Solar Battery Size Calculator: How Much Storage Do You Need?

Getting a battery quote is easy. Knowing whether the number on the quote is right for your home is the hard part. A solar battery size calculator removes the guesswork: enter how much energy you need to back up, and it tells you how much battery storage (in kWh) to buy.

A 10 kWh battery rarely gives you 10 kWh of usable power. Depth of discharge, round-trip efficiency, and your backup days all shrink the usable share. This guide walks you through the real formula, step by step, with sourced 2026 numbers. When you are done, run your own numbers through our solar battery size calculator for your exact size.

How Does a Solar Battery Size Calculator Work?

A battery calculator runs one core formula you can also do by hand:

Battery size (kWh) = daily backup energy (kWh) × backup days ÷ depth of discharge ÷ efficiency

That is it. Everything else on this page just helps you fill in each blank correctly, and this same math sits behind the published sizing tools and guides I cross-checked.

Here is what each input means:

• Daily backup energy (kWh): the energy your chosen loads use per day. Find it on your electricity bill (monthly kWh ÷ 30), in your smart meter app, or by adding up appliances: watts × hours used ÷ 1,000 = kWh per appliance.
• Backup days (days of autonomy): how many days you want the battery to carry you with no sun and no grid. One day covers most short outages; storm-prone areas often plan for two or three.
• Depth of discharge (DoD): the share of the battery you can safely use before recharging. Modern lithium batteries allow 80-95%; lead-acid allows about 50%.
• Efficiency: energy lost as heat when charging and discharging. Most modern batteries run at 85-95% round-trip efficiency.

Quick hand check: a home needing 10 kWh a day with one day of backup and a 90% DoD, 90% efficiency lithium battery needs 10 ÷ 0.9 ÷ 0.9 = 12.3 kWh of rated storage. The same home on lead-acid at 50% DoD would need 10 ÷ 0.5 ÷ 0.9 = 22.2 kWh, more than twice the sticker size.

What Is Depth of Discharge, and Why Does It Change the Answer?

Depth of discharge (DoD) is the percentage of a battery’s rated capacity you can safely use without shortening its life. It is why two “10 kWh” batteries can behave very differently, and ignoring it is the most common sizing mistake.

Battery chemistry	Usable depth of discharge
Lithium iron phosphate (LiFePO4)	80-95%
Flooded lead-acid	~50%
Sealed lead-acid (AGM/gel)	~50%

Source: Unbound Solar battery bank sizing guide.

A 10 kWh lithium bank at 90% DoD gives you about 9 kWh of usable power, while a 10 kWh lead-acid bank safely gives up only about 5 kWh. You buy far less lithium for the same usable energy.

On top of DoD, every charge-discharge cycle loses a little energy as heat. Most modern batteries run at 85-95% round-trip efficiency, so a slightly larger rated size is needed to deliver your planned usable power (source: Story of Solar).

How Do You Decide Which Loads to Back Up?

This is the biggest lever in the whole calculation: backing up only critical loads (fridge, lights, WiFi, a few outlets) can need as little as 4-5 kWh, while whole-home backup with air conditioning can need 25-40 kWh or more. List what must stay on during an outage and skip the rest, then work out each load’s daily energy:

Daily energy (kWh) = watts × hours used per day ÷ 1,000

Your electricity bill is the fastest source for the total (monthly kWh ÷ 30), but the appliance-by-appliance method shows you exactly which loads are worth backing up.

Illustrative worked example (stated assumptions): essential loads only, one day of backup, LiFePO4 battery at 90% DoD and 90% efficiency. Wattages below are assumed averages, not measured values; the fridge’s 150 W is its assumed average draw since it cycles on and off.

Load	Assumed watts	Hours/day	Daily kWh
Refrigerator	150 W	24	3.60
LED lights (5 × 10 W)	50 W	5	0.25
WiFi router	15 W	24	0.36
LED TV	100 W	4	0.40
Laptop	50 W	4	0.20
Ceiling fan	75 W	8	0.60
Total			5.41

Apply the formula: 5.41 ÷ 0.9 ÷ 0.9 = 6.7 kWh, so this homeowner should shop for roughly a 7 kWh battery target. A fridge and a few basics need far less storage than most people expect.

What Size Battery Do Most Homes Actually Buy?

Most residential battery units sit in the 5-15 kWh range. Above that, you buy two or more stackable units wired together, which is normal for whole-home backup. Popular 2026 home battery sizes include:

• 5.0 kWh: Enphase IQ Battery 5P (scales to 40 kWh with eight units)
• 9.7 kWh: SolarEdge Home Battery, stackable up to three per inverter
• 13.5 kWh: Tesla Powerwall 3, the most common single-unit size, with 11.5 kW continuous output
• 15 kWh: FranklinWH aPower 2
• 9-18 kWh per cabinet: Generac PWRcell 2 (3 kWh modules)
• Up to 18 kWh per cabinet: Panasonic EVERVOLT

Sources: EnergySage Tesla Powerwall 3 review, PowerOutage.us 2026 home battery comparison.

