How to Size a Solar Battery System for Your Home

You bought a 13 kW solar array, you’re stoked about the $200 power bill credits, and now your installer is asking how big a battery you want. Suddenly the spreadsheet matters. Undersize the battery and you’ll watch grid power kick on at 9pm because you didn’t have enough storage for the evening peak. Oversize it and you’ve spent $12,000 on capacity you’ll never cycle. This guide walks through the actual math for sizing a solar battery system in the Southeast — kWh load, depth of discharge, autonomy days, and the inverter conversation most installers skip.

Last updated: May 2026

How big a battery do you actually need?

Most Southern homeowners need 20–40 kWh of usable battery storage to cover overnight load and a partial outage. The exact number depends on three inputs: your daily kWh consumption, how many hours of backup you want, and whether your battery will be paired with grid-tied solar (smaller battery) or off-grid solar (larger battery).

The shortcut answer for a 1,800–2,500 sq ft Georgia home with central AC, electric water heater, and 2–3 occupants is one or two stacked Tesla Powerwall 3 units (13.5 kWh each), or the equivalent in Bluetti, EcoFlow, or Zendure stacks. That covers overnight loads and gives you 12–24 hours of essentials backup during a grid outage.

Step 1: Find your real daily kWh consumption

You can’t size a battery without knowing your load. Three ways to find your daily kWh, ranked by accuracy:

• Best: pull 12 months from your utility account. Log into Georgia Power (or your provider) and download monthly kWh totals. Average them, then divide by days in the month. Most Southern homes land between 25 and 50 kWh per day, with 30–35 kWh average.
• Decent: read your last 6 bills. Add the kWh, divide by days. Misses summer/winter swings if you don’t go back a full year.
• Rough: square footage × 0.018 kWh/sq ft/day. A 2,000 sq ft Georgia home → 36 kWh/day. Useful for new construction with no bill history.

Once you have your daily average, look at the summer peak separately. July/August in Georgia regularly pushes daily consumption to 55–70 kWh on AC-heavy days. Your battery doesn’t need to cover the whole peak day — it needs to cover the night-time fraction of it after solar production stops.

Step 2: Figure out what hours your battery is actually working

Solar produces from roughly 8am to 6pm in Georgia. Your battery’s job is everything else — overnight, early morning before sunrise, and grid outages.

For grid-tied solar + battery:

• Daytime: solar runs the house, surplus charges battery
• Evening (6pm–11pm): battery runs the house
• Overnight (11pm–6am): battery continues, switching to grid only when battery hits its reserve floor
• Early morning (6am–8am): battery + grid as solar ramps up

For most Southern homes, the evening + overnight load is roughly 35–45% of total daily kWh. So if you use 35 kWh per day, your battery needs to deliver 12–16 kWh per evening cycle. That’s the bottom of the sizing range.

Step 3: Add autonomy days for outage backup

If you only want load shifting (running off battery in the evening to avoid peak utility rates), the math above is enough. If you want hurricane/storm outage protection, multiply your essentials load by the days you want to be self-sufficient.

Use case	Battery sizing approach	Typical kWh needed
Time-of-use shifting only	Cover evening + overnight load (1 cycle)	12–18 kWh
Solar self-consumption (max grid offset)	Cover all non-solar hours, full daily load	20–35 kWh
Essentials backup, 1-day outage	Refrigerator + lights + WiFi + minimal AC, 24 hrs	15–25 kWh
Essentials backup, 3-day outage	Same essentials, 72 hrs (assumes solar recharge)	30–50 kWh
Whole-home backup, 1-day outage	Full home including AC, 24 hrs	40–70 kWh

For most Southeast hurricane/storm scenarios, essentials backup with 24–48 hours of autonomy plus solar recharge is the sweet spot. That’s 25–40 kWh of usable storage.

Step 4: Convert “usable” to “nameplate” kWh

Battery specs are usually quoted as nameplate capacity, but you can’t actually use 100%. LiFePO4 (lithium iron phosphate) batteries — which is what every modern home battery uses — have a usable depth of discharge of 80–90%.

• Tesla Powerwall 3: 13.5 kWh nameplate, ~13.5 kWh usable (100% DoD)
• Enphase IQ Battery 5P: 5 kWh nameplate, 5 kWh usable
• EcoFlow PowerOcean: 5 kWh per module, ~95% usable
• Zendure SolarFlow: 1 kWh per module, ~96% usable
• Bluetti EP900: 5 kWh per module, ~90% usable

So if you need 30 kWh of usable storage, you need at least 33 kWh nameplate (assuming 90% DoD). For Powerwall, 30 kWh usable = three units (40.5 kWh nameplate, with all three you have 40.5 kWh usable since they spec 100% DoD). The math is generally close to 1:1 for residential LiFePO4 in 2026.

Step 5: Match the inverter to the battery

Battery capacity is half the answer. The other half is inverter continuous wattage — this determines what you can run simultaneously off the battery.

