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There is no universal “right” battery size. Size a home backup in two separate passes: calculate the kilowatt-hours (kWh) your chosen essential loads will consume during the outage, then verify that the battery inverter can supply their combined kilowatt (kW) demand and any motor-starting surge. A refrigerator-only plan may need a few kWh; a plan that includes a well pump, HVAC, electric cooking or long outages can require much more energy and output.
The sizing method in one view
Start with the circuits you genuinely need during an outage, not your normal whole-home electricity bill. For every selected appliance, estimate energy use over the planned outage, add those values, and compare the total with the battery’s usable AC energy. Then make a separate power check: add the running wattage of loads that may operate at the same time and compare that total with the system’s continuous AC output. Finally, check the starting surge of compressors and pumps against the specific inverter and installation design.
The core calculation is:
Required energy (kWh) ≈ sum of each essential appliance’s outage-period energy use
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If the selected loads use 5 kWh per day and you want two days with no recharge, the basic energy target is 10 kWh. That figure still needs the system’s own allowance for conversion losses, reserve capacity and limits on usable energy. A battery’s advertised kWh rating is not automatically the same as the AC energy available to your appliances.
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Tesla’s official home-battery sizing guide, published August 6, 2025, summarizes the distinction this way: “While energy (kWh) tells you how long a battery can run, power (kW) determines how many things it can run at the same time.” Starting surge and system configuration are additional checks, not optional details.
Step 1: Define the outage you are designing for
Write down the duration and operating pattern before looking at products. A six-hour plan is one quarter of a day; a two-day plan is two daily energy totals. Decide whether the calculation assumes no solar recharge, reliable solar recharge, or a conservative storm scenario in which solar contributes little.
- Short interruption: A few hours may only require refrigeration, lighting, communications and device charging.
- Overnight outage: Add the loads needed for sleeping, medical equipment, heating or cooling and water service.
- Multi-day outage: Include food preservation, pumping, HVAC strategy, cooking, hot water and a charging plan. Energy use can rise substantially when people remain at home.
State the duration beside every example. “A 10 kWh battery” has no useful runtime meaning without a load schedule.
Step 2: Build an essential-load list
Choose loads deliberately and identify the circuits that will remain energized. Common candidates are a refrigerator or freezer, a few lights, internet equipment, phone and computer charging, selected medical equipment and, where applicable, a well pump. Treat HVAC, electric cooking, resistance water heating, laundry, a pool pump and EV charging as separate decisions because each can change the energy target, the required output, or both.
The following values are illustrative figures in Tesla’s 2025 guide, not guarantees for every appliance or household:
| Example load | Illustrative energy use | What can change it |
|---|---|---|
| Refrigerator | 1.9 kWh per day | Model, temperature setting, room temperature, door openings and compressor efficiency |
| Wi-Fi equipment | 0.5 kWh per day | Router, modem, network switches and whether they run continuously |
| Lighting | 0.8 kWh per day for 70 W used 12 hours | Number and type of lamps and the hours occupied |
| Cooking range | 1.4 kWh per day | Cooking method, element duty cycle and meals prepared |
| 1 horsepower well pump | 2 kWh per day when used for two hours | Pump efficiency, pressure settings, well depth and actual run time |
That guide’s cited critical-load set totals approximately 4–6 kWh per day. Use it as a worked example, not as a standard allowance. A modern refrigerator may cycle rather than draw its nameplate wattage continuously, while a pump or heater can run for long periods under unusual conditions.
Step 3: Estimate each appliance’s energy
Use measured or manufacturer energy data first
If a device has measured kWh data or a manufacturer’s daily-use estimate, use that instead of multiplying its running wattage by 24 hours. Refrigerators, freezers, heat pumps and other cycling equipment spend part of the day off or at reduced output. The nameplate running wattage is still useful for the power check, but it can greatly overstate daily energy when treated as a constant draw.
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For a load that really does run at a known draw for a known time:
Watt-hours = watts × hourskWh = watt-hours ÷ 1,000
For example, 70 W of lights used for 12 hours consumes 840 Wh, or 0.84 kWh, which rounds to the 0.8 kWh illustrative value above. If a 500 W appliance runs for two hours, its estimate is 1,000 Wh, or 1 kWh.
