How Does a Home Solar Battery Work?
A home battery stores the solar your house doesn’t use at midday and gives it back later — in the evening or when the grid goes down. Inside, lithium cells store energy as direct current; an inverter turns it into the alternating current your home uses, and a controller decides when to charge and discharge. You get back about 89–90% of what goes in. In Los Angeles, 79% of new solar homes on SCE added one in 2025, most often a single 13.5 kWh unit.
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- Two numbers: capacity (kWh) is how much it holds; power (kW) is how much it can run at once. LA’s median battery in 2025: 13.5 kWh and 11.5 kW.
- Some energy is lost: Tesla rates the Powerwall 3 at 89% solar-to-battery-to-home, FranklinWH the aPower 2 at 90% round trip.
- AC or DC coupled: in 40% of 2025 LA battery systems the Powerwall 3 was also the solar inverter (DC-coupled); the rest connected on the AC side.
- Backup needs a gateway: it detects the outage, isolates the house and lets the battery and panels keep running.
A day in the life of a battery
| Time | Solar | Battery | Grid |
|---|---|---|---|
| Morning | Rising, covers part of the house | Holds its charge or tops up | Supplies the rest |
| Midday | More than the house uses | Charges from the surplus | Takes exports once the battery is full |
| 4–9 p.m. | Fading to zero | Discharges to run the house | Little or none while the battery lasts |
| Night | None | Keeps a reserve for outages | Supplies the house |
| Outage | Keeps producing, if the system can island | Runs backed-up circuits, recharged by the panels | Disconnected |
That evening discharge is where the money is on SCE’s Solar Billing Plan: a summer midday export earns about 6¢, while power bought from 4 to 9 p.m. on TOU-D-PRIME costs about 61¢. On LADWP, exports net at the retail price under its net metering rider, so the battery mostly sits charged for outages. See do you need a battery? and do solar panels work at night?
Inside the box
Cells
Charging pushes lithium ions from one electrode to the other; discharging lets them flow back, driving current through your house. FranklinWH, for one, lists lithium iron phosphate (LFP) cells (datasheet).
Battery management
Electronics watch every cell’s voltage and temperature and keep the pack within safe limits; output can be reduced in heat — above 104°F for the Powerwall 3.
Inverter
Turns the cells’ DC into 240 V AC and back again for charging. In a hybrid like the Powerwall 3 the same inverter also handles the panels. See what a solar inverter does.
Controller and app
Decides when to charge and discharge — self-use, time-of-use, a backup reserve — based on your settings and rate.
Capacity, power and efficiency
Capacity (kWh) is the size of the tank: one Powerwall 3 holds 13.5 kWh. Power (kW) is the size of the pipe: 11.5 kW continuous, with a rating to start motor loads up to 185 LRA (datasheet). Run essentials only — fridge, lights, Wi-Fi, a furnace fan, about 7.2 kWh a day — and one full battery lasts about 1.9 days without sun.
Efficiency: some energy turns to heat on the way in and out. Tesla puts solar-to-battery-to-home at 89% and solar straight to the home at 97.5%; FranklinWH puts the aPower 2’s round trip at 90%. So 13.5 kWh of midday solar comes back as about 12 kWh in the evening.
The batteries LA homes installed in 2025
| Measure | Value |
|---|---|
| New solar homes with a battery | 12,743 of 16,118 (79.1%) |
| Capacity: middle half | 10.0–18.0 kWh (median 13.5) |
| Power: median | 11.5 kW |
| 15 kWh or less / 15–30 / over 30 | 74.4% / 22.9% / 2.7% |
| Powerwall 3 also serving as the solar inverter | 40.4% |
| Storage per kW of solar: median | 1.85 kWh |
Source: California DG Stats, Interconnected Project Sites, SCE, data through August 31, 2026; Powerwall 3 identified by Tesla part number 1707000. Cali Energy count; LADWP is not in this dataset.
Three in four homes chose one battery’s worth of storage, enough to shift the evening peak and keep essentials on through an outage night. For brand-by-brand specifications, see Powerwall 3 vs Enphase vs FranklinWH; to size your own, the battery sizing calculator.
AC-coupled vs DC-coupled
DC-coupled
Panels and battery share one hybrid inverter, and solar can charge the battery before any conversion. Typical of new systems built around a Powerwall 3, which accepts up to 20 kW of panel DC.
