Home Battery Sizing Calculator

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

Battery federal credit eligibility was affected by Public Law 119-21 (OBBBA, July 2025). The calculator shows a 30% ITC estimate for batteries paired with solar — this is based on the surviving Section 25D provisions, but eligibility for solar-paired batteries in 2026+ depends on the specific IRS guidance issued under OBBBA. Standalone batteries without any solar connection do not qualify. See the IRS OBBBA FAQ and consult a tax professional before filing.

Tax-credit rules updated May 2026, reflecting OBBBA Public Law 119-21. Battery model specifications sourced from manufacturer data, verified May 2026. Residential electricity rate benchmarks from EIA state electricity data.

Last reviewed: May 2026.

The Homeowner's Guide to Home Battery Storage

Home battery storage has moved from a niche product into a mainstream purchase decision, driven by a combination of falling prices, more frequent grid outages, and the rapid adoption of residential solar. But the market is still confusing enough that most homeowners end up buying the wrong system — either too small to matter or far more expensive than their actual needs require. The single most important thing you can do before calling an installer is understand what problem you are actually trying to solve.

Backup power vs. daily cycling — two different batteries

The question "how big a battery do I need?" has two completely different answers depending on why you are buying it. If your goal is grid outage resilience — keeping your refrigerator running, your medical devices powered, and your router connected during a storm or a planned utility shutoff — you are sizing for backup. If your goal is to capture every kilowatt-hour your solar panels produce and use it after dark instead of buying power from the grid, you are sizing for daily cycling. These are not the same calculation, and confusing them is the most common mistake homeowners and even some installers make.

A backup-only battery is sized around your critical loads and your target outage duration. It may sit at full charge for weeks or months between uses, experiencing relatively few full discharge cycles over its lifetime. A daily-cycling battery, by contrast, goes through a complete charge-discharge cycle every single day — roughly 365 cycles per year. That difference in cycle count affects which products make economic sense, what warranty terms to pay attention to, and how deeply you should discharge the battery on a regular basis.

The most popular configuration — and the one most installers will quote you — is a hybrid: solar paired with a battery that handles both daily self-consumption and provides backup when the grid goes down. This is a great system, but it is also the most demanding to size correctly. It needs to be large enough to absorb a meaningful portion of your daily solar production, large enough to cover your critical loads through an overnight outage, and ideally sized to a standard product unit count rather than some awkward fraction. We will walk through both use cases.

Nameplate capacity vs. usable capacity — the number that matters

Every battery product has two capacity numbers: the nameplate (or gross) capacity printed on the spec sheet, and the usable capacity — the amount of energy you can actually draw on. Manufacturers deliberately reserve 10 to 20 percent of the total battery capacity to protect the cells from the stress of operating at the extremes of their charge range. Running a lithium cell fully dead and then charging it to 100 percent on every cycle dramatically shortens its life. The reserved buffer is what keeps your ten-year warranty achievable.

This distinction matters enormously when you are comparing products. A battery advertised as 16 kWh may deliver only 13.5 kWh of usable energy. Another product at 15 kWh nameplate may offer 14 kWh usable. If you compare nameplate numbers, you will draw the wrong conclusion. Always ask for the usable kWh figure and compare those.

Here are the usable figures for the major residential products as of mid-2026. The Tesla Powerwall 3 offers 13.5 kWh usable per unit, stackable up to four units for a maximum of 54 kWh. The Enphase IQ Battery 5P provides 5.0 kWh usable per module, with a maximum of four modules per system (20 kWh total). The Franklin aPower 2 delivers approximately 15 kWh usable per unit. These numbers come from manufacturer specifications and should be verified at the time of your purchase, since manufacturers do update firmware that can adjust usable capacity.

Round-trip efficiency is a related figure that matters primarily for daily-cycling applications. A battery with 90 percent round-trip efficiency loses 10 percent of every kilowatt-hour it stores to heat during charging and discharging. If your solar panels push 20 kWh into the battery over a day, only 18 kWh comes back out. Over a year, that inefficiency adds up to a real cost. Most modern lithium iron phosphate (LFP) and nickel manganese cobalt (NMC) batteries achieve 90 to 95 percent round-trip efficiency — good enough that it is usually not the deciding factor, but worth checking when comparing similar-priced products.

