What Happens When a Home Battery Is Full?

The question people mean when they ask this is not really about the battery. The battery part is dull and well documented: charging stops. The interesting part is what happens to the solar kilowatt-hour that arrives one second later, because there are only four places it can go and three of them are worth different amounts of money. Everything below is quoted from manufacturer documents and from the California Public Utilities Commission, read August 19, 2026. We have not tested any of these products.

Charging stops on a threshold, not a clock

Lithium batteries are charged in two stages. Constant current first, pushing a fixed current into the pack while the voltage climbs. Then constant voltage, holding the pack at a set voltage while the current it accepts falls away. The charge ends when that current drops below a cutoff value.

Manufacturers rarely put this on a datasheet, but two of them publish the exact numbers inside their warranty documents, because the test method for a capacity claim has to specify them.

Qcells states its battery performance measurement conditions as, verbatim, "Charge: 0.25C-Rate CC/CV, Cut-off current 0.05C at 262.8 V" with a discharge cutoff at a minimum cell voltage of 2.5 V. That is the whole mechanism in one line: constant current, then constant voltage at 262.8 V, terminating when the accepted current falls to five percent of the C-rate.

BYD's US limited warranty describes the same sequence in words for its capacity test: "Charge the battery with constant current and constant charge voltage to its full capacity," with a table of constant charge voltages per model.

Fortress Power's warranty goes further and publishes the settings an installer is expected to program into the inverter. For its 8,000 cycle configuration: bulk voltage 54.4 V, absorb voltage 54.4 V, recharge voltage 52 V, high battery cut out 61 V, and float voltage "Turn off, if possible."

That last set is the most revealing for this question. Once a Fortress pack reaches 54.4 V it is done, and it does not start charging again until the pack has fallen back to 52 V. Fortress explicitly advises turning float off rather than holding the pack at the top. A full battery is not a battery being continuously topped up. It is a battery being left alone until it has drifted down enough to justify another charge.

Then the array has nowhere obvious to be

Once the battery has stopped accepting energy, your solar output has three remaining destinations, in a strict priority order that every system follows.

Your house loads. Whatever the refrigerator, the air conditioning and the standby draw are consuming at that moment is served first, because that is the cheapest kilowatt-hour in the system: it avoids a retail purchase.

The grid. If your interconnection permits export, everything above house load flows out and earns a credit.

Nowhere. If export is not permitted, or you are off-grid, the system reduces production to match the load.

Which of those applies is not a setting you pick freely. It is set by your utility at permission to operate, and it is the single largest determinant of what a full battery means for your bill.

What the exported kilowatt-hour is worth

The concise answer, for California, comes from the CPUC's own net energy metering and net billing page.

The CPUC states that under the net billing tariff, "compensation for excess generation exported to the electric grid is applied to a customer's bill at a rate reflecting the value of this generation to the grid," that "the value of the export compensation (which the IOUs term 'Energy Export Credits') is usually lower than the retail rate," and that it "can rise above the retail rate on late summer evenings." Its own comparison table gives the basis for those credits as CPUC Avoided Cost Calculator values, against import rates under the older NEM 1.0 and NEM 2.0 tariffs.

Two consequences follow, and both bear directly on a full battery.

First, there is no single export rate to plan against. The Avoided Cost Calculator produces hourly values, which utilities publish as schedules varying by month and hour, so a midday export and a 7 p.m. export are not worth the same thing. We are not printing a number here for that reason.

Second, the CPUC says outright why storage changes the calculation: "Customer-generators can maximize bill savings under the NBT by installing battery storage along with their generation, so they can use or export stored energy during these high-value hours." The point of the battery is to move energy out of the low-value part of the curve into the high-value part. A battery that fills at 10 a.m. and then watches the rest of the day export at midday values is doing half its job. The full mechanics are in our NEM 3.0 guide and the short definition is in the NEM 3.0 glossary entry. Outside California the structure differs by utility, but the underlying question is identical: what is an exported kilowatt-hour credited at, and at what hour.

When export is blocked, the array is turned down

This is the case that surprises people, and Tesla documents it in two places with unusual clarity.

On its Advanced Settings support page, Tesla describes a Permission to Export setting available on systems with an integrated solar inverter. With it set to No, Tesla states: "Powerwall will reduce solar production to prevent solar export." The longer description is more specific: "When set to 'No,' Powerwall will charge from solar production. When Powerwall is fully charged, solar output will be reduced to match your home loads, preventing export. This will reduce overall solar generation." With it set to Yes, Tesla states that "Powerwall will not adjust charging or solar production to prevent export." Tesla adds that permission to export is generally granted by the local utility, and that on systems installed by a certified installer the owner has to change the setting once permission to operate is granted.

The mechanism becomes explicit in Tesla's description of its off-grid mode, where export is impossible by definition. Asked what happens to excess solar in that state, Tesla answers: "When your Powerwall is fully charged while using Go Off-Grid, excess solar production will have no available destination, as it would otherwise be sent to the grid. To prevent overcharging or other system damage, your Powerwall will raise your system frequency to turn off your solar inverter."

Raising the frequency is how a battery inverter tells a grid-tied solar inverter to stop. Grid-following inverters disconnect when frequency leaves their allowed band, so shifting the frequency deliberately is the standard way to curtail production in an islanded system. It is also why Tesla notes that flickering lights can occur during off-grid operation with a full battery, and why on a Powerwall 3 the compliance list includes UL 1741 PCS, the power control systems standard that covers managed output.

The practical takeaway for an off-grid or non-export system: a battery that hits full early in the day is production you never harvested. Tesla's own advice on the off-grid page is to reconnect to the grid before the Powerwall fully charges if you want to maximize solar production.

Scheduling so the battery is full at the right time

Everything above is why discharge scheduling exists, and why the interesting setting is not "charge the battery" but "when."

Tesla exposes the trade directly through an Energy Exports setting with two options, available only where the utility permits export for time-of-use purposes. Set to Solar, Tesla states the Powerwall "will only export solar production to the grid during high-cost time periods," and will use stored energy to match house consumption when energy is expensive. Set to Everything, it "will export both solar production and stored Powerwall energy to the grid during high-cost time periods" and will continue discharging down to the configured backup reserve. One keeps energy at home for independence, the other sells it into the peak. Which one wins depends entirely on the spread between your import price and your export credit in those hours.

The backup reserve interacts with this too. A reserve set high means the battery reaches a floor early in the evening and then sits, waiting; a reserve set low means more of the pack is available to arbitrage but less is held for an outage. Neither is wrong, and the correct answer follows from whether you bought the battery