AC-coupled
The battery has its own inverter and connects on the household side, so it works with any existing solar — the usual retrofit, and how FranklinWH sells the aPower 2 (DOE). See adding a battery to existing solar.
What happens in a blackout
When the grid fails, a grid-tied inverter must disconnect within two seconds (SCE Rule 21). With backup equipment — for Tesla, a Gateway 3, Backup Switch or Backup Gateway 2 — the system detects the outage, separates the house from the grid and switches to battery power; the panels then keep recharging the battery by day. Without that equipment a battery cannot back up the house. Whether it covers the whole home or chosen circuits depends on how it is wired; see whole-home vs critical-loads backup and batteries in outages and PSPS.
Operating modes and the warranty
Most batteries offer the same three modes: self-consumption (store surplus solar, use it later), time-based control (discharge in the expensive hours) and backup reserve. Tesla’s warranty allows unlimited cycles in exactly those three; add another use, such as a virtual power plant, and a 37.8 MWh throughput cap applies. See how long home batteries last and what a battery earns in a VPP.
Where to go next
Cali Energy installs home batteries in Los Angeles, AC- and DC-coupled, for SCE and LADWP customers. Call +1-323-844-7777 or ask about a battery — savings and backup figures are estimates, not guarantees.
Frequently asked
How does a home solar battery work?
It charges from solar the house doesn’t use at midday, stores it as DC in lithium cells, and an inverter turns it back into AC for the evening or an outage. A controller decides when to charge and discharge.
How much energy do you lose in a battery?
About 10%. Tesla rates the Powerwall 3 at 89% solar-to-battery-to-home, FranklinWH the aPower 2 at 90% round trip; solar used directly reaches the house at about 97.5%.
What’s the difference between AC-coupled and DC-coupled batteries?
DC-coupled batteries share a hybrid inverter with the panels; AC-coupled batteries have their own inverter and connect on the household side, which suits adding storage to existing solar.
What size battery do LA homes install?
In 2025 the median on new SCE solar homes in Los Angeles County was 13.5 kWh and 11.5 kW, and 74% had 15 kWh or less.
Will a home battery power my whole house in an outage?
Only with backup equipment, and only up to its power and capacity: one Powerwall 3 delivers 11.5 kW and holds 13.5 kWh, enough for essentials for about 1.9 days without sun but not for AC, a water heater and EV charging at once.
Can a battery charge from the grid?
Yes. The CPUC lets NEM-paired batteries charge from the grid before a Public Safety Power Shutoff (D.20-06-017), and time-based modes may use grid power too. Whether it may export that energy, and how it is credited, depends on your tariff.
What chemistry do home batteries use?
Lithium-ion; lithium iron phosphate (LFP) is common in home batteries for its thermal stability. FranklinWH lists LFP cells on its datasheet.
Related reading
Sources & methodology
Figures on this page come from the primary sources below and, where noted, from Cali Energy calculations using the stated assumptions. Rates, incentives, and program terms change; each was verified October 5, 2026.
- Tesla — Powerwall 3 datasheet (13.5 kWh, 11.5 kW, 185 LRA, 89% / 97.5% efficiency, islanding devices)
- Tesla — Powerwall Limited Warranty (USA), Rev. 2.6 (operating modes, throughput cap)
- FranklinWH — aPower 2 datasheet (LFP, AC-coupled, 90% round trip)
- California DG Stats — Interconnected Project Sites (SCE, data through August 31, 2026; Cali Energy count for Los Angeles County)
- U.S. Department of Energy — Solar-Plus-Storage 101 (AC vs DC coupling)
- SCE — Rule 21, Generating Facility Interconnections (anti-islanding, two seconds)
- SCE — How the Solar Billing Plan works
- LADWP — Service Rider NEM, Net Energy Metering
Put your solar to work after dark
Get a battery sized for your home’s power, capacity and backup goals, AC- or DC-coupled. Estimates, not guarantees.
Get a free estimatePrepared by Cali Energy, October 5, 2026. This article is for general educational purposes only and is not legal, tax, financial, engineering, or utility advice. Rates, incentives, codes, permit requirements, equipment specifications, prices, and program terms may change; figures and timelines are estimates, not guarantees. Confirm current requirements with the applicable utility, AHJ, program administrator, manufacturer, or a licensed professional. See our Content Disclaimer. Cali Energy · 19201 Parthenia St, Unit E, Northridge, CA 91324 · CSLB #1032379 (B, C-10, C-39)