How a home battery actually works: cells, BMS, inverter, and your panel

The four boxes inside the appliance

A Base-installed home battery looks like a single sealed ground-mounted appliance set near the electric meter along your exterior wall. Inside, it's four functional layers stacked together. The cell is the chemistry layer. The pack and BMS are the safety layer. The inverter is the conversion layer that turns DC into the AC your house runs on. The transfer panel is the wiring layer that decides which Texas circuits get power when the grid is down.

We install LFP-chemistry residential systems certified to UL 1973, UL 1741, UL 9540, and UL 9540A, with active fire suppressants built into the enclosure. They run from Round Rock and Plano to Houston, San Antonio, and the rest of the served Oncor, CenterPoint, AEP Texas, TNMP, and cooperative territories. The 12-hour ice-storm window your Williamson County house actually cares about is governed by the same four boxes regardless of who installed it; we walk through them so you know what we're putting at your home.

Each box has its own sizing failure mode. Customers who shop on kWh alone discover the inverter's kW limit when the central A/C tries to start. Customers who shop on warranty-cycle-count alone discover the BMS's thermal-derate behavior in a Texas summer⁴. We size all four boxes for Texas service and the trade-offs your house creates — that's the point of the rest of this page.

The cell: where the energy actually lives

Base installs lithium iron phosphate (LFP) cells — the residential default in 2026 across the systems we ship and most of the rest of the Texas market¹³. A single LFP cell is a metal can about the size of a thicker paperback book². Its nominal voltage is 3.2V. Charged it sits near 3.4V; depleted it drops to ~2.5V. That's not enough to do anything useful. To run a 240V central A/C compressor in your Plano home⁹, we stack cells in series until the pack voltage gets to ~48V, which the inverter then steps up.

A "kWh" of capacity is a chemistry quantity. A 100 amp-hour LFP cell at 3.2V holds 320 watt-hours, or 0.32 kWh. The 25 kWh single ground-mounted system Base ships is roughly 78 such cells wired into a series-parallel arrangement. Round Rock's August modeled household draw is about 1,625 kWh for the month⁴; a 25 kWh battery fully charged carries roughly 15 hours of that month's average draw, but the actual runtime during a 4 PM peak at 4 kW is much shorter — closer to 6 hours. The 50 kWh dual doubles both. We size for the peak hour, not the monthly average.

Cell life is counted in cycles. An LFP cell rated for 6,000 cycles to 80% retained capacity will, in a Texas household that fully cycles it once a day, hit that point in 16+ years². NMC cells — the older residential chemistry, still in some 2018–2022 installs around Dallas — run 4,000–5,000 cycles, typically. We ship LFP for residential service for reasons covered in the chemistry deep-dive; the rest of this article assumes LFP.

The pack and BMS: thermal management for a Texas exterior install

Cells are not exposed to your exterior wall's 130°F afternoon air. The systems Base installs seal the cells inside an enclosure with passive thermal management plus active fire suppressants built into the unit itself. The BMS is the integrated controller that monitors every cell's voltage and temperature, balances charge across the pack, and shuts the system down before any single cell exceeds its safe operating envelope. The enclosure plus the BMS is what makes UL 1973, UL 1741, UL 9540, and UL 9540A certifications hold in a Texas summer.

Texas climate is the reason this layer matters more here than in, say, Seattle. NOAA-station ambient highs cross 95°F across most of Williamson, Travis, and Dallas counties for 30+ days each summer⁴; enclosure-adjacent ambient temperatures run hot through summer afternoons. PNNL battery aging studies show calendar-aging Arrhenius behavior — capacity fade roughly doubles for every 18°F sustained ambient above 77°F³. The BMS's thermal-derate logic — slowing or stopping charge above a threshold — is what keeps a hot Round Rock exterior install from cooking itself in year three.

UL 9540A — the thermal-runaway propagation standard every residential battery sold in Texas must pass⁶ — is one of four UL listings the Base systems carry: UL 1973, UL 1741, UL 9540, and UL 9540A. The full-stack listing exists because Texas's and California's heat profiles made exterior residential installs the regulatory pivot point. The 2026 systems Base ships clear all four; the active fire suppressants built into the enclosure are belt-and-suspenders for the same case.

The inverter: where buyers under-spec without realizing it

The cells deliver DC at ~48V at the pack level. Texas single-family homes run on 240V split-phase AC⁷. The inverter is the box that does the conversion, and its rating is the most common silent under-spec in residential battery purchases.

The continuous AC rating is what the battery can deliver to your house at any given second — a power rating, distinct from the energy rating in kWh. A 5-ton central A/C compressor — common in 2,000-sqft Plano homes — pulls about 6.5 kW running and 12–15 kW LRA (locked-rotor amperage) for ~5–10 seconds at compressor startup⁹. The 25 kWh and 50 kWh systems Base installs are sized with whole-home inverters that handle that startup transient. We don't ship under-spec'd inverters to houses that need to start central A/C — that's the most common spec mistake we see customers walk in with from other installers.

The kWh tells you how long. The kW tells you what's possible at all. An under-spec'd inverter powers lights, fridge, Wi-Fi, and a window A/C — but not central air at startup. Texas buyers shopping elsewhere on energy capacity alone often discover this on the day the A/C tries to start during an outage and the inverter trips. Inverter- driven and two-stage A/Cs draw lower startup surge and pair more easily; older single-stage systems are the hardest case, and one of the things we look at during the home survey before quoting size.

