Home Battery Backup Calculator

Appliance	Watts	Hrs/day	Qty	kWh/day	Remove
Daily total	—			—

📋

Walk-through

How to Use This Calculator

4 steps

1

Build your outage load

In the Load Builder tab, start from a preset — Essentials only, Whole home, or Medical-critical — then add or remove appliances. For each row, set the running watts, the hours per day you expect it to run during an outage (this captures duty cycle, so a fridge that cycles on and off might be 8 effective hours), and the quantity. The calculator sums Σ(watts × hours × qty) into your daily energy need in kWh and shows your continuous and peak-surge watts.

2

Set your battery spec

In Battery & Runtime, enter the nameplate capacity per battery (a Powerwall 3 is ~13.5 kWh), the usable depth of discharge (90% for lithium LFP, ~50% for lead-acid), and round-trip efficiency (~90% for lithium). Usable energy per battery = capacity × DoD × efficiency. Then choose how many days of backup you want without solar — 1–2 days is typical for grid-outage resilience.

3

Read your sizing result

The hero result shows how many batteries you need: ceil(daily need × backup days ÷ usable per battery). The stat cards show usable energy per battery, single-battery runtime, daily need, and total bank size. The runtime-vs-batteries chart shows how adding batteries extends backup hours so you can trade cost against autonomy.

4

Check the inverter and add solar

In Inverter & Solar, enter your inverter's continuous and surge watt ratings. The calculator flags ✓ or ⚠ for whether it covers your running load and your largest startup surge. Optionally add a solar array size and peak sun hours — if daily harvest meets or beats your load, backup becomes effectively indefinite because the bank refills each day.

⚡

Reference

Formula & Methodology

6 formulas

Daily Energy Need

Need (kWh/day) = Σ(Watts × Hours × Qty) ÷ 1000

Sum each appliance's running watts times the hours it runs per day times how many you have, then convert watt-hours to kilowatt-hours. Example: a 150 W fridge for 8 h + two 60 W bulbs for 5 h = (150×8) + (60×5×2) = 1,200 + 600 = 1,800 Wh = 1.8 kWh/day.

Usable Energy per Battery

Usable (kWh) = Nameplate × DoD% × Efficiency%

Nameplate capacity is rarely fully usable. Multiply by depth of discharge (how deep you can safely drain it) and round-trip efficiency (conversion losses). A 13.5 kWh battery at 90% DoD and 90% efficiency delivers 13.5 × 0.90 × 0.90 = 10.94 kWh of real, usable energy.

Batteries Needed

Batteries = ceil( Need × Days ÷ Usable per battery )

Divide the total energy you must store (daily need times backup days) by the usable energy per battery, then round up because you can't buy a fraction of a battery. Example: 12 kWh/day × 2 days ÷ 10.94 = 2.19 → 3 batteries.

Backup Runtime

Runtime (hours) = Usable total ÷ Daily need × 24

How long a given bank lasts at your load. 10.94 kWh usable ÷ 1.8 kWh/day × 24 ≈ 146 hours (about 6 days) for a tiny essentials load; a 30 kWh/day whole-home load on the same battery lasts under 9 hours.

Inverter Surge Check

Surge load = Continuous + max( Watts × (SurgeFactor − 1) )

The inverter must momentarily supply the running load plus the single largest motor startup. A fridge (150 W, 3× surge) and well pump (1,000 W, 3.5× surge) running together peak at 1,150 W continuous + 1,000 × 2.5 = 2,500 W extra inrush = 3,650 W. If that tops the inverter's surge rating, it can trip.

Solar Offset

Net daily draw = Need − (SolarkW × SunHours × Derate)

Daily solar harvest offsets the load. At 6 kW array, 4.5 peak sun hours, and 0.75 system derate you harvest 20.25 kWh/day. If that meets or exceeds your load, the battery refills daily and backup is indefinite; otherwise the net draw is how fast the bank still drains.

✓

YMYL-adjacent

Trust, Methodology & Sources

Maintained by Calculover Editorial · Updated 2026-06-13 · 4 sources

Editorial accountability

Owner: Calculover Editorial - Editorial owner

Last reviewed: 2026-06-13

Last verified: 2026-06-13

Methodology

Sizing follows standard residential backup-power practice. Daily energy need is the sum of (running watts × hours/day × quantity) divided by 1000. Usable energy per battery is nameplate capacity × depth-of-discharge × round-trip efficiency. Batteries needed is the ceiling of (daily need × backup days ÷ usable per battery). Backup runtime is usable energy ÷ daily need × 24. The inverter surge check models the worst realistic case — total continuous running load plus the single largest appliance startup inrush (running watts × (surge factor − 1)) — and compares both the continuous and surge totals against the inverter's rated continuous and surge watts. Solar offset uses array kW × peak sun hours × a 0.75 system derate and reports indefinite backup when daily harvest meets or exceeds daily load.