Use utility bills only as a baseline
Divide the kWh on a recent bill by the number of days in that billing period to find average daily purchases. Then remove loads that will be switched off and estimate the remaining essential circuits. Whole-home purchases are not the same thing as outage-load consumption. The U.S. Energy Information Administration notes that on-site solar can make purchased electricity differ from total household consumption.
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Step 4: Multiply by the target duration
Add the outage-period energy for each load. If you have daily estimates, multiply the daily total by the number of days with no expected recharge:
Target energy before allowances = daily essential-load kWh × outage days
For a six-hour target, multiply a daily estimate by 6/24. For a 36-hour target, multiply by 1.5. Keep the schedule realistic: a pump that runs two hours every day may run longer if a storage tank is empty, and a refrigerator may consume more during a hot summer outage.
Worked example: refrigerator, communications and lights
Using the illustrative values above, a refrigerator (1.9 kWh/day), Wi-Fi (0.5 kWh/day) and lighting (0.8 kWh/day) total 3.2 kWh/day. A 12-hour outage is half a day, so the basic energy estimate is 1.6 kWh before conversion losses and reserve. The battery still needs enough inverter output for the refrigerator’s running load and its compressor-starting event.
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Worked example: a two-day critical-load plan
Using the guide’s approximately 4–6 kWh/day critical-load range, a two-day no-recharge plan requires approximately 8–12 kWh of load energy before system-specific allowances. Tesla’s guide illustrates a 5 kWh/day, two-day case with a 13.5 kWh Powerwall 3 rated for 11.5 kW of power. That is the manufacturer’s example configuration, not a promise that every home using 10 kWh of arithmetic will receive the same runtime or that a 10 kWh-rated battery delivers 10 kWh of usable AC energy.
Step 5: Check usable energy, losses and reserve
Compare your calculated load energy with the system’s published usable energy, not only its nameplate capacity. Batteries and inverters consume some energy while converting and managing power, and systems may retain a reserve or limit discharge to protect the battery. Ask the installer or manufacturer which kWh figure is usable on the AC side, under what conditions, and whether it changes with temperature, power level or battery age.
Do not silently add an arbitrary percentage and present it as a universal rule. Instead, use the manufacturer’s documented usable-energy figure and any stated reserve or efficiency assumptions. If those values are not stated, label your result as a preliminary load estimate rather than a guaranteed runtime.
Step 6: Verify simultaneous running power
Energy answers “how long”; power answers “how many loads at once.” List the appliances that could overlap and add their running demand in watts. Convert to kW and compare with the battery system’s continuous AC output. Include the inverter, transfer equipment and any circuit-level limits in the final design.
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| Question | Energy (kWh) | Power (kW) |
|---|---|---|
| What does it measure? | Total energy consumed over time | Instantaneous rate of delivery |
| What determines it? | Load draw multiplied by operating hours | Loads running at the same moment |
| Typical failure when undersized | Battery empties before the outage ends | Inverter overloads or refuses to start a load |
A battery can have ample kWh yet shut down when a refrigerator, pump and HVAC compressor start together. Conversely, a high-kW system can run heavy loads briefly but run out of stored energy quickly.
Step 7: Account for starting surge
Compressor and motor loads can briefly demand substantially more power at startup than while running. Refrigerators, freezers, well pumps, sump pumps, air conditioners and some furnaces are common examples. Obtain the equipment’s documented starting or locked-rotor requirement where possible, and ask the installer to confirm that the specific inverter can supply it.
Tesla’s Powershare installation documentation warns that motor inrush can overload that system and calls for a home load calculation and compliance with local codes. Those published thresholds apply only to that configuration; do not generalize them to another battery or inverter. A qualified installer may use load management, staggered starts, a soft-start device or a different circuit arrangement, but the remedy depends on the equipment and system.
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What changes the size most?
Refrigerators and freezers
They are often essential but cyclical. Use measured daily kWh or a manufacturer estimate when available. Keep the compressor’s starting surge in the power calculation, and remember that a second freezer or an older appliance changes both energy and startup requirements.