Inverter continuous output	What it runs simultaneously
3 kW	Refrigerator + lights + WiFi + small fan; no AC, no electric water heater
5 kW	Above + microwave OR window AC unit OR electric stove burner
7.6 kW (Powerwall 3)	Above + 2-ton central AC OR electric water heater (not both at once)
11.5 kW (2× Powerwall 3)	Whole-home modest use, including 3-ton AC + most appliances

A common mistake: buying 30 kWh of battery on a 5 kW inverter. You’ll have plenty of stored energy but you can’t pull it fast enough to run the house — your AC trips offline because surge demand exceeds what the inverter can deliver. Match inverter capacity to peak instantaneous load, not average.

Battery brand recommendations for Southern homes

For most Georgia, Alabama, Tennessee, and Florida buyers in 2026, three options cover 90% of use cases:

Modular incremental: Zendure SolarFlow. Best if you want to start small and scale up over time. Stacks in 1 kWh increments per module up to 7.6 kWh per tower; multiple towers supported. Lower upfront cost than committing to a 13.5 kWh Powerwall, and the modules can be added years later without replacing inverter electronics. The Zendure SolarFlow system works with most existing solar arrays via AC coupling. See our full Zendure SolarFlow review for performance details.

Higher-capacity all-in-one: Bluetti EP900. If you need 15–30 kWh of storage and want fewer cabinets, the Bluetti EP900 stacks 5 kWh modules under a single 9 kW inverter. Good fit for whole-home backup ambitions on a 1,800–2,400 sq ft home. The Bluetti EP900 system ships with smart load management to cycle non-essentials off during outage mode.

Where most installers oversize

The solar industry has a structural incentive to oversize batteries — bigger system = bigger commission. Three places to push back:

• Don’t size for “whole home off-grid forever.” If you have grid available, you don’t need 5 days of autonomy. The grid usually comes back in 12–48 hours even after major Southern storms.
• Don’t oversize because of EV charging “future-proofing.” EVs charge primarily overnight on grid power; battery isn’t the right bucket for that load.
• Don’t include AC in essentials sizing. Central AC is a 4–8 kW load that drains a 13.5 kWh battery in 2–3 hours. For outage planning, accept that you’ll skip AC for 24 hours or use one window unit on a smart plug.

The federal tax credit changed in 2026

The 30% federal residential solar tax credit (ITC) expired December 31, 2025 for cash and loan purchases of solar panels. Battery storage may qualify separately under the residential energy efficient property rules — but the rules are murky enough in 2026 that you should not assume the credit applies. Check with a CPA before signing a contract that’s been priced “after credit.” Solar leases and PPAs still qualify through 2027 via the business-side 48E credit.

Frequently Asked Questions

How many kWh of battery do I need for a 2,000 sq ft home?

For a 2,000 sq ft Southern home with central AC, plan on 20–30 kWh of usable battery for solar self-consumption (covering all non-solar hours), or 13–20 kWh for essentials-only outage backup. That’s typically one or two Tesla Powerwall 3 units, or the equivalent in modular systems like Zendure or Bluetti.

Can a Tesla Powerwall power my whole house?

One Powerwall 3 (13.5 kWh, 11.5 kW continuous) can power a typical 2,000 sq ft home for 8–12 hours including some AC use, or 24+ hours on essentials-only mode. For whole-home backup including all-day AC during a multi-day outage, you generally need two stacked Powerwalls or a comparably sized alternative.

Is it worth getting a solar battery without solar panels?

It can be, for time-of-use rate shifting in markets where peak/off-peak pricing differs significantly. In most Southern utility territories the rate spread is too small for battery-only to pay back inside 10 years. Battery makes the most financial sense paired with solar where you’re storing free midday production for evening use.

How long do home solar batteries last?

LiFePO4 home batteries (Powerwall 3, Enphase 5P, Zendure, Bluetti, EcoFlow) carry 10-year warranties and typically retain 70–80% capacity at year 15 with normal cycling. That’s roughly 4,000–6,000 cycles. Heat shortens lifespan — installation in a conditioned garage or interior closet outlasts outdoor wall-mounted installs in Southern summers.

Should I size my battery to cover summer peak load?

No — sizing for July peak overspends for 11 months of the year. Size for average annual daily load and a partial peak (cover essentials during the worst week, accept some grid pull). A common Southern strategy is 2 Powerwalls for the year, with grid backup taking the load on the 8–12 hottest peak days each summer.

Bottom Line

Most Southern homeowners need 13–30 kWh of usable battery storage, paired with at least 7.6 kW of inverter continuous output. The right number depends on your daily kWh, how many days of autonomy you want during outages, and whether you’re including central AC in your backup plan. Don’t let an installer talk you into 50+ kWh “for resilience” if you have grid available — the math almost never works. Start with the load data, work backward to the battery, then pick the brand that scales the way you want to grow.