Size home battery backup from the circuits you want to run, their energy use and the outage duration you want to cover. Battery capacity in kWh tells you how much energy can be stored; inverter power in kW tells you which loads can run together. You need both.
This is a planning worksheet, not a home electrical design. Ask an installer to verify starting loads, supported circuits, transfer equipment and whether solar can recharge the battery during an outage.
Reviewed October 17, 2026. All numbers below are hypothetical; no rebate, equipment performance or outage duration is promised.
1. Build a critical-load energy budget
List only the loads you intend to back up. For each, multiply watts by equivalent operating hours and divide by 1,000 to get kWh. A refrigerator cycles, so use measured daily energy or equivalent run time rather than assuming its rated power runs continuously.
- Refrigerator: 150 W × 10 equivalent hours = 1.50 kWh.
- Router: 15 W × 24 hours = 0.36 kWh.
- Lights: 60 W × 5 hours = 0.30 kWh.
- Charging: 100 W × 3 hours = 0.30 kWh.
- Furnace fan: 400 W × 3 hours = 1.20 kWh.
- Other outlets: 200 W × 2 hours = 0.40 kWh.
Total: 4.06 kWh per day. Actual appliances, weather and usage can differ substantially. Electric heating, air conditioning, water heating or EV charging can change the budget completely.
2. Convert the energy budget into capacity
For a simplified example, assume 90% of nameplate capacity is available after your reserve setting and 90% delivery efficiency from that available energy to the loads. With no solar recharge:
Nameplate capacity = load energy × backup days ÷ (available fraction × delivery efficiency).
One day in this example needs 4.06 ÷ (0.90 × 0.90) = about 5.01 kWh nameplate; two days need about 10.02 kWh. These fractions are illustrative, not specifications for a product. If the manufacturer’s usable AC energy figure already includes a limit or loss, do not deduct it again.
Keep a separate allowance for uncertainty and aging. Do not turn the minimum arithmetic into a guarantee that a particular battery will last a full outage.
3. Check power, surge and recharge
A battery with enough kWh can still fail to start a motor or support simultaneous loads. Ask for continuous and surge power, backup-panel scope and operating limits in writing. Solar panels connected to the grid should not be assumed to operate during an outage without equipment designed for that purpose.
Solar recharge depends on season, weather, shade and system configuration. Use the official PVWatts production model for location-specific production context, then ask the installer to model the intended backup configuration. An annual average alone cannot establish multi-day winter resilience.
4. Keep backup sizing separate from financial payback
A battery stores energy rather than creating it. Its additional bill benefit depends on when energy is charged and discharged, losses, import prices and export credits you forgo. Do not add the entire value of stored solar to savings if that same energy was already counted in the solar-only case.
For a hypothetical $14,000 installed battery with no verified incentive and $500 in additional annual bill reduction, simple cost divided by benefit is 28 years, before upkeep, replacement, degradation or discounting. That is not evidence the battery will operate for 28 years or recover its cost. Backup value may matter personally, but it is not a utility-bill saving.
The IRS says Section 25D is unavailable for property placed in service after December 31, 2025. The historical battery-capacity eligibility rule does not make a new 2026 homeowner battery eligible. Verify any state or utility program separately; do not subtract an assumed 30% credit.
Use the right next step
Review battery economics and battery quote costs separately. Normalize the underlying cash offers in the quote comparison tool; it does not size a home battery or simulate its dispatch.
The Department of Energy homeowner guide provides background on solar and storage. MySolarROI’s methodology explains the limits of its planning calculators.