Round-trip efficiency is the inverter's tax. NREL's 2024 ATB pegs residential battery round-trip efficiency at 89–95% for current systems². For a 25 kWh single, that's 22–23.8 kWh actually delivered to your Round Rock loads per full cycle — meaningful for the VPP-revenue math we price into the bundled rate, mostly invisible to a household running essentials during an outage.

The transfer panel: anti-islanding and the 20-millisecond switch

When Oncor's wires go down at 3:14 AM during a Williamson County ice storm — the Winter Storm Uri pattern of February 2021, when 4.5 million Texans lost power — the battery has to decide what to do in milliseconds. Anti-islanding is the rule that says: if the grid is down, disconnect from it, because line workers may be touching the wires. PUCT Substantive Rule §25.211 codifies this for every distributed-generation interconnection in Texas⁵. The battery accomplishes it with a transfer switch.

Texas residential installs use one of two wiring topologies. Whole-home backup wires the battery to feed every circuit in the house, with a generation-side disconnect that opens the moment the grid drops. Critical-loads sub-paneling runs a smaller panel containing only the circuits the household has decided matter — fridge, well pump, a couple of lights, the home-office router, maybe one window A/C, the CPAP machine.

Whole-home is what most Round Rock and Plano buyers want and what most marketing copy implies. It also requires a battery sized to handle every circuit's worst-case simultaneous load — which, for a Sugar Land house with central A/C plus an electric oven plus a well pump, can mean stacking two batteries. Critical-loads sub-paneling is the practical compromise: a single 13.5 kWh battery that keeps a fridge cold for 24–36 hours instead of running the entire house for 4. The sizing guide walks the math through three real Texas household profiles.

Transfer-switch latency is what determines whether you notice the outage. The systems Base installs transfer within milliseconds — fast enough that lights flicker but most household electronics keep running. Houses with always-on critical electronics — a CPAP machine, a home- office workstation, medical devices — get paired with a small uninterruptible power supply alongside the battery for guaranteed seamless transition; that's part of how we spec the install.

Three operating modes, often in the same week

A battery in a Texas household isn't a single-use device. The Base systems we install run in three modes, often switching between them in the same week during a typical Round Rock summer.

Normal grid-up operation. The battery charges from the grid (or a paired solar array, when present) and discharges into our Virtual Power Plant during high-value ERCOT ancillary-market hours. Customers who buy a non-VPP battery elsewhere never see it move on most days; the pack sits at 90%+ state of charge waiting for an outage that doesn't come. Base's bundled product is structurally different — every battery we install is automatically enrolled in the Base VPP, so the battery is doing economic work essentially every day. See how our VPP works.

Outage operation. The inverter detects grid loss within milliseconds¹ and the transfer switch opens. The battery is now the household's sole power source. The BMS holds a reserve floor — typically 10–20% — below which it won't discharge to preserve cell health and reserve a buffer for restoration. A 13.5 kWh pack at a 20% reserve is effectively 10.8 kWh of usable capacity in this mode.

VPP dispatch. ERCOT, through Base's VPP controller, signals the battery to charge fast (when wholesale prices are negative on a sunny April afternoon) or discharge fast (when ancillary-service markets clear high during an EEA Watch in August)¹⁰. Sub-second response is required to participate in fast-frequency-response and regulation services. Your reserve floor is preserved — the battery never dispatches at the cost of leaving your Williamson County house unable to ride out a sudden outage.

How Base sizes the four boxes for your house

The reason we walk through the four boxes is that all four decisions get made for your specific Texas home before we install. Three concrete things we look at on the home survey:

1. The kW spec, not just the kWh. If your home has central A/C, we size the inverter to start it. The 25 kWh single and 50 kWh dual configurations Base ships are spec'd for whole-home Texas A/C loads. We don't sell under-spec'd inverters to homes that need whole-home backup.
2. LFP chemistry, ground-mounted near the meter. Base ships LFP across our Texas footprint because the chemistry handles 130°F ambient at the install location⁴ better than NMC does³. Ground-mounted exterior placement along the wall near your meter is the standard install; a 20 kWh wall-mounted variant exists for tighter footprints.
3. Whole-home vs critical-loads decided before signing. Whole-home backup needs the battery sized to your worst-case simultaneous load. Critical-loads sub-paneling lets a single 13.5 kWh pack go further by running fewer circuits longer⁵. We walk the sizing math with you so the decision is yours, not a default.

When a home battery isn't the right answer

We tell customers when a battery doesn't make sense. A Texas household with one Williamson County outage in five years, a 1,200-sqft footprint, a gas-cooktop kitchen, and no medical electronics has a low backup case for the cost. A small portable power station and a plan to relocate to a friend's house during multi-day events is often the more honest answer for that profile, and we say so.

The Texas households where the four boxes do real work, today, are the ones with central A/C they don't want to lose during a derecho-season outage, well pumps that stop working when grid power drops, electric vehicles they need to charge through a multi-day event, or VPP-revenue economics that change the payback math entirely (covered in the payback pillar). That's the customer Base built its bundled rate + battery + VPP product for.