How to Size a Home Battery Backup System for a Texas Home
Most battery sizing guides are written for the US average. The problem is that Texas isn’t average. Texas homes use more electricity than nearly any other state, summer heat turns air conditioning into a critical load, and ERCOT grid outages can last 40 hours or more during a bad storm.
The short answer: most Texas homeowners need between 10 and 30 kWh of battery storage. The actual number depends almost entirely on one question. Do you need the air conditioner to run during an outage?
This guide walks through the 3-step sizing formula, explains how AC changes the calculation, and covers why battery chemistry matters more in Houston than it does in Seattle.
Key Takeaways – Texas homes average 36 kWh per day, 24% above the US national average, so national sizing guides consistently undersize for Texas homeowners (U.S. Energy Information Administration, 2025). – Adding central AC to your backup plan roughly triples the battery capacity you need. – LiFePO4 batteries lose 20-30% of cycle life in extreme heat versus 40-50% for NMC, making chemistry choice a meaningful decision in Texas.
Why Texas Homes Require a Different Sizing Approach
Texas residential customers use an average of 1,096 kWh per month, which works out to about 36 kWh per day, compared to the US national average of 886 kWh per month, or about 29 kWh per day (U.S. Energy Information Administration, 2024 Residential Energy Data). That 24% gap exists because Texas summers are long, hot, and humid, which means air conditioning runs harder and longer here than in most other states.
The ERCOT grid serves most of Texas as an isolated system, with less interconnection to neighboring grids than most US states. That isolation increases vulnerability during peak demand events. During Winter Storm Uri in 2021, 4.5 million Texas homes lost power, and the average outage lasted approximately 42 hours. For Gulf Coast homeowners, storm season adds another layer of risk that homeowners in other regions don’t face the same way.
ERCOT projects that electricity demand in Texas will nearly double by 2030 as the population grows and electric vehicle adoption increases. Grid stress events during summer peaks are already more frequent than they were five years ago. A battery system sized for today’s needs could feel undersized before the decade is out.
For all these reasons, Texas homeowners sizing a battery backup system should start with higher baseline assumptions than what most national guides recommend.
Step 1: What Do You Actually Need to Power?
Before any calculation can happen, you need a load list. This is the step where most homeowners discover they’ve been overestimating. The goal is to identify which circuits genuinely need to work during an outage, not to replicate your normal electricity use on battery power.
There are two categories to choose between: essential-only backup and comfort backup. Essential-only means keeping the fridge cold, lights on, devices charged, and the router working. Comfort backup adds climate control to that list, and that decision drives the sizing more than anything else.
The jump from a window AC unit to central AC is the number that changes everything in this calculation. A window unit for one room draws roughly the same power as 100 LED light bulbs. A central AC system draws three to five times more.
How to Build Your Load List
Add up the wattage of everything you want to run, then multiply each appliance by the number of hours per day you expect it to operate. That gives you watt-hours (Wh) per day. Divide by 1,000 to convert to kilowatt-hours (kWh).
Example: a refrigerator running 24 hours at an average 200W draws 4.8 kWh. Add lights, charging, and a router and your essential-only daily load lands around 5-6 kWh. That’s your baseline if you’re not planning to run air conditioning.
If you’re not sure what your appliances draw, check the label on the back, search the model number, or pick up a plug-in watt meter at any hardware store for under $20.
Step 2: How Long Does Your Backup Need to Last?
Texas outage history gives a practical answer here. Short outages under 4 hours are the most common event type and any modern battery system handles them without issue. Storm-related outages, the ones that actually threaten households, typically run 12-48 hours. Sizing for a 24-48 hour window covers the large majority of realistic scenarios.
Three-day-plus outages happen, but they’re less common and usually involve major infrastructure damage. If you want coverage for that scenario without solar, the battery count climbs fast. Most homeowners address extended outages through solar pairing rather than stacking additional batteries.
The math here is direct: multiply your daily critical load (in kWh) by the number of backup days you want.
Example: 6 kWh/day critical load x 2 backup days = 12 kWh gross capacity needed.
That 12 kWh is your starting point. You’ll adjust it in the next step based on how deeply the battery can discharge before it shuts off.
Step 3: Calculate the Battery Size You Need
Once you have your gross capacity number, one more adjustment brings you to the actual battery size to buy: depth of discharge (DoD).
Batteries can’t always use 100% of their rated capacity without shortening their lifespan. The usable percentage depends on battery chemistry. LiFePO4 batteries, the chemistry used in most quality home backup systems, can safely discharge 80-90% of their rated capacity. Most manufacturers rate their systems at 90% usable capacity.
The formula:
Battery size to buy = Gross capacity needed / usable percentage
Worked example 1: Essential loads, no AC
- Daily critical load: 6 kWh (fridge, lights, devices, router)
- Backup duration: 2 days
- Gross capacity: 12 kWh
- With 90% DoD adjustment: 12 / 0.90 = 13.3 kWh
One standard 10-13 kWh battery unit covers this scenario.
