How many kWh of battery do you actually need? A Texas sizing walk-through

The two numbers Base sizes for — kWh and kW

Every residential battery carries two numbers worth reading: kWh and kW. They mean different things and they fail at sizing in different ways. kWh is energy — how long the battery can run a given load. kW is power — how big a load the battery can run at any instant. The 25 kWh single and 50 kWh dual configurations Base ships are sized so the inverter can carry whole-home loads through a Texas summer outage; smaller batteries with under-spec'd inverters (common from other installers) trip when central A/C tries to start.

The kWh question is "how long will it last?" Round Rock's August average household draw is 1,625 kWh ÷ 30 days ÷ 24 hours = 2.26 kW continuous⁵. A Base 25 kWh single pack at that draw runs ~11 hours. During a 4 PM peak when the A/C is hammering, the same household pulls 4–5 kW continuous; the same battery runs ~5–6 hours. The kWh number, divided by your real kW load, is the runtime — and it's why the 25 kWh single hits the ~8+ hour floor that covers ~97% of TX outages.

The kW question is "what can it actually start?" A 5-ton central A/C in a Plano single-family home pulls about 6.5 kW running and 12–15 kW for 5–10 seconds at compressor startup³. That startup surge is what trips under-spec'd inverters. Base sizes its 25 kWh and 50 kWh systems with inverters rated for whole-home loads in Texas single-family homes — the central A/C is the binding case we design for. The inverter, not the kWh, is the gate that decides whether your air conditioner runs during an outage.

Texas load math: what your central A/C is doing to your battery

Texas residential electricity is dominated by cooling load May through September⁵. Round Rock and Plano both run 30+ days each summer above 95°F ambient²; Sugar Land runs higher and longer. The load profile of a Texas single-family with central A/C looks nothing like the EIA "average household" — A/C compressor cycling creates 4–7 kW step changes through the afternoon and evening.

ASHRAE-cataloged load characteristics for residential A/C compressors³ break down as:

• 5-ton single-stage (common in 2,000+ sqft TX homes built 2010 and earlier): 6.5 kW running, 12–15 kW LRA for 5–10 seconds at startup. Hardest case for battery inverters.
• 5-ton two-stage (common in 2010–2020 builds): 5.5 kW running, 10–12 kW startup. Easier; some lower-spec inverters can start it.
• 4–5-ton inverter-driven (premium and most new construction post-2020): 4.5–6 kW running, 6–8 kW startup (no large surge). Pairs comfortably with smaller battery inverters.

Two follow-on loads matter for many Texas homes. A typical 1-HP well pump pulls ~1 kW running and 4–5 kW LRA at startup — important in Williamson, Comal, and Hays county homes on private wells. A Tesla Wall Connector EV charger draws 11.5 kW continuous when active. A 13.5 kWh battery cannot meaningfully charge an EV during an outage; it can keep the EV from losing the trickle that maintains 12V auxiliary systems.

Three real Texas household profiles, sized as Base would

House 1 — 1,400 sqft Round Rock townhouse

Williamson County townhouse, single 3-ton inverter-driven A/C, gas range and water heater, no pool, no well, no EV. Annual usage 9,500 kWh; August peak day runs ~50 kWh, peaking at ~3.5 kW continuous in late afternoon⁵. Critical-loads list: fridge, microwave, Wi-Fi router, two LED-light circuits, the bedroom window A/C, the tankless gas water heater's igniter circuit. Critical-loads continuous draw: ~1.2 kW.

What Base would install: a single 25 kWh ground-mounted battery, $695 install + $19/mo, on the bundled 8¢/kWh + delivery rate. The whole-home inverter Base ships with the 25 kWh handles the 3-ton A/C startup. On reduced consumption, the 25 kWh single runs ~24 hours of backup — well past the ~2.5-hour Williamson County average outage and inside the ~97% coverage band. For a Uri-class 72-hour event, the household runs essentials for the first 24 hours and falls back to the gas range and a portable phone-charger battery for the remainder.

House 2 — 2,400 sqft Plano single-family

Collin County single-family, 5-ton single-stage central A/C, electric range, electric water heater, two-car garage, one EV on a Wall Connector. Annual usage 14,500 kWh; August peak day runs ~70 kWh, peaking at ~5 kW continuous⁵. The 5-ton single-stage compressor's 12–15 kW startup surge³ is the binding constraint.

