A home backup battery can keep essential loads operating during a wildfire-related outage or wider grid failure—but only when storage capacity, inverter output and recharge capability are matched to the loads that actually matter.
This guide provides a transparent engineering method for estimating the battery capacity required for critical household loads. Start with the equipment you need to keep operating, calculate the energy demand, then compare storage systems that fit that requirement.
Start with your critical loads
During a wildfire or grid outage, the goal is usually not to power the entire home exactly as normal. Separate essential loads from optional loads and decide how long each one must remain available.
| Critical load | What to measure | Why it matters |
|---|---|---|
| Refrigerator / freezer | Daily energy use and compressor starting surge | Food preservation may require continuous availability even though the compressor cycles on and off. |
| Air purifier | Watts at the fan speed used during smoke conditions | A purifier may operate for many hours continuously during wildfire smoke events. |
| Router, modem and communications | Combined continuous watts | Low-power communications can become important during an emergency. |
| Lighting | Total LED watts and expected hours | Small loads can be maintained for long periods with relatively little stored energy. |
| Water pump or other motor load | Running watts and starting surge | The inverter must be capable of starting the motor, not only supplying its normal running power. |
First battery-sizing estimate
A useful first estimate is:
Required battery capacity (kWh) ≈ daily critical-load energy (kWh) × required autonomy (days) ÷ usable battery fraction ÷ system efficiency
Example: if your critical loads require 6 kWh per day and you want approximately two days of autonomy, the loads require 12 kWh of delivered energy. Assuming 90% usable battery capacity and 90% overall conversion efficiency, the required nominal storage would be approximately 14.8 kWh.
This is a planning estimate, not an electrical design. Real systems also require checks for inverter output, surge demand, battery operating limits, charging power, wiring, protection devices and local electrical requirements.
Optional load builder
Don’t know your kWh per day? Build the load profile first
Enter the running power and expected daily operating time for the loads you want to keep available. Use measured or manufacturer values where possible. Rows with empty power or hours are ignored.
| Load | Running power | Hours / day | Quantity | May run simultaneously | |
|---|---|---|---|---|---|
| W | h | ||||
| W | h | ||||
| W | h | ||||
| W | h |
The load builder estimates energy from running watts × hours. It does not calculate compressor or motor starting surge. Keep the inverter-output input conservative and verify surge separately.
Interactive engineering tool
Home backup battery sizing calculator
Move the sliders or enter exact values. The result updates automatically and adjusts for usable battery capacity and conversion losses.
Systems currently listed
Compare systems against your calculated requirement
All listed systems remain visible. The calculator screens both nominal storage and continuous inverter output, distinguishes systems that meet the calculated minimum from systems that also satisfy your selected design reserve, and marks one as the closest technical fit. You remain free to inspect any product or current price.
Technical fit score: 0–100 is calculated only from storage and continuous-output fit against the preferred design target created from your current inputs and reserve margin. Brand, price, commission and affiliate status are not included in the score.
Matching is affiliate-independent: capacity and continuous-output fit determine the result, not price, brand or commission.
How the technical fit score works
100 means the listed storage or inverter value is very close to the preferred design target while still meeting it. Undersized systems are penalized because they do not satisfy the entered target; increasingly oversized systems gradually lose fit points.
Overall fit combines storage fit (70%) and power fit (30%). If either storage or continuous output is below the preferred design target, the overall score is capped below the recommended-fit range. The score is a sizing aid, not a product review.
Sizing landscape
See your minimum, design target and listed systems on the same scale
Markers update with the calculator. Product markers reflect the systems currently visible under the market and match filters; select a marker to jump to its full comparison card.
Side-by-side comparison
Compare up to three systems
Select products below to compare capacity, inverter output and fit against your current calculation and design reserve.
DATOUBOSS
8 kWh + 3.3 kW hybrid inverter
Compare this system with the calculated minimum and preferred design target above.
DATOUBOSS
16 kWh + 6.2 kW hybrid inverter
Compare this system with the calculated minimum and preferred design target above.
DATOUBOSS
16 kWh + 11 kW hybrid inverter
Compare this system with the calculated minimum and preferred design target above.
DATOUBOSS
32 kWh + 11 kW hybrid inverter
Compare this system with the calculated minimum and preferred design target above.
Affiliate disclosure: Retail price buttons use affiliate tracking. Tecnobosque may earn a commission from a qualifying purchase at no additional cost to you. Highlighting is driven only by the storage, continuous-output and selected reserve inputs above; it is not a safety certification, endorsement or final system design. Starting surge, battery limits, voltage, transfer equipment, installation rules, warranty and regional availability must also be checked.
How the match highlighting works
Best fit is the listed system with the strongest technical-fit score against the preferred design target. Meets design target means it satisfies both storage and continuous-output targets after the selected reserve margin. Meets minimum · reserve short means it reaches the calculated minimum but not all of the selected engineering headroom. This is a screening aid, not a product recommendation or electrical design.
This calculator is a planning tool, not an electrical design. It does not independently size inverter surge capability, conductors, overcurrent protection, transfer equipment, solar array capacity or installation requirements.
Decision notes
What the calculator can — and cannot — tell you
How many kWh does a home backup battery need?
There is no universal number. The storage target depends on the energy used by your selected critical loads, the autonomy you want, the fraction of the battery you plan to use and system losses.
Does enough battery capacity guarantee that a pump or refrigerator will start?
No. Battery energy and inverter power are different constraints. Motor and compressor loads can have short starting surges that must be checked against the exact inverter and appliance specifications.
Why can two systems with similar kWh receive different match results?
The screening also checks the continuous inverter-output input. A system may have enough stored energy but still fall below the continuous power you entered.
Is the highlighted product a recommendation?
No. Highlighting is a transparent screening result based on the values you entered. It does not replace checks for voltage, surge, installation, warranty, regional availability, transfer equipment, safety certification or electrical code requirements.