Sizing for backup — critical loads vs. whole home

The math for backup sizing is straightforward once you have identified which loads you actually need to protect. Start by listing every appliance you want to keep running during an outage, and look up or measure its wattage. A standard refrigerator draws roughly 150 watts continuously. A home internet router runs at about 30 watts. LED lighting for a few rooms might total 150 to 300 watts depending on how many fixtures you leave on. A gas furnace's blower motor — the fan that distributes heat even when the furnace burner is gas-fired — typically draws 400 to 800 watts while running.

Once you have your load list, multiply each appliance's wattage by the number of hours per day it runs, then sum those watt-hours. A refrigerator at 150 watts running 24 hours contributes 3,600 watt-hours. A well pump at 1,500 watts running two hours a day contributes 3,000 watt-hours — less total energy than the refrigerator, despite its much higher wattage, because it only runs intermittently. This distinction between continuous draw and duty cycle is what this calculator is doing when you enter hours per day for each load.

After summing your watt-hours, divide by 1,000 to convert to kilowatt-hours, then add a 20 percent safety margin to account for motor startup surges. Motors — well pumps, furnace blowers, window AC units — draw two to three times their running wattage for a half-second when they start. The battery's peak power output rating (in kilowatts) must be able to handle these surges, and your usable capacity estimate needs a cushion to absorb them gracefully. As a practical example: 1,000 watts of continuous critical loads running 24 hours produces 24,000 watt-hours, or 24 kWh. With inverter losses and a depth-of-discharge buffer, you need roughly 29 kWh of usable battery — two Powerwall 3 units, or six Enphase 5P modules.

Whole-home backup is tempting to think about but rarely practical at reasonable cost. A typical American home draws between 1.2 and 2 kilowatts continuously when averaged across a day. Over 24 hours, that is 30 to 50 kilowatt-hours. Add air conditioning on a summer day or electric heat on a winter night and the number climbs fast. Four Powerwall 3 units at 54 kWh usable total might cover a modest home for a day — at an installed cost in the range of $40,000 to $55,000 before any credits. Most homeowners find the critical-loads approach far more rational: protect the 10 to 15 things that genuinely matter, stay comfortable and safe for 24 to 72 hours, and let the rest of the house be dark until the grid comes back.

Sizing for solar and battery daily cycling

When a battery is paired with solar for daily energy management, the sizing question changes from "how long do I need to last through an outage?" to "how much of my solar production can I capture and use after dark?" These are related but different problems.

The practical rule of thumb for daily-cycling sizing: match your battery's usable capacity to roughly half of your daily solar production, or to your average evening-and-overnight consumption — whichever is smaller. If your 8-kilowatt solar system produces 35 kWh on a sunny day and you use 18 kWh between sunset and the next morning, you need about 18 kWh of usable battery to capture your full self-consumption opportunity. In reality, you might size to 13.5 kWh (one Powerwall 3) and accept that you will export some excess solar to the grid on good production days. That is often the right economic call.

If your utility offers time-of-use (TOU) pricing — lower rates during off-peak hours and higher rates during evening peak hours — a battery can participate in arbitrage even without solar. The battery charges from the grid at 3 cents per kilowatt-hour at 2 AM and discharges into your home at 7 PM when your utility would otherwise charge you 22 cents. The economics depend on the spread between your off-peak and on-peak rates. Check your utility tariff before assuming this strategy works — some utilities have closed the spread specifically to prevent battery arbitrage.

For households doing daily cycling, pay particular attention to the product's cycle life warranty. A battery warrantied for 4,000 cycles at 70 percent end-of-life capacity will perform better over ten years of daily cycling than one with 3,000 cycles at the same degradation threshold. Divide the warranted cycle count by 365 to get a rough sense of how many years of daily cycling the manufacturer is standing behind.

The federal tax credit situation for home batteries

The tax credit picture for residential home batteries changed significantly in 2025. The One Big Beautiful Bill Act (Public Law 119-21, signed July 4, 2025) terminated the Section 25D residential clean energy credit for expenditures made after December 31, 2025. That credit had previously covered both residential solar and batteries paired with solar at a 30 percent rate. As of 2026, the credit no longer applies to new residential solar or solar-paired battery installations.

What survived is limited. The Section 25D geothermal heat pump provision remains active through 2032 at a 30 percent rate. Batteries paired specifically with a geothermal heat pump system may qualify under that surviving provision — but this is a narrow case that applies to a small fraction of homeowners. Standalone batteries without any solar or geothermal connection do not qualify for any federal residential credit under current law.