Worked example 2: Essential loads plus a window AC unit
- Window AC running 8 hours: 1,100W x 8h = 8.8 kWh/day
- Total daily load: 6 + 8.8 = 14.8 kWh
- 2-day gross: 29.6 kWh
- With 90% DoD: 29.6 / 0.90 = 32.9 kWh (roughly two to three standard units)
LiFePO4 Chemistry in Texas Heat
Battery chemistry matters more in Texas than in most of the country. High ambient temperatures accelerate degradation in all lithium batteries, but the degree varies by chemistry. NMC (nickel manganese cobalt) batteries lose 40-50% of their rated cycle life when operating consistently at 45°C (113°F), temperatures that a Houston garage can reach on an August afternoon (EVE Energy, 2024). LiFePO4 batteries lose roughly 20-30% under the same conditions.
That gap compounds over thousands of charge cycles. LiFePO4 systems carry rated cycle life of 6,000-10,000 cycles. NMC systems rate at 1,000-3,000. For a system installed in a Texas garage or utility closet, choosing LiFePO4 can extend the usable life of the system by a decade or more.
Does Your AC Need to Run? The Texas Sizing Question
Air conditioning is the single variable that separates a modest battery investment from a significant one in Texas. Adding central AC to your backup plan doesn’t incrementally increase the size you need. It roughly triples it.
Central AC systems draw 3,000-5,000 watts continuously while running. At that draw rate, a single 13.5 kWh battery runs your AC for 3-5 hours before it’s empty. To get 24 hours of AC backup, you’d need 18-30 kWh for the AC alone, before adding any other loads. Most installers in the Houston area have the same conversation with homeowners: if you want to run central AC through a multi-day summer outage, you’re pairing with solar. Battery-only AC backup for extended outages gets expensive fast.
There’s a practical middle ground worth considering. A window unit for one bedroom draws 900-1,400 watts instead of 3,000-5,000 watts. In a multi-day summer outage, that trade-off, one cool room instead of whole-house comfort, extends your runtime by a factor of three or more.
Should You Pair Your Battery with Solar?
A battery-only system has a fixed energy supply. Once it’s depleted, you wait for the grid to restore. Solar pairing changes that entirely: your panels recharge the battery each day, extending backup indefinitely as long as the sun is out.
Houston receives 4.5-5.5 peak sun hours per day on average (TurbineGenerator.org). A 6-8 kW solar array in Houston produces enough daily energy to fully recharge a 13.5 kWh battery and still export power under clear conditions.
For homeowners who want more than 48 hours of backup, especially during summer storm season when outages and heat overlap, solar pairing is usually more cost-effective than buying additional batteries. Three or four extra batteries add up quickly. A solar array sized for your home’s consumption solves the extended-outage problem while also reducing your monthly bill during normal operation.
For help deciding between battery-only and solar-plus-battery systems, see our guide to choosing a home battery system.
Ready to See Which System Fits Your Home?
Sizing a battery system doesn’t have to be complicated, but getting it right matters. An undersized system leaves you without power when you need it most. An oversized system costs more than necessary.
We size every Eos system based on a homeowner’s actual critical loads, their home’s outage history, and whether solar pairing makes sense for their situation. View our plans to see standard system tiers, or reach out and we’ll run the numbers for your home.
Frequently Asked Questions
How many kWh of battery backup does a Texas home need?
Most Texas homes need 10-30 kWh of battery storage, depending on whether air conditioning is included in the backup plan. For essential loads only (refrigerator, lights, devices, and router), 5-10 kWh covers 24-48 hours. Central AC backup requires a minimum of 20-30 kWh to cover a meaningful window.
What size battery backup do I need to run AC in Texas?
Central AC systems draw 3,000-5,000 watts continuously. To run central AC for 24 hours on battery alone, you need 18-30 kWh for AC plus additional capacity for essential loads. Most installers recommend pairing with solar if 24-hour AC backup during summer is the goal.
Is a 10 kWh battery enough for a Texas home?
For essential loads only, yes. A 10 kWh battery covers roughly 24-48 hours of refrigerator, lights, router, and device charging. It’s not enough to run central AC for more than 2-3 hours. It’s a solid starting point for homeowners who want reliable essential backup without climate control.
What battery chemistry works best in Texas heat?
LiFePO4 (lithium iron phosphate) is the right choice for Texas installations. It degrades 20-30% less in high heat than NMC chemistry, carries cycle ratings of 6,000-10,000 cycles, and has a higher thermal runaway threshold, which matters when systems are installed in garages that can reach 113°F in August.
The Short Version
Texas battery sizing starts with three numbers: your critical load in kWh per day, your target backup duration, and your depth-of-discharge adjustment. Multiply the first two, divide by 0.90 for LiFePO4, and you have your minimum battery size.
Air conditioning is the variable that changes everything. Essential-only backup is achievable with one standard battery unit. Whole-home backup with central AC requires multiple units and, for extended outages, a solar pairing.
LiFePO4 is the chemistry that holds up in Texas heat. It’s what Eos installs on every job.
When you’re ready to price out a system, understanding what a battery installation actually costs is a useful next step.