Home Battery Sizing Calculator

Find out exactly how much battery storage you need to keep your lights on, your refrigerator running, and your medical devices powered during a grid outage. Select your critical loads and backup duration — the calculator handles the rest, including inverter efficiency, depth-of-discharge limits, and federal tax credit eligibility.

Select the loads you need to power during an outage

Peak load: 260 W
EnableApplianceWattsHrs/dayRemove
Refrigerator150 W
Internet / Router30 W
Lights (LED, 5 fixtures)50 W
Furnace Blower600 W
Well Pump1500 W
Window AC Unit1000 W
CPAP Machine60 W
Oxygen Concentrator350 W
Sump Pump800 W
Phone / Tablet Charging30 W

How This Calculator Works

Watt-Hour Formula

For each enabled load, we calculate the energy consumed during your backup window:
Wh = wattsContinuous × hoursPerDay × (backupDuration / 24)
A refrigerator rated at 150 W that runs around the clock contributes 150 × 24 = 3,600 Wh over a 24-hour backup period. A well pump rated at 1,500 W that runs 2 hours per day contributes only 1,500 × 2 = 3,000 Wh — even though its wattage is much higher — because it cycles infrequently.

Inverter Efficiency (92%)

Battery systems store energy as DC but your home runs on AC. The conversion through the inverter is not lossless — we apply a 92% efficiency factor consistent with modern string and hybrid inverters. This means the battery needs to store roughly 8% more energy than your loads will actually consume.

Depth-of-Discharge Buffer (90%)

Lithium battery cells last longest when they are not fully drained on every cycle. Most manufacturers recommend a 10–20% state-of-charge floor to preserve long-term cycle life. We apply a 90% usable-capacity assumption, meaning a battery rated at 10 kWh should reliably deliver 9 kWh per cycle. The combined efficiency formula is:
Required kWh = total Wh ÷ 1,000 ÷ (0.92 × 0.90)

Model Selection Logic

The calculator finds every battery model that can meet your required capacity within its maximum stackable unit count. Models are shown sorted by estimated installed cost (ascending) so you can immediately compare the most affordable option against premium alternatives. If no single model can cover your load within its stacking limit, the model with the highest achievable capacity is shown with a note.

Federal ITC Eligibility

Under the Inflation Reduction Act, a home battery qualifies for the 30% Residential Clean Energy Credit (ITC) when installed alongside a solar system. The battery does not need to be installed at the exact same time as solar — batteries added to an existing solar system also qualify. Standalone batteries (without any solar) are not eligible under current law. The 30% credit rate applies through 2032. Consult a tax professional and see IRS Publication on the Residential Clean Energy Credit .

Frequently Asked Questions

How many kWh do I need for home backup power? ⌄

The answer depends entirely on which appliances you need to power and for how long. A minimal critical-loads setup — refrigerator, internet router, LED lights, and phone charging — typically requires 5–7 kWh for a 24-hour outage. Adding a well pump, furnace blower, or window AC unit pushes that closer to 15–30 kWh or more. Medical equipment like an oxygen concentrator (running around the clock) is a major contributor despite its modest wattage. Use the checklist above to get a number specific to your home.

What is the difference between the Tesla Powerwall and Enphase IQ Battery? ⌄

The Tesla Powerwall 3 stores 13.5 kWh per unit with up to four units stackable (54 kWh total). It includes an integrated inverter, which simplifies installation but ties you to Tesla's ecosystem. The Enphase IQ Battery 5P offers 5 kWh per module, also stackable up to four units (20 kWh). Enphase's modular approach lets you start smaller and expand; it also works well with any solar inverter brand. Pricing varies significantly by region and installer — always get multiple quotes.

Does the federal tax credit apply to home batteries? ⌄

Yes — when paired with solar. A battery installed alongside a new or existing solar system qualifies for the 30% federal Residential Clean Energy Credit (ITC). The credit applies to the full installed cost of the battery, potentially saving thousands of dollars. Standalone batteries without any solar connection do not currently qualify. The 30% rate is in effect through 2032. Because eligibility depends on your specific tax situation, see IRS guidance and consult a tax professional before filing.

Can I run my whole house on a battery backup? ⌄

Technically yes — economically, probably not at the residential scale. The average US home draws 1.2–2 kW continuously, which sounds modest, but over 24 hours that's 30–50 kWh. A whole-home backup that includes HVAC, water heating, and cooking would require even more. Most homeowners find that a critical-loads approach — powering only the refrigerator, lights, router, and medical devices — delivers most of the value at a fraction of the cost. This calculator defaults to that philosophy: select the things that truly matter during an outage.

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