The calculator on this page shows a 30 percent ITC estimate when you indicate a solar-paired installation. That figure reflects the pre-OBBBA credit rate and is displayed for informational context, not as a guarantee of eligibility. The IRS has published guidance clarifying which property types remain eligible under the surviving 25D provisions. Before making any filing decisions, consult the IRS OBBBA FAQ and a licensed tax professional who can review your specific situation.

The practical implication for your purchase decision: do not count on a tax credit when building your battery budget for a 2026 or later installation. Size for what makes financial sense at full cost. If a credit does apply to your situation, treat it as a bonus that improves your return — not a prerequisite that makes the project viable.

Top battery products — what to know before you buy

The residential battery market has consolidated around a few dominant products, with meaningful differences in how they integrate with solar, what kind of installation they require, and what they cost.

The Tesla Powerwall 3 is the current flagship from the brand that effectively created the consumer battery market. Each unit provides 13.5 kWh of usable storage and includes a built-in hybrid inverter — meaning it can connect directly to your solar panels on the DC side and handle the AC conversion in one box. This simplifies installation and eliminates a separate inverter purchase, but it also means you are committing to Tesla's ecosystem. The Powerwall 3 handles peak power output of 11.5 kilowatts, enough to manage most motor startups without issue. Installed cost typically runs $11,000 to $14,000 per unit depending on region, existing panel capacity, and whether trenching is required.

The Enphase IQ Battery 5P takes the opposite approach: a modular 5 kWh unit that you can stack up to four of, starting smaller and expanding as your needs or budget allow. Enphase integrates tightly with its own IQ8 microinverter solar systems, and the whole system is managed through the Enphase app and Envoy hub. If you already have an Enphase solar system or are installing one, the 5P is a natural complement. It also works as an AC-coupled add-on to other solar systems. Installed cost per module typically runs $4,500 to $6,000 before any incentives, making a two-module (10 kWh) system comparable in price to a single Powerwall 3 while offering more flexibility.

The Franklin aPower 2 is a newer entrant worth considering, particularly for buyers who want more usable capacity per unit than the Powerwall offers. The aPower 2 delivers approximately 15 kWh usable in a DC-coupled configuration. Franklin has been aggressive on pricing, and in competitive markets their installed cost has come in meaningfully below Tesla. The caveat is installer availability — Franklin's certified installer network is smaller than Tesla's or Enphase's, which can make service and warranty support more variable depending on where you live.

Pricing in this market moves faster than in almost any other major home improvement category. The estimates above should be treated as directional — get quotes from at least two installers for any product you are seriously considering, and ask each one to break out equipment cost, installation labor, permit fees, and any electrical panel upgrade costs separately. That breakdown will tell you more than a single installed-price number.

How to read your sizing results

The calculator above produces a "required usable capacity" number in kilowatt-hours. This is your minimum — the smallest amount of usable storage that can cover your selected loads for your chosen backup duration, after accounting for inverter losses and depth-of-discharge buffer. Round up to the next available product configuration, not down. A system that falls 0.8 kWh short of your requirement will leave you without power before your target backup window expires.

When reviewing the recommended products, look at two numbers together: the total usable kWh and the peak power output in kilowatts. The kWh number tells you how long the system can run your loads. The kW number tells you whether the system can handle your loads simultaneously — specifically, whether it can absorb the startup surge when your well pump or furnace blower kicks on. A system with plenty of energy storage but insufficient peak power will trip its overload protection the first time your pump starts. For most critical-loads backup setups, look for at least 7 to 10 kW of peak output.

Installation costs deserve serious attention because they vary more than the equipment itself. In a straightforward installation — a modern electrical panel with available capacity, a garage or utility room location, no trenching required — labor and permitting might add $2,000 to $3,500 to your total. In a more complex situation — an older panel that needs upgrading, a difficult mounting location, conduit runs through finished walls — those costs can reach $5,000 to $8,000 or more. Installation represents 30 to 50 percent of total project cost in many cases. Ask your installer for a written scope of work before signing anything, and make sure you understand what is and is not included in their quote.

Finally, talk to your utility before installing. Some utilities require notification or even approval before connecting a battery system to their grid. Others offer demand response programs that will pay you to let the utility draw on your battery during peak grid stress events — a revenue stream that can meaningfully improve your system's economics over time. Your installer should know your local utility's interconnection requirements, but it is worth verifying independently.