why “one battery” is not a standard unit: different products emphasize modularity, power, integration and installation architecture.
Using the illustrative 6.16kWh daily essential-load profile above:
- A 5kWh module would not cover a full day after reserve and conversion losses.
- A 13.5kWh-class battery might cover roughly one and a half to two days of those loads, depending on usable capacity, efficiency, reserve and conditions.
- Solar can recharge the battery during an outage only when the system is designed and commissioned for backup operation.
These are planning examples, not performance promises.
Portable power stations: useful but different
For a side-by-side current comparison, see our 2026 portable power station guide. A portable station can keep internet, phones, a CPAP, laptop or small refrigerator running without becoming part of the home’s permanent electrical system. The EcoFlow RIVER 3 Plus (paid link) is a small 286Wh unit for light loads. The Jackery Explorer 2000 v2 (paid link) and EcoFlow DELTA 3 Max (paid link) are closer to the 2kWh class.
Do not backfeed a home panel with a portable station. Use manufacturer-approved transfer equipment and a qualified electrician. Portable stations also have different grounding, neutral, charging and weather limitations from permitted home batteries.
Questions to answer before requesting quotes
- Which circuits must stay on?
- How many hours or days of autonomy do you want?
- Which loads have large startup surges?
- Is solar installed now, planned later or not part of the project?
- Can large loads be shed automatically?
- Does the utility offer a battery program or time-of-use rate?
- Where can equipment be installed under local code and manufacturer rules?
- What happens when the battery reaches its reserve level?
Turn the load list into a project brief
Use ConvertGreen’s options form to record backup goals, solar status, utility and timing. Where service coverage exists, the request can be matched to an appropriate provider after the referral relationship is disclosed.