Well pumps and sump pumps
Pumps combine meaningful daily energy with high starting demand. Estimate actual run time from pressure-tank behavior and household water use rather than assuming continuous operation. Confirm horsepower, voltage, phase and startup current with the installer. A battery that can run lights may still be unable to start a pump.
Heating and air conditioning
HVAC can dominate both kWh and kW. Decide whether the outage plan requires whole-home temperature control, one mini-split, a furnace blower or only a safe-temperature strategy. Heat pumps in cold climates can be challenging for solar-plus-storage backup because of the storage required, although they generally perform better than electric-resistance heating. This finding comes from Lawrence Berkeley National Laboratory’s 2023 modeled study of representative building stock in ten U.S. locations; it is a scenario result, not a personalized forecast.
Electric cooking and water heating
Ranges, ovens, electric resistance water heaters and clothes dryers use high power while operating. Excluding them can make a partial-home system practical; including them may require load management, a larger inverter and substantially more kWh. Decide whether the outage plan uses a gas appliance, a small induction unit, stored hot water or another limited-duty option, and model that schedule explicitly.
Medical and communications equipment
Identify medically necessary devices by their actual wattage and hours, including any heated humidifier or battery charger. Routers, modems and network switches are small individually but run continuously. Keep a separate runtime calculation for life-safety equipment and follow the equipment maker’s backup-power requirements.
Solar recharge: a separate scenario, not a free capacity upgrade
If solar is expected to recharge the battery, calculate a second case rather than subtracting an optimistic daily solar number from the battery requirement. Solar output depends on location, array size, orientation, season, shading and weather. Tesla cautions that storms or snow may produce minimal generation, exactly when backup power is most valuable.
- Verify that the solar inverter and battery are configured to operate while the grid is down.
- Check whether the battery can accept the array’s output while serving household loads.
- Model at least a poor-weather day instead of assuming clear-sky production.
- Ask how the system prioritizes essential loads, battery charging and export restrictions during an outage.
A design that works only when the array produces its best daily output is not a two-day backup design. Keep enough stored energy for the no-recharge case unless your risk plan explicitly accepts a shorter runtime in severe weather.
Partial-home or whole-home backup?
Partial-home backup sends power to selected circuits, usually through a backup gateway, transfer switch or load center. It lets you reserve energy for refrigeration, lighting, communications and chosen pumps while excluding large discretionary loads. Whole-home backup keeps more circuits available but raises the simultaneous-power, surge and energy requirements and may require load-shedding controls.
Manufacturer documentation describes both approaches and notes that climate, home orientation and energy-use goals make the final design site-specific. A Tesla system, for example, may use a Backup Gateway or Backup Switch, with configuration depending on the model, solar equipment and site. Treat these as system-specific installation facts, not universal architecture for every battery.
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Capacity alone is an incomplete comparison. Use the following axes when reviewing a residential battery or solar-plus-storage proposal:
| Comparison axis | What to verify |
|---|---|
| Usable energy | AC-side kWh available to your selected loads, including stated reserve and efficiency conditions |
| Continuous output | kW available for the circuits that may run simultaneously |
| Motor starting | Documented surge capability for refrigerator, HVAC and pump startup |
| Backup scope | Exact circuits in partial-home backup or whether whole-home load shedding is included |
| Recharge behavior | Solar operation during grid outages, poor-weather assumptions and charge-rate limits |
| Electrical compatibility | Voltage, phase, service rating, transfer equipment, utility approval and local-code requirements |
| Expansion | Whether added battery units increase usable energy, output, or both; the manufacturer must state the limits |
Powerwall 3 is one manufacturer example cited in Tesla’s guide: 13.5 kWh of storage and 11.5 kW of power. The same guide describes a 13.5 kWh expansion unit that shares power from the lead unit. Verify current specifications and the effect of expansion for the exact installation; an added kWh figure does not by itself prove greater surge capability or compatibility.
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Installation and electrical checks
Residential battery systems are not plug-in appliances. Before committing to a design, have a qualified installer perform a load calculation and identify the backed-up circuits. The design may require a gateway or transfer switch, service-panel work, utility approval, permits, grounding and compliance with local electrical and fire codes.
- Provide the installer with appliance model numbers, motor horsepower, voltage and any available startup data.
- Show which loads must operate together, such as a well pump and pressure-control equipment.
- Confirm whether solar is allowed to operate in islanded mode during an outage.
- Ask for the usable AC kWh, continuous kW and surge rating in writing.
- Request the expected behavior when the battery is low: automatic load shedding, shutdown thresholds and restart sequence.
Manufacturer guidance recommends consulting an advisor or certified installer for a final design. That is appropriate because circuit selection, site conditions and local rules cannot be resolved from a battery label alone.
Efficiency and flexibility can reduce the required battery
The Lawrence Berkeley National Laboratory’s November 2023 study modeled statistically representative buildings in ten U.S. locations. It found that efficiency measures and load flexibility can reduce storage needs or extend the interruption conditions a system can cover. Practical examples include improving building insulation, pre-cooling or pre-heating when power is available, lowering water-heater demand, and scheduling flexible loads outside the backup window.
These are scenario findings, not a calculation for a particular address. They are most useful before purchasing: reducing the essential-load list or its duty cycle can be less expensive than adding storage, while a highly electrified home may need both more energy and more inverter power.
National averages are context, not a sizing answer
National statistics help explain why household bills cannot determine a battery size by themselves:
- The U.S. Energy Information Administration reports 10,791 kWh purchased per residential electric-utility customer in 2022, about 899 kWh per month. On-site solar can make purchased electricity differ from total household consumption.
- EIA reports approximately 5.5 hours of electricity interruptions per U.S. customer in 2022. That national average includes major events and does not predict the duration or frequency at a particular home.
- Tesla’s 2025 guide reports approximately 10,500 kWh in 2023, or about 29 kWh per day, citing an EIA figure. Keep that 2023 figure separate from EIA’s 2022 average-purchases statistic; they are different years and measures.
Your outage plan should use the loads you intend to protect and the duration you are willing to cover, not an annual national average.
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Using the whole utility bill as the outage load
Problem: Average daily household purchases include loads you will not run during an outage.
Correction: Build a circuit-level essential list and calculate only those loads.
Multiplying every nameplate wattage by 24 hours
Problem: Cycling appliances appear much larger than their actual daily energy use.
Correction: Use measured or manufacturer daily kWh, while retaining nameplate and surge data for the power check.
Checking kWh but not kW
Problem: The battery has enough stored energy but the inverter trips when loads overlap.
Correction: Add simultaneous running watts and compare with continuous output.
Ignoring motor inrush
Problem: A refrigerator, pump or compressor fails to start even though its running wattage looks acceptable.
Correction: Obtain startup requirements and have the installer verify surge capability and any soft-start or load-management solution.
Assuming solar always recharges
Problem: The plan works on a clear day but fails during storm clouds or snow.
Correction: Keep a no-recharge case and verify that the solar-plus-storage configuration can operate while the grid is down.
Treating a product example as a universal answer
Problem: A cited 13.5 kWh battery is presented as suitable for every home.
Correction: Recalculate for your load list, duration, usable energy, simultaneous output and startup surge.
A worksheet you can hand to an installer
- Write the target outage duration and whether the base case assumes no solar recharge.
- List every essential appliance and the circuit that supplies it.
- Record measured or manufacturer daily kWh; if unavailable, record watts and expected hours.
- Calculate each load’s outage-period kWh and add them.
- Mark which loads can overlap and add their running watts to obtain the simultaneous kW requirement.
- Record compressor, pump or other motor startup requirements.
- Compare the results with the proposed system’s usable AC kWh, continuous kW and surge rating.
- Confirm backup topology, solar-islanding behavior, transfer equipment, permits, utility approval and local-code compliance.
- Run a poor-weather solar scenario and a no-recharge scenario.
- Ask for the expected runtime and load-shedding behavior in writing, with the assumptions attached.
Bottom line
Size the battery to a stated load list and outage duration, not to a generic household number. Add the selected appliances’ kWh for the energy requirement; separately verify continuous kW for overlapping loads and surge capability for motors. Then account for usable AC energy, reserve, solar uncertainty, circuit configuration and local installation rules. That process tells you whether a refrigerator-only plan, a 4–6 kWh/day critical-load plan, or a larger whole-home design is realistic without pretending that any single battery capacity fits every house.
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