How Many Solar Batteries Do I Need to Power a House?

Most homes need one battery to keep the essentials running and two to three batteries to back up the whole house with central air. Exactly where you land depends on three things: what you want the battery to do, how much electricity you use, and how many hours you want it to last.

The good news is that you don't need an installer to get a solid estimate. Grab a recent electric bill, follow the four steps below, and you'll know your number in about five minutes.

Step 1: Pick your goal

The single biggest factor in battery count isn't your house size. It's what you're asking the battery to do. Most people fall into one of these groups:

• Essentials backup. You want the fridge, lights, internet, ceiling fans and medical devices to keep running during an outage. This is the most common and most affordable goal.
• Whole-home backup. You want the house to feel normal during an outage, including central air. In Texas, A/C is what turns a one-battery job into a two- or three-battery job.
• Bill savings. You want to store midday solar and use it in the evening. In Texas, most retail plans don't reward this much, so it's rarely the main reason to buy a battery here.
• Off-grid. No utility connection at all, like a ranch cabin or workshop. This needs the most storage per kWh used, because there's no grid to fall back on.

Step 2: Find your daily kWh

Look for "kWh used" on your electric bill and divide by 30. That's your average daily use. The average Texas home uses about 1,094 kWh a month, or roughly 36 kWh a day, but summer months with heavy A/C can run far higher.

For a backup goal, you don't need your whole-house number. You only need the loads you actually want running. The chart below shows typical daily use for common appliances.

Notice the gap. Everything you need to be comfortable and safe adds up to about 7 to 10 kWh a day. Central air on a hot day can use three to four times that by itself. That's why the A/C decision matters more than anything else in this guide.

Step 3: Decide how many hours to cover

• With solar panels: your batteries mostly need to get you from sunset to sunrise, about 14 hours. The panels recharge them the next morning.
• Without solar, or on cloudy days: plan for a full 24 hours or more per day of outage.
• Off-grid: plan for 2 to 3 days of "autonomy" so a stretch of bad weather doesn't leave you dark.

If you remember the February 2021 winter storm, you'll know Texas outages don't always end in a few hours. If long outages are your main worry, size for at least a full day.

Step 4: The battery sizing formula

Now put it together:

kWh needed = daily kWh of your backed-up loads × (hours to cover ÷ 24)

Number of batteries = kWh needed ÷ (usable kWh per battery × 0.9)

The 0.9 keeps a 10% reserve so you never drain the battery to zero, which also helps it last longer. Always round up.

Example: your essentials use 10 kWh a day and you want to cover a full 24-hour outage. With a 13.5 kWh battery: 10 ÷ (13.5 × 0.9) = 0.82, so one battery does the job, with a little to spare.

Modular batteries like the Enphase 5P come in smaller blocks, so you'll need more units for the same storage. Three 5P units give you 15 kWh, about the same as one Powerwall 3.

Don't forget power (kW)

Capacity (kWh) is how long a battery can run. Power (kW) is how much it can run at once. A battery can have plenty of stored energy and still trip off if you ask too much from it at the same moment.

This matters most with big motor loads like central air conditioners and well pumps, which pull a surge of power when they start. A 3-ton A/C unit typically draws around 3 to 4 kW while running, and several times that for a second at startup. Before you finalize your battery count, ask your installer to confirm:

• The battery's continuous and peak kW can handle your A/C's start-up surge
• Whether a soft starter on the A/C would let fewer batteries do the job (it often can, for a few hundred dollars)
• Which circuits go on a backup panel if you're not backing up the whole house

3 real Texas examples

1. Dallas, 1,800 sq ft, essentials backup

The owners want the fridge, lights, Wi-Fi, ceiling fans and gas-furnace blower running during storms. Those loads add up to about 9 kWh a day. With solar recharging by day, one 13.5 kWh battery easily covers the night, and even without sun it lasts more than a full day. Answer: 1 battery.

2. Houston, 2,600 sq ft, whole home with A/C

This family wants everything on, including a 4-ton A/C in August. Their summer use is about 45 kWh a day, and roughly half of it falls between 6 p.m. and 8 a.m., or about 22 kWh. With solar recharging during the day: 22 ÷ (13.5 × 0.9) = 1.8. Answer: 2 batteries, plus a soft starter on the A/C. If they want to ride out multi-day hurricane outages with cloudy skies, a third battery is worth considering.

3. Austin, 2,000 sq ft, evening bill savings

The homeowner is on a plan with higher evening rates and wants to use stored solar from about 5 to 10 p.m. Their use in that window is about 8 to 10 kWh. Answer: 1 battery, or two Enphase 5P units if they'd rather start smaller and add more later.

Small and off-grid systems

The same formula works for smaller setups like a cabin, RV, shed or workshop, except you'll usually work with standard lithium (LiFePO4) batteries rated in amp-hours. To convert, multiply amp-hours by volts:

• A 12V 100Ah LiFePO4 battery stores about 1.28 kWh
• A 48V 100Ah server-rack battery stores about 5.12 kWh

Example: a weekend cabin uses 3 kWh a day and you want 2 days of autonomy. That's 6 kWh ÷ 0.9 = 6.7 kWh, so two 48V 100Ah batteries (10.24 kWh) give you a comfortable margin. For an RV using 2 kWh a day, two 12V 100Ah batteries (2.56 kWh) will cover a day with a small reserve.

If you're building a small system yourself, our solar parts and accessories checklist covers the cables, fuses and mounting hardware you'll need.

Common sizing mistakes

1. Sizing from the whole-house number. If you only want essentials backed up, using your full 36 kWh a day will make you buy two or three times more battery than you need.
2. Forgetting the A/C surge. Enough kWh doesn't help if the battery can't deliver the startup kW.
3. Ignoring the season. A July bill and a March bill can differ by 2× in Texas. Size for the season you care about most.
4. Draining to zero every night. An undersized battery that's emptied daily wears out faster. See how long solar batteries last.
5. Not planning for growth. If an EV or pool is coming, choose a battery system you can expand later.

Bottom line

For most Texas homes, the answer is simple: one battery if you want to keep the essentials running, two if you want the whole house with A/C and have solar to recharge, and three or more if you want to ride out long outages without much sun. Start with your goal, use the formula, and confirm the kW with your installer before you sign. For a quick estimate with your own numbers, try our solar battery calculator.