What Base would install: a 50 kWh dual ground-mounted stack, $995 install + $29/mo, bundled 8¢/kWh + delivery. Two 25 kWh packs along the exterior wall handle the 5-ton compressor startup and run whole-home loads for the August peak. Up to 48 hours of backup with reduced consumption — past the median CenterPoint and Oncor outage and well past the 2.5-hour ~97% threshold. For a Uri-class event, the household runs whole-home essentials through the first 24+ hours and consciously trims load (cycling the A/C higher) to extend through 48+ if needed.

House 3 — 3,800 sqft Sugar Land single-family with pool

Fort Bend County single-family in CenterPoint territory, two 4-ton single-stage central A/Cs, electric range, electric water heater, 2-HP pool pump, one EV. Annual usage 22,000 kWh; August peak day 110 kWh, peaking at 9 kW continuous⁵. Two compressor-startup surges can stack within seconds if both A/Cs cycle simultaneously — the worst-case instantaneous load can hit 25–30 kW.

What Base would install: a 50 kWh dual ground-mounted stack, sometimes with optional load-management on the second A/C circuit. Base wires the pool pump off the backup panel (it's optional load); we wire the EV charger off as well unless the household specifically wants outage charging. With pool and EV off-backup, 50 kWh runs whole-home non-pool/non-EV through the median outage. Most Sugar Land outages are sub-4-hour CenterPoint events; the May 2024 derecho was the outlier. For Uri-class tail risk we tell the customer up front the household will need to manage load consciously after the first day regardless of battery size — honest expectation, not a marketing claim.

How long is a Texas outage, really?

PUCT's Annual Reliability Report on TX investor-owned utilities publishes per-TDU SAIDI and SAIFI annually⁹. Across the major deregulated TDUs in 2024, average SAIDI ran ~180 minutes per customer per year — the typical customer's total minutes of outage across the whole year. Most of that is short events; the median individual outage clears in under 4 hours.

The tail is where battery sizing has to make a choice. Three 2021–2024 Texas events broke the SAIDI distribution:

• Winter Storm Uri, February 2021. 4.5 million customers without power; many for 36–72+ hours. Combined price-and-outage event.
• Houston derecho, May 2024. ~2.2 million CenterPoint customers without power; 48–96 hour restoration for the long tail. Outage-only event.
• Hurricane Beryl, July 2024. Subsequent Houston-area outages on top of the May derecho recovery; some customers logged 7-day outages. Outage-only event.

Sizing for the median TX outage (under 4 hours) is well inside Base's 25 kWh single (~24 hours of reduced-consumption backup) — that single configuration covers ~97% of TX outages per the average outage duration of 2.5 hours. Sizing for the tail (Uri-class 72-hour event) means the 50 kWh dual plus either critical-loads sub-paneling or paired solar to recharge during sunny hours. Sizing for "every possible event with central A/C running the whole time" requires more battery than most household budgets support — the right answer is usually whole-home backup for the median plus a conscious load-trim plan for the tail. Read more on event-type differences in outage vs ERCOT price spike.

The Base VPP override: when "right-sized" means up-sized

Backup-only sizing answers "how big a battery do I need?" Base's bundled product answers a different question: "what battery size makes economic sense given automatic VPP enrollment?" Those answers can disagree.

Every battery Base installs is automatically enrolled in our VPP. Revenue scales with battery size — a 50 kWh dual stack dispatches twice as much energy into ERCOT ancillary markets as a single 25 kWh pack. We price that revenue back into the bundled rate (8¢/kWh + utility delivery on the standard plan, ~13.8¢/kWh all-in on Oncor) rather than paying it out per dispatch. For households on the bubble between the 25 kWh single and 50 kWh dual, the VPP economics often push the decision toward the dual — and we walk that trade-off on every quote.

For House 1 (Round Rock townhouse), the VPP-economic right-size still lands at 13.5 kWh — backup needs are modest, and the second pack would be running idle most days. For House 2 (Plano single-family), 27 kWh is both the backup answer and the VPP answer. For House 3 (Sugar Land), 40 kWh covers the backup case; sizing further (54+ kWh) is a pure VPP-revenue play that works only if the household has the available wall space. The VPP pillar walks the revenue math.

When the right answer is "no battery"

Honest accounting: a battery isn't the right answer for every Texas household. A 1,200-sqft Williamson County apartment renter, a small single-occupant home with one Uri-class outage in five years, or a household with a backyard generator and reliable gas service — these profiles often net out cheaper with a 1.5 kWh portable power station and a friend's house to relocate to during major events.

The Texas profiles where the four boxes (cell, BMS, inverter, transfer panel — see how home batteries work) do real work, today, are owners of single-family homes with central A/C they want running through a derecho, well pumps that stop without grid power, electric vehicles they need to charge through multi-day events, or households wanting the VPP-driven payback math regardless of backup needs.