Is a 13.5 kWh Home Battery Too Big? Measured Daily Solar Surplus and Night Use in 103 LA Homes
For a typical Los Angeles solar home, a 13.5 kWh battery is more than the daily routine needs. Across 103 homes without a battery, the panels sent a median 16 kWh a day to the grid and the home bought back 13 kWh at other hours. The energy a battery could actually move from one to the other — the smaller of the two each day — was a median 8.5 kWh. A 13.5 kWh battery would have filled and emptied completely on only 18% of days, and been about 61% used on an average day; a 10 kWh battery about 75%. Bigger households and backup for outages are the cases where more storage earns its keep.
A battery earns its keep by storing solar the house would otherwise export and using it later instead of grid power. How much it can do in a day is capped by both sides: the surplus available to charge it and the evening and night use available to discharge into.
A day’s surplus and a night’s need
103 homes without a battery, every clean day from January 2023 to mid-October 2026.
| Measure | Median home | Middle half of homes | Top tenth of homes |
|---|---|---|---|
| Home use a day | 25.9 kWh | 15–36 kWh | 51 kWh or more |
| Sent to the grid a day (surplus) | 15.9 kWh | 9–24 kWh | 29 kWh or more |
| Bought from the grid a day (need) | 13.2 kWh | 9–20 kWh | 31 kWh or more |
| Could be shifted by a battery (smaller of the two) | 8.5 kWh | 5–12 kWh | 17 kWh or more |
| Shiftable on a big day (each home’s 90th-percentile day) | 16.5 kWh | 9–23 kWh | 31 kWh or more |
| Each home’s median day, then the median across homes. A median day is lower than the yearly average: the same kind of homes used an average 31 kWh a day over a year, see home electricity use. “Shiftable” ignores battery losses of 10–20% and the order of the hours, so it is an upper bound for what a battery could do. | |||
https://cali-energy.com/research/home-battery-size-daily-solar-surplus-los-angeles. Aggregated data are free to use under CC BY 4.0 with a link to this page.How much of each battery size a home would use
| Battery size | Days it would fill and empty completely | Middle half of homes | Average share used each day | Middle half of homes |
|---|---|---|---|---|
| 5 kWh | 79% | 48–91% | 91% | 79–96% |
| 10 kWh | 36% | 6–65% | 75% | 51–87% |
| 13.5 kWh | 18% | 1–42% | 61% | 38–78% |
| 20 kWh | 3% | 0–18% | 43% | 26–61% |
| Average share used = mean over days of min(shiftable energy, battery size) ÷ battery size. | ||||
For the median home, a 5 kWh battery would be put to near-full use (91%) and a 10 kWh battery would still be about 75% used; a 13.5 kWh battery 61%, and a 20 kWh battery under half. That does not make a bigger battery wrong. The top quarter of homes would use a 13.5 kWh battery 78% or more of its capacity, an EV or heat pump can change the picture overnight, and a battery for outages is sized to the loads it must carry for a day or two, not to the daily surplus. What it should not be is sized by default.
Month by month
Median home, energy a day.
| Month | Sent to the grid | Bought from the grid | Could be shifted |
|---|---|---|---|
| January | 11.0 kWh | 13.6 kWh | 7.7 kWh |
| February | 14.9 kWh | 12.0 kWh | 7.6 kWh |
| March | 19.6 kWh | 10.4 kWh | 7.7 kWh |
| April | 23.6 kWh | 8.7 kWh | 7.4 kWh |
| May | 22.0 kWh | 9.8 kWh | 6.9 kWh |
| June | 21.9 kWh | 12.4 kWh | 8.8 kWh |
| July | 17.6 kWh | 17.4 kWh | 10.8 kWh |
| August | 14.4 kWh | 20.7 kWh | 10.2 kWh |
| September | 14.6 kWh | 16.4 kWh | 9.7 kWh |
| October | 15.0 kWh | 12.1 kWh | 8.7 kWh |
| November | 11.4 kWh | 12.2 kWh | 7.5 kWh |
| December | 9.5 kWh | 14.5 kWh | 6.7 kWh |
The need peaks in summer, when air conditioning runs into the evening: the median home bought 20.7 kWh a day from the grid in August and could have shifted 10.8 kWh a day in July. In spring there is plenty of surplus but little evening use to fill, and in winter the surplus itself runs short: 9.5 kWh a day sent to the grid in December. A battery sized for August sits partly idle in May and December.
How full real batteries get
18 batteries in our fleet that cycle 100 or more times a year, 2023–2026.
| Measure | Median battery | Middle half |
|---|---|---|
| Charged per day, share of capacity | 82% | 58–89% |
| Days charged to 90% or more | 27% | 8–40% |
| Winter day (November–January) | 79% | 48–88% |
| Summer day (May–August) | 86% | 66–92% |
| Most of these batteries hold 10.08 or 20.16 kWh. Charge includes the energy a battery draws to stay ready. | ||
The batteries already installed in our fleet, most of them 10 or 20 kWh, tell a similar story. On a typical day the median one took in 82% of its capacity, and it reached 90% or more on 27% of days. How many cycles that adds up to, and what it means for the warranty, is measured in how many cycles a year a home battery really does; what a battery changes in a home’s energy flows is in how much solar a home actually uses. To size your own: the battery sizing calculator.
Methodology
- Homes
- 103 Enphase-monitored homes with a consumption meter and no battery, at least 300 clean days from January 2023 to mid-October 2026, with the data checks of the self-consumption study.
- Shiftable energy
- For each day, the smaller of the energy sent to the grid and the energy bought from it: the most a battery of unlimited size could have moved that day, ignoring losses and the order of the hours.
- Battery sizes
- For a size X, the share of days with shiftable energy of X or more, and the average of min(shiftable, X) ÷ X.
- Real batteries
- 18 Enphase batteries with at least 300 clean days since 2023 and 100 or more cycles a year; daily charge ÷ usable capacity.
- Limits
- Rates, outage backup and future loads such as an EV are not modeled.
Frequently asked questions
Is a 13.5 kWh battery too big for my home?
For a typical LA solar home it is more than the daily routine fills: in our data it would be used about 61% on an average day and fill completely on 18% of days. Larger households, EVs and backup needs can justify it.
How much solar surplus is there to store each day?
The median home sent 16 kWh a day to the grid, but bought 13 kWh at other hours; the energy a battery could actually shift was about 8.5 kWh a day.
What battery size do most LA solar homes need for daily use?
Around 10 kWh covers most days for the median home (about 75% used on average); the middle half of homes could shift 5–12 kWh a day.
Do home batteries fill up every day?
No. Batteries in our fleet that cycle charged to a median 82% of capacity a day and reached 90% or more on 27% of days.
When does a battery help most?
In summer: the median home could shift 10.8 kWh a day in July, against 6.7 in December.
More research
Next steps
Sizing a battery for your home?
Prepared by Cali Energy, October 15, 2026. Figures describe systems in our monitoring fleet and are not a guarantee of what any specific roof will produce; orientation, tilt, shade, equipment and weather change results. Not engineering, financial or utility advice. See our Content Disclaimer.