Assumption: Appliance running watts and motor surge factors use typical residential reference values (for example, refrigerator ~150 W and ~3x surge, half-HP well pump ~1,000 W and ~3.5x surge); your equipment's nameplate data may differ.

Assumption: Default battery assumptions (90% depth of discharge, 90% round-trip efficiency) reflect modern lithium-iron-phosphate home batteries; lead-acid systems require roughly 50% DoD and ~80% efficiency.

Assumption: The hours-per-day input is meant to capture real duty cycle (a refrigerator does not run continuously), and all listed loads are treated as potentially simultaneous for the inverter continuous check.

Assumption: Solar harvest uses a flat 0.75 derate for inverter, wiring, soiling, and temperature losses; actual production varies with weather, season, and shading.

Limitations & guidance

This is a planning estimate, not an electrical engineering design or a substitute for a site assessment by a licensed electrician or battery installer.

It does not size conductors, breakers, transfer switches, or critical-load subpanels, and does not account for battery temperature derating, age-related capacity fade, or specific manufacturer firmware limits.

Surge handling varies by inverter model and by whether soft-starters are installed; confirm motor-start compatibility with your equipment's documentation.

For life-critical loads such as medical devices, always maintain an independent backup plan and professional verification — do not rely on this estimate alone.

Professional guidance: This calculator provides general educational estimates for sizing a home battery backup system. It is not professional electrical, engineering, or safety advice. Battery and inverter installation is governed by electrical code and should be designed and installed by a licensed professional. Verify all sizing for your specific equipment and loads before purchasing or installing a system, especially where medical or life-safety loads are involved.

Primary sources

Should You Get Battery Storage for Your Home Solar System? - U.S. Department of Energy — Energy Saver

Tesla Powerwall Specifications - Tesla

Energy Storage Research - National Renewable Energy Laboratory (NREL)

Estimating Appliance and Home Electronic Energy Use - U.S. Department of Energy — Energy Saver

📖

Glossary

Key Terms Explained

7 terms

Depth of Discharge (DoD) The percentage of a battery's nameplate capacity you can safely use before recharging. Lithium-iron-phosphate (LFP) home batteries support 90–100% DoD; lead-acid should be limited to about 50% to avoid sharply shortening cycle life. Higher usable DoD means fewer batteries for the same backup.

Round-Trip Efficiency The share of energy you get back out of a battery versus what you put in, after charge/discharge conversion losses. Lithium achieves 90–95%; lead-acid 80–85%. Lower efficiency means you must store more to deliver the same usable energy.

Usable Capacity The energy actually available from a battery: nameplate × DoD × round-trip efficiency. A 13.5 kWh battery may only deliver ~11 kWh usable. Always size backup against usable capacity, not the nameplate number on the box.

Continuous (Running) Watts The steady power an appliance draws while operating. Your inverter's continuous rating must cover the sum of all running loads you intend to power at once.

Surge (Starting) Watts The brief inrush when a motor starts — refrigerators, well pumps, sump pumps, and air conditioners can pull 2.5–3.5× their running watts for a fraction of a second. The inverter's surge rating must cover the largest single start happening while everything else runs.

Inverter The device that converts the battery's DC power into household AC. It has two key limits — a continuous watt rating and a higher, short-duration surge rating. A battery can hold plenty of energy yet still be unable to start a motor if the inverter's surge rating is too low.

Days of Autonomy How many days the battery bank can carry your load with no recharge from solar or grid. Backup resilience typically targets 1–2 days; off-grid living in cloudy regions may need 3–5 days of storage.

👥

Scenarios

Real-World Examples

3 worked examples

🔦

Dana, riding out a short neighborhood outage

Dana wants to keep the essentials running during occasional 1-day outages: refrigerator, a couple of LED lights, the Wi-Fi router, and phone/laptop charging. She has one Powerwall (13.5 kWh, 90% DoD, 90% efficiency) and its built-in 5 kW / 7 kW inverter.

Load preset Essentials only (~6–7 kWh/day) Battery 13.5 kWh, 90% DoD, 90% eff Backup target 1 day

Her essentials load is light, so a single battery covers more than a day — the runtime card shows roughly 1.5–2 days on one Powerwall. The continuous load (a few hundred watts) sits far inside the 5 kW inverter, and the only real surge is the fridge compressor, well within 7 kW. One battery is plenty; she could even skip a second unit and rely on the wide margin.

🏠

Marcus, whole-home backup in a storm zone

Marcus wants near-normal living through multi-day outages — fridge, lights, TV, furnace blower, well pump, central AC, and the microwave — about 26 kWh/day, with the central AC dominating the load. He's sizing a bank of Powerwalls and wants two days of autonomy.

Load preset Whole home (~26 kWh/day) Battery 13.5 kWh, 90% DoD, 90% eff Backup target 2 days

Usable per battery is 10.94 kWh, so 26 × 2 ÷ 10.94 = 4.75 → 5 batteries for two days. That's a big, expensive bank — the interpretation panel nudges him to trim high-draw loads (the central AC alone is over half his energy) or add solar. The AC's 3× surge stacks with the well pump start, so he must confirm the combined surge stays under his inverter's peak rating or stagger their startups.

🩺

Priya, powering a medical device

Priya relies on a CPAP/oxygen concentrator and must keep a medication fridge cold. Her critical load is small but absolutely cannot fail. She adds a 4 kW solar array to recharge during the day.

Load preset Medical-critical (~5–6 kWh/day) Battery 13.5 kWh, 90% DoD, 90% eff Solar 4 kW, 4.5 sun hours

Her load is ~5–6 kWh/day, so one battery alone lasts ~2 days. With 4 kW of solar harvesting about 13.5 kWh/day (4 × 4.5 × 0.75), daily harvest far exceeds her load — the solar verdict reads 'indefinite,' meaning the bank refills every sunny day and the device runs as long as the sun returns. For life-critical loads she should still keep a margin and a backup plan for extended overcast stretches.

📄

Deep Dive

How to Size a Home Battery Backup From Your Actual Appliance Load

4 chapters · 7 min read

Most battery shopping starts from the wrong number — the nameplate kWh on the box, or a vague 'how many Powerwalls do I need?' The reliable way to size a backup system is bottom-up: list the appliances you must keep running during an outage, total their real energy use, and work back to usable capacity, batteries, and the inverter that has to drive them. This calculator does exactly that, and it deliberately differs from a solar-battery ROI tool — it's about keeping the lights on, not about time-of-use arbitrage.

Sizing a backup vs. sizing a solar battery

A solar-battery economics tool starts from your whole-house daily kWh and your utility's time-of-use rates to estimate payback and grid independence. Backup sizing is a different question: which specific loads must survive an outage, for how long, and can your inverter physically start them? A whole-home profile might be 30 kWh/day, but a well-chosen critical-loads profile — refrigerator, lights, internet, phone charging, a furnace blower — is often just 6–10 kWh/day. That distinction is the single biggest lever on cost: cutting the load roughly cuts the number of batteries. Start from the Essentials or Medical-critical preset, add only what you truly need, and you'll usually find one or two batteries cover a realistic outage instead of the five or six a whole-home target demands. If you also want the financial picture — payback, TOU savings, ROI — use the companion Solar Battery Storage Calculator.

Why usable kWh is always less than the nameplate

Two factors shrink a battery's nameplate capacity into the energy you can actually use. Depth of discharge (DoD) is how deep you can safely drain the pack: modern lithium-iron-phosphate (LFP) batteries — Powerwall, Enphase IQ, Franklin — tolerate 90–100% DoD, while lead-acid must be limited to about 50% or its cycle life collapses. Round-trip efficiency is the energy lost converting in and out: lithium loses about 5–10%, lead-acid 15–20%. Multiply them: a 13.5 kWh lithium battery at 90% DoD and 90% efficiency delivers about 10.9 kWh usable; the same nameplate in lead-acid at 50% DoD and 80% efficiency yields only 5.4 kWh. Sizing against nameplate instead of usable capacity is the most common way people under-build a backup and run out of power mid-outage. This calculator always works from usable capacity, so the battery count it returns reflects the real world.

Surge vs. continuous watts — the spec that trips inverters

Energy (kWh) tells you how long the battery lasts; power (watts) tells you whether the inverter can run the load at all, and these are independent. An inverter has two ratings: continuous watts (the steady load it can carry indefinitely) and surge watts (a brief, higher burst). Resistive loads — lights, heaters, chargers — draw their rated watts and nothing more. But motor loads spike on startup: a refrigerator compressor, a ½-HP well pump, a sump pump, or an air conditioner can momentarily pull 2.5–3.5× their running watts for a fraction of a second. The danger case is when one motor starts while others already run — the combined peak can exceed the surge rating and trip the inverter even though the battery is nearly full. This tool models that worst case (continuous load plus the single largest startup surge) and flags it. Fixes when you exceed the rating: stagger appliance startups, add a soft-starter to the AC or pump, or upsize the inverter.

How long will a Powerwall actually last?

Runtime is just usable energy divided by load, but the load assumption is everything. A single Powerwall's ~10.9 kWh usable runs a 30 kWh/day whole-home load for under 9 hours, a 12 kWh/day reduced load (no AC, dryer, or range) for about 22 hours, and a 6 kWh/day essentials load for nearly two days. The fastest way to extend backup is not buying more batteries — it's removing the energy hogs. Central AC (3–5 kW), electric resistance heat and space heaters (1.5 kW each), electric ranges (2–4 kW), and electric dryers (5 kW) dominate a home's draw; shedding them during an outage can triple your runtime. Pairing the battery with solar changes the math entirely: once your daily solar harvest meets your daily load, the bank refills every sunny day and backup becomes effectively indefinite. Without solar, a battery gives you exactly one tank of stored energy — size that tank against the load you'll actually carry, with a margin for a longer-than-expected outage.

❓

Questions

Frequently Asked Questions

7 questions

How long will a Powerwall last my house during an outage?+

A single Tesla Powerwall delivers about 13.5 kWh usable. Runtime equals usable kWh divided by your load. A whole-home load of 30 kWh/day drains it in roughly 10–11 hours, while a critical-loads-only profile of 6–8 kWh/day (refrigerator, lights, Wi-Fi, phone charging) stretches it to 1.5–2 days. Cutting high-draw appliances — central AC, electric dryer, electric range — is the single biggest lever on backup runtime.

What is the difference between continuous watts and surge watts?+

Continuous (running) watts is the steady power an appliance draws while operating. Surge (starting) watts is the brief inrush when a motor spins up — refrigerators, well pumps, sump pumps, and air conditioners can momentarily pull 2.5–3.5× their running watts for a fraction of a second. Your inverter must satisfy both: the continuous rating must cover the sum of running loads, and the surge rating must cover the largest single motor start happening while everything else runs.

Do I need solar panels to recharge a backup battery?+

No. A battery can be charged from the grid and used purely as backup. But without solar it provides only one discharge of stored energy during an outage — once depleted in a multi-day blackout it cannot refill. Pairing the battery with solar lets it recharge each day; when your daily solar harvest meets or exceeds your daily load, backup becomes effectively indefinite for as long as the sun keeps the bank topped up.

How many batteries do I need for whole-home backup?+

Divide your daily energy need by the usable capacity per battery (nameplate × depth of discharge × round-trip efficiency), multiply by the number of backup days you want, and round up. A 30 kWh/day whole-home load needs about three 13.5 kWh batteries for one day of autonomy at 90% DoD and 90% efficiency (about 10.9 kWh usable each). Sizing for critical loads instead of the whole home typically cuts that to one or two batteries.

Can my inverter run my well pump or air conditioner?+

Only if its surge rating covers the appliance's startup inrush. A ½-HP well pump runs at roughly 1,000 W but can surge to 3,000–3,500 W on start; a window AC runs at ~1,200 W and surges near 3,600 W. If those starts happen while other loads run, the inverter's surge rating must exceed the combined peak. When it doesn't, you'll trip the inverter — stagger startups, add a soft-starter, or upsize the inverter.

What depth of discharge and efficiency should I assume?+

Modern lithium-iron-phosphate (LFP) home batteries — Powerwall, Enphase IQ, Franklin — support 90–100% usable depth of discharge and 90–95% round-trip efficiency, so a 90% / 90% assumption is realistic and slightly conservative. Lead-acid is very different: limit it to about 50% depth of discharge and 80% efficiency to avoid sharply shortening its cycle life. This calculator multiplies nameplate capacity by both factors to report the energy you can actually use.

Is this estimate good enough to buy a system?+

Treat it as planning guidance, not a final design. It gives an accurate first-order size — usable kWh, batteries, runtime, and an inverter surge check — but real installations depend on appliance nameplate data, simultaneous-use assumptions, wiring, code, and your specific equipment. Before purchasing, have a licensed electrician or battery installer verify your critical-load panel, inverter sizing, and surge handling for the exact appliances you plan to back up.

Download a one-page PDF of your numbers instantly. Add your email to also get our occasional calculator tips — no spam, unsubscribe anytime. Privacy.