“Batteries are like a generator. I knew going in those don’t pay for themselves.” That’s how I explain my two Powerwall+ units to friends, and nearly three years of data back it up. Here’s what they cost, what they actually do on FPL net metering, how efficient they really are, and why I keep them at a 100% reserve.
What they cost
The two Powerwall+ units were $18,700 on my July 2023 contract, after a $5,200 bundle discount for buying them with solar. The 30% federal tax credit took that to about $13,090*. A buyer today pays the full price, because the credit ended for systems installed after 2025. Together they store about 27 kWh.
Why they don’t save money on FPL net metering
FPL credits every kWh I send to the grid 1:1 against a kWh I buy later, at the same retail rate. So storing noon solar in a battery to use at 8 p.m. is worth exactly the same as exporting it at noon and importing it at 8 p.m. The battery doesn’t add value — it only adds its own losses. That’s why my break-even numbers are figured on the solar cost alone.
That changes if FPL ever moves to time-of-use rates for homes, or pays less for exported solar than it charges for imported power. Then a battery could start earning its keep. Today, on my bill, it doesn’t.
Think of them as a generator
What the Powerwalls do is the job of a whole-home standby generator: when FPL goes down, the Backup Gateway disconnects the house from the grid and the home keeps running on batteries and solar. There’s no fuel to store, no engine noise, no exhaust and nothing to start — it switches over automatically, and solar recharges the batteries the next morning. In a multi-day outage after a storm, that last part matters most. A generator runs until its fuel runs out; my panels can refill 27 kWh on most days, even cloudy ones (a heavily overcast October 18, 2026 still made about 40 kWh).
For comparison, Generac says a home standby generator typically costs $8,000–16,000 installed, including the generator itself (Generac). My batteries were about $13,090 after the tax credit*; without the credit, today’s buyer would be at the full $18,700, above that range. I’m not claiming one is better. A generator can run central air around the clock as long as the fuel lasts, and 27 kWh can’t: my house uses roughly 70–135 kWh a day depending on the season, so in a long outage you manage loads. But it’s the right comparison. These are a resilience purchase, not an investment.
* Includes the 30% federal tax credit Rick received for his 2023 install. That credit isn’t available for systems installed after 2025 — see break-even without it.
What they’ve actually done
How much energy came out of the batteries
kWh per month
Tesla app monthly “From Powerwall” (0.1 kWh precision). Single spikes are likely outages or Storm Watch; sustained months are daily cycling at a lower reserve.
Show the numbers
| Month | From Powerwall (kWh) |
|---|---|
| 2023-12 | 158.4 |
| 2024-01 | 287.6 |
| 2024-02 | 15.5 |
| 2024-03 | 246.2 |
| 2024-04 | 28.6 |
| 2024-05 | 0.0 |
| 2024-06 | 0.1 |
| 2024-07 | 0.9 |
| 2024-08 | 0.1 |
| 2024-09 | 0.0 |
| 2024-10 | 0.2 |
| 2024-11 | 0.0 |
| 2024-12 | 25.2 |
| 2025-01 | 0.0 |
| 2025-02 | 0.0 |
| 2025-03 | 0.1 |
| 2025-04 | 85.5 |
| 2025-05 | 0.0 |
| 2025-06 | 98.4 |
| 2025-07 | 21.9 |
| 2025-08 | 346.5 |
| 2025-09 | 571.4 |
| 2025-10 | 553.7 |
| 2025-11 | 599.9 |
| 2025-12 | 183.0 |
| 2026-01 | 0.0 |
| 2026-02 | 77.6 |
| 2026-03 | 213.6 |
| 2026-04 | 519.8 |
| 2026-05 | 0.0 |
| 2026-06 | 0.0 |
| 2026-07 | 0.0 |
| 2026-08 | 19.2 |
| 2026-09 | 0.0 |
| 2026-10 | 0.0 |
Most months the batteries deliver almost nothing, because they sit at a 100% reserve waiting for an outage. The exceptions:
- Dec 2023–Mar 2024 (158 / 288 / 16 / 246 kWh): commissioning, likely before I set the reserve to 100%.
- Apr 2025, Jun 2025, Jul 2025, Aug 2026 (86 / 98 / 22 / 19 kWh): single events — likely outages or Storm Watch.
- Aug–Dec 2025 (347 / 571 / 554 / 600 / 183 kWh) and Mar–Apr 2026 (214 / 520): daily cycling at a lower reserve.
Over about 2.9 years that’s only around 165 full cycles. The batteries mostly sit idle, which is what a backup system should do.
100% reserve vs. a low reserve: I tested it the expensive way
From August to November 2025 I ran the batteries with a lower reserve (roughly 30%, judging by how deep they went), so they discharged every night — about 2,072 kWh over four months, roughly 17 kWh a day. It felt productive. My self-consumption (the share of solar I used myself) jumped to 75–80%, up from 60–70% the year before.
Under 1:1 net metering, though, that shift saved nothing. What it did do was lose energy on every charge and discharge. At 81–90% round-trip efficiency, a 20% reserve run year-round would waste about 690–1,450 kWh a year, roughly $85–180 at my FPL rate (estimate). At a 100% reserve, the only cost is idle top-ups, about $26 a year. And at 100% I always have a full battery when a storm shows up with little warning.
So my reserve is back at 100%, and it stays there.
Efficiency: 81% measured vs. 90% on the spec sheet
Tesla rates the Powerwall+ at 90% round trip, measured AC to battery to AC at 77°F and 3.3 kW. My lifetime totals through October 23, 2026: 5,499.2 kWh charged and 4,454.0 kWh discharged — 81.0%. The two units are nearly identical (81.2% and 80.8%), so neither one is weak.
Round trip: spec vs. reality
Lifetime charge/discharge totals per Powerwall from the Tesla gateway via Home Assistant. “Cycling only” removes an estimated 420–730 kWh of standby top-ups (ESTIMATE).
Most of the gap is idle draw, not bad batteries. Over a measured week at a 100% reserve, each Powerwall slowly drifted down about 0.36 kWh, then topped back up. Together that’s 0.58 kWh a day, or about 213 kWh a year (estimated from a mild October week; summer is probably higher because of battery cooling). Those top-ups count as “charged” with nothing ever discharged. Take out an estimated 420–730 kWh of them over the system’s life and the cycling efficiency comes out around 88–93% — right on Tesla’s spec.
The “DC charging” myth
Some installer sites say Powerwall+ is “DC-coupled,” so solar charges the battery more efficiently. Tesla’s own documents say otherwise. The Powerwall+ datasheet says it “has two separate inverters, one for battery and one for solar.” And in the install manual’s step for connecting the solar and battery assemblies, the installer plugs in the “battery assembly AC wiring harness” plus a data cable. There’s no DC link between the two.
So solar charging inside a Powerwall+ goes panels (DC) → solar inverter (AC) → battery inverter (back to DC). It’s AC-coupled inside the box. In practice, panels wired to the Powerwall+ have no efficiency advantage over panels on a separate inverter.
Sources: Tesla Powerwall+ datasheet; Tesla Powerwall+ install manual pages on the solar and battery assemblies and the connection step.
Practical tips
- Price batteries like a generator, not an investment. On FPL net metering they won’t pay for themselves.
- Keep the reserve at 100% unless your rate plan rewards evening use. Cycling just burns energy.
- Leave Storm Watch on. It tops the batteries off before named storms.
- Know your outage loads. 27 kWh goes fast with central air running, so decide ahead of time what you’ll turn off.
- Check the lifetime charge/discharge totals once a year. Both units should track each other.
Related: how net metering shows up on my FPL bill.