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Size a whole-house battery in two dimensions: usable energy capacity (kWh) for runtime and power output (kW) for the loads that can operate together. Then check motor-starting surge, backup-panel limits, solar recharge, and future loads. A battery with enough kWh can still fail to start an air-conditioner or well pump if its inverter lacks sufficient continuous or peak power.
The two numbers that determine whether backup works
Usable kWh is your runtime budget. It represents the energy available to your backed-up loads. If the loads consume 10 kWh per day and the battery has 20 kWh usable, the arithmetic indicates about two days before accounting for conversion losses, operating limits, weather, or reserve settings.
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Continuous kW is the simultaneous-load limit. It determines how much power the inverter can deliver while appliances are running. Peak or inrush capability handles the brief surge when a compressor, pump, or other motor starts. As Tesla explains, energy in kWh indicates how long a battery can run, while power in kW determines how many things it can run at the same time. EnergySage similarly distinguishes peak power, which supplies startup surges, from continuous power for steady loads.
| Question | Measurement to use | What it controls |
|---|---|---|
| How long should backup last? | Usable kWh | Runtime before the battery is depleted or reaches its reserve limit |
| Can several appliances run together? | Continuous kW | Total steady demand at one moment |
| Will a motor start? | Peak kW, locked-rotor amps (LRA), or equivalent surge rating | Short startup demand from compressors, pumps, and similar equipment |
Step 1: Establish your daily energy use
Start with utility-bill data
Add the kWh shown on a recent bill and divide by the number of days in that billing cycle:
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average daily use = billed kWh ÷ billing-cycle days
Tesla’s example is 900 kWh over 30 days, or 30 kWh per day. For a U.S. reference point, the U.S. Energy Information Administration reports 10,500 kWh of annual household electricity use in 2023, which is about 29 kWh per day when averaged across a year. That national figure is a benchmark, not a design value for your house.
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Use the right season
A yearly average can hide the conditions that matter during an outage. Air-conditioning, electric resistance heat, heat pumps, pool pumps, and other seasonal equipment can raise consumption substantially. Review bills from the hottest and coldest months, and identify whether your planned outage season is likely to coincide with a high-use period.
If your bill combines multiple buildings, a detached workshop, or an electric vehicle, separate those loads where possible. Conversely, do not remove a load merely because it is inconvenient; if you expect the battery to run it during an outage, include it in the design case.
Step 2: Choose an outage target before choosing a battery
Model a short interruption
For an outage measured in hours, multiply the daily energy you intend to use by the outage duration divided by 24:
target energy = outage-use kWh per day × outage hours ÷ 24
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Model a multi-day event
For an outage lasting days, multiply the planned daily outage use by the number of days. At 12 kWh per day, two days requires 24 kWh before the design accounts for conversion losses, temperature, reserve settings, or charging limitations.
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The EIA reports an average U.S. outage duration of 5.5 hours for 2022, but that average is not a safe whole-home guarantee. Severe hurricanes and other major events can leave homes without grid service for more than two weeks. Decide whether you are designing for an ordinary interruption, a one- or two-day storm, or an extended emergency. The longer the target, the more important solar recharge and load conservation become.
Define what “whole house” means for your project
Some owners mean every circuit, including HVAC, electric cooking, pumps, and an EV charger. Others mean the entire house wiring with automatic management that prevents selected large loads from operating together. Write down the intended outcome before requesting quotes; otherwise, two contractors may use “whole-home backup” to describe different systems.
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Record energy by appliance
List refrigerators and freezers, internet equipment, lighting, cooking appliances, heating and cooling, well or sump pumps, water heating, laundry, garage equipment, medical devices, and EV charging. For each item, record its wattage, expected hours per day, and whether it has a motor or compressor.
Tesla’s critical-load examples illustrate the method: a refrigerator uses about 1.9 kWh per day, Wi-Fi about 0.5 kWh per day, 70 W of lighting operated for 12 hours about 0.8 kWh per day, a cooking range about 1.4 kWh per day, and a 1-horsepower well pump operated for two hours about 2 kWh per day. That selected group totals roughly 4–6 kWh per day. Actual consumption varies with appliance efficiency, thermostat settings, duty cycle, and household behavior.
| Load category | What to capture | Why it matters |
|---|---|---|
| Refrigeration | Running watts, startup surge, hours of compressor operation | Usually modest daily energy but a motor-start event |
| Heating and cooling | Equipment type, rated input, heating or cooling season, thermostat strategy | Often the largest continuous and daily loads |
| Pumps | Horsepower, voltage, running amps, LRA, starts per hour | High inrush can trip an inverter even when daily kWh is low |
| Cooking and water heating | Electric range, induction, oven, resistance or heat-pump water heater | Large coincident demand can exceed continuous kW |
| EV charging | Charger amperage, charging schedule, desired miles per outage day | Can consume more energy than essential household loads |
| Networking, lighting, and controls | Always-on watts and planned operating hours | Small individual loads become significant over several days |
Measure instead of guessing when the load is uncertain
Use appliance nameplates, electrical schedules, smart-meter data, or a qualified electrician’s measurements. A nameplate may show maximum input rather than typical consumption, so distinguish rated power from expected duty cycle. For HVAC, pumps, and compressors, obtain both running and startup information from the equipment documentation or installer.