Roughly: 10-13.5 kWh covers essential backup for an average home for about a day; 25-40+ kWh (two to four stacked units) covers whole-home backup; off-grid homes often size at two to three times daily usage.

How Much Does a Solar Battery Cost Per kWh?

Size and cost move together, so compare quotes in cost per kWh of storage. Per EnergySage Marketplace data from the first half of 2026, a typical 13.5 kWh home battery costs $15,647 installed on average before incentives. Prices vary widely by brand: budget options run under $500 per kWh, premium brands top $3,000 per kWh, with MidNite Solar at about $342/kWh and Fortress Power at about $3,505/kWh. Source: EnergySage, updated July 2026.

Typical installed cost ranges by backup goal (before incentives):

Backup goal	Typical setup	Installed cost
Essential loads only	1 battery (10-13.5 kWh)	$5,000-$18,000
Whole-home backup	2-3 batteries	$24,000-$40,000

Source: EnergySage emergency backup guide.

These are U.S. figures; get local quotes in other markets. Oversizing has a real cost too: capacity you rarely use only stretches your payback period.

How Many Days of Backup Should You Plan For?

One day of backup covers most short outages, which is where grid-tied homes should start. Storm-prone areas often plan two to three days; off-grid homes size at two to three times daily usage.

Sizing a battery for a week of cloudy weather costs far more than sizing it for one day and keeping a generator as a backstop for the rare long stretch. Also, your solar array must produce enough daily energy to refill the battery in a day of sunlight, or you will top up from the grid anyway.

How Do You Convert kWh to Amp-Hours?

Battery spec sheets often list capacity in amp-hours (Ah) at a given system voltage, while your calculator result is in kilowatt-hours (kWh). One line of math converts between them:

kWh = (Ah × volts) ÷ 1,000, so Ah = (kWh × 1,000) ÷ volts

Whole-home banks usually run at 48 volts: a 12.5 kWh target equals 12,500 Wh ÷ 48 V = 260 Ah at 48 volts. Now you can read any spec sheet: a 100 Ah, 48 V battery stores 4.8 kWh, so a 24 kWh target means five of them wired together.

What Are the Most Common Battery Sizing Mistakes?

• Ignoring depth of discharge. Buying based on rated capacity instead of usable capacity is the single most common sizing error. A “10 kWh” lead-acid bank is really a 5 kWh bank.
• Sizing for the worst imaginable outage. Most outages are short. Size for realistic needs unless outages where you live are genuinely frequent and long.
• Forgetting future loads. An EV, a heat pump, or an electric water heater added later can blow past a battery sized for today’s loads. Retrofitting costs more than sizing right the first time.
• Sizing by square footage. There is no size-to-square-footage formula. Energy use depends on occupants, appliances, and climate, not floor area. Use your actual daily kWh.
• Skipping a professional review. A calculator gives a solid estimate; a licensed installer confirms it against your panels, inverter, and local code before you spend thousands.

The Bottom Line

Sizing a solar battery is one formula and four inputs: daily backup energy, backup days, depth of discharge, and efficiency. Most homes land at 10-13.5 kWh for essential backup and far higher for whole-home coverage, with lithium (LiFePO4) now the default because its 80-95% usable depth of discharge keeps the rated size close to your real need.

Run your own numbers through our solar battery size calculator to get your starting estimate in under a minute, then take that number to a licensed installer for a final check against your panels, inverter, and local code. The calculator gets you close; a professional site assessment closes the last gap.

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Solar Battery Sizing FAQs

How long will a 10 kWh battery last? At 90% usable depth of discharge, a 10 kWh lithium battery delivers roughly 9 kWh. For essential loads only, that can cover a full day; for whole-home use with air conditioning, it may last just a few hours.

What is the difference between kW and kWh? kW (kilowatts) measure power, the rate energy flows, which decides how many appliances can run at once. kWh (kilowatt-hours) measure stored energy, which decides how long they run. Check both on any spec sheet: plenty of kWh with too little kW may not start a large air conditioner.

Can I add more batteries later? Usually yes, if your inverter and system design allow expansion, since most modern home batteries are stackable modules. Plan it upfront anyway: mixing much older and newer modules can shorten battery life.

Should I buy a bigger battery for cold weather? Not for the size itself. A LiFePO4 battery still delivers most of its capacity in deep cold, but most lithium batteries cannot safely charge below freezing. The fix is a conditioned or insulated space (or cells with low-temperature protection), not extra kWh.

Does a bigger battery always save more money? No. An oversized battery costs more upfront, takes longer to pay back, and often sits half-empty. Sizing accurately beats sizing big “just in case.”

What size solar battery do I need for load-shedding? This solar battery size calculator guide recommends 10-13.5 kWh for most homes. Use the solar battery size calculator formula: daily backup kWh x days / DoD / efficiency.

Last reviewed: 11 October 2026 By TheSolarCost Editorial Team