Step 4: Check continuous demand and startup surge separately
Add likely simultaneous running loads
Choose a realistic worst-case combination, not just the sum of every device in the house. For example, during a summer outage the central heat pump may run while the refrigerator, lighting, network equipment, and a well pump operate. Add their running watts and convert to kW:
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simultaneous running kW = (running watts for coincident loads) ÷ 1,000
Compare that result with the battery system’s continuous off-grid output. A 200-amp service panel does not make 200 amps available from the battery; the inverter, transfer equipment, and configured backup circuits impose their own limits.
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Check the largest motor start
Startup demand can be several times running demand. EnergySage gives a refrigerator example of roughly 100–250 W while running with up to an 800 W startup surge. It lists a central heat pump at approximately 2,000–5,000 W and a television at about 60–150 W. These are reference ranges, not measurements of your specific equipment.
Check each compressor and motor’s LRA or startup specification, then compare the aggregate starting scenario with the inverter’s peak rating. If two large motors could start at once, ask the installer how the system handles that coincidence rather than checking only one appliance.
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Tesla’s Powerwall 3 documentation states that each unit supports a largest backup load or breaker up to 60 A and provides 185 A of LRA motor-inrush capability. If an air-conditioner or other motor exceeds the aggregate limit, the documented options include load shedding, moving that load outside the backup circuit, or adding Powerwalls. Those limits apply to that product’s documented configuration; use the current datasheet for any battery you are considering.
Step 5: Select whole-home or partial-home architecture
Whole-home backup
A whole-home design uses transfer and backup equipment so the house can be powered during a grid outage. Tesla describes its Powerwall 3 whole-home system as storing energy from the grid or solar and powering the entire home during an outage. “Entire home” does not mean unlimited simultaneous operation: inverter output, breaker limits, motor-start capability, and load-management rules still govern what can run.
Partial-home backup
A partial-home design backs up selected circuits, such as refrigeration, lighting, networking, a furnace blower, and a sump pump, while excluding electric ranges, dryers, water heaters, or EV chargers. This can reduce both required kWh and inverter power. It may also be the practical choice where the service equipment, local code, or an oversized motor makes unrestricted whole-home operation impractical.
Verify the transfer equipment and code path
Ask the installer to identify the backed-up panel, transfer switch or gateway, breaker arrangement, neutral and grounding treatment, and permitted location for batteries and disconnects. The quote should state whether the design is whole-home or partial-home, which circuits are included, and which loads are intentionally excluded or controlled.
Step 6: Use load management to control peaks
Energy-management systems can sequence appliances or shed nonessential loads when demand approaches the inverter limit. A controller might delay a water heater while a heat pump is running, disconnect an EV charger during a motor start, or prevent two large loads from operating together.
- Load shedding: turns off a selected circuit when the system approaches its power limit.
- Load sequencing: starts appliances in an order that avoids coincident surges.
- Scheduled operation: restricts discretionary loads to periods when solar production or battery state of charge is favorable.
- Manual conservation: gives occupants a clear outage operating plan for cooking, laundry, and charging.
Management can reduce the inverter power you need, but it does not create additional energy. If a heat pump still runs for many hours, the battery must have enough usable kWh for that runtime. Expansion batteries generally add duration; they add inverter capacity only when the system architecture adds the corresponding power hardware.
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Step 7: Plan how the battery will recharge
Grid-only systems
EnergySage notes that a standalone battery cannot recharge until the grid returns. Such a system can cover the modeled outage duration, but it cannot extend a multi-day event without another charging source or a deliberate reduction in loads.
Solar-coupled systems
Tesla states that solar can serve daytime loads and recharge the battery during an outage. For multi-day resilience, compare expected storm-season solar production with daytime consumption and the energy you need to reserve for overnight loads. Do not assume normal solar output during smoke, snow, hurricanes, heavy cloud, or a utility safety shutoff. Solar production may also be limited by the system’s islanding and charging configuration.
A useful planning test is to model a poor-solar day, not only a clear day. If daytime loads consume all available generation, there may be little energy left to restore the battery. Decide whether discretionary loads such as EV charging, laundry, and electric cooking should be disabled until the battery reaches a chosen state of charge.
Step 8: Leave room for future loads
Tesla recommends planning for changes such as an EV, electric water heating, induction cooking, a pool, or other household additions. Include projects that are already budgeted and identify likely future upgrades separately. A system sized tightly to today’s bills may require another inverter or a different backup-panel design later, not merely additional battery modules.
Worked sizing examples
Essentials for two days
Assume your outage plan uses 5 kWh per day for refrigeration, communications, lighting, a gas-furnace blower, and limited cooking. For two days:
5 kWh/day × 2 days = 10 kWh
Tesla uses this 10-kWh example and states that one Powerwall 3, rated at 13.5 kWh and 11.5 kW, covers that example. It is an illustration of the arithmetic, not a guarantee that one unit will run every home’s HVAC, pumps, cooking, or other large loads for two days.
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Tesla states that operating large loads can require 40–90 kWh over two to three days. That range demonstrates why a home with sustained HVAC, electric cooking, pumps, and EV charging may need several units, aggressive load management, solar recharge, or a reduced outage-load plan. The correct quantity depends on your measured daily use and simultaneous power calculation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Do not mix specifications between battery models
Compare the exact model and current datasheet. Tesla’s Powerwall 2 specification lists 13.5 kWh usable energy, 7 kW maximum continuous off-grid power, 10 kW peak power for 10 seconds, and 90% round-trip efficiency at beginning of life. Tesla’s Powerwall 3 sizing example uses 13.5 kWh and 11.5 kW. Those figures should not be combined as if they describe one device.
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| Specification | Powerwall 2 value | How to use it |
|---|---|---|
| Usable energy | 13.5 kWh | Compare with your outage-energy target |
| Maximum continuous off-grid power | 7 kW | Compare with coincident running loads |
| Peak power | 10 kW for 10 seconds | Compare with documented startup events |
| Round-trip efficiency | 90% at beginning of life | Use the qualified manufacturer figure when estimating delivered energy |
Compare quotes using the same checklist
| Quote item | Question to ask |
|---|---|
| Usable capacity | How many usable kWh are available, and what reserve or operating limits apply? |
| Continuous output | What is the continuous off-grid kW rating for the proposed configuration? |
| Peak and inrush | What surge rating, duration, and motor-start limits apply? |
| Backup architecture | Is this whole-home or partial-home, and which breakers are included? |
| Solar recharge | Can solar operate during an outage, and what happens during low production or a grid safety shutoff? |
| Expansion | How many additional batteries or inverters can the system accept? |
| Efficiency and temperature | What round-trip efficiency, operating-temperature range, and derating conditions are stated? |
| Warranty | What energy-throughput, capacity-retention, and calendar limits apply? |
| Installation and compliance | Who handles permits, utility approval, inspection, and code-compliant disconnects? |
| Total installed cost | What equipment, labor, electrical upgrades, permits, and jurisdiction-specific incentives are included? |
Request that every quote use the same outage scenario and load list. Otherwise, a lower price may simply reflect fewer backed-up circuits, a smaller inverter, or no provision for motor starting.
Common sizing mistakes and fixes
Mistake: sizing only by kWh
Symptom: the battery appears large enough on paper, but the inverter trips when HVAC or a pump starts. Fix: calculate coincident continuous kW and startup surge separately.
Mistake: treating the service-panel rating as battery capacity
Symptom: a 200-A panel is assumed to mean the battery can supply every circuit at full output. Fix: use the inverter, transfer equipment, breaker, and load-management limits in the design.
Mistake: using annual average consumption for a peak-season outage
Symptom: the design works in mild weather but depletes quickly during a heat wave or freeze. Fix: use seasonal bills and a high-use outage scenario.
Mistake: counting solar as guaranteed recharge
Symptom: the battery is expected to recover every day despite smoke, storms, snow, or a utility shutoff. Fix: model poor-solar days and set a conservation plan.
Mistake: adding batteries without checking power hardware
Symptom: extra storage increases runtime but does not solve an inverter overload. Fix: determine whether expansion adds inverter capacity or only usable kWh; use load management or additional power hardware when needed.
Mistake: ignoring future electrification
Symptom: a new EV, induction range, heat-pump water heater, or pool makes the original backup plan inadequate. Fix: identify near-term projects and leave an expansion path in the electrical design.
A practical handoff for your installer
- Provide 12 months of utility bills, highlighting the highest-use season.
- State the outage target in hours or days and identify the loads that must remain on.
- Supply equipment model numbers or nameplate data for HVAC, pumps, water heating, cooking, and EV charging.
- Ask for a written simultaneous-load calculation and a motor-start or LRA check.
- Require the quote to identify whole-home versus partial-home coverage, backed-up breakers, and load-management behavior.
- Request a solar-recharge scenario for a poor-weather day if multi-day resilience is important.
- Confirm permits, utility requirements, operating-temperature limits, warranty terms, expansion limits, and total installed price for your jurisdiction.
Bottom line
Choose enough usable kWh for the outage duration you actually want, enough continuous kW for the loads that will run together, and enough peak capability to start the largest motors. Then verify the backup architecture, load management, solar-recharge assumptions, and future-load margin. A professional design that documents all four—energy, continuous power, startup power, and system controls—will produce a more reliable whole-house backup than selecting a battery from its kWh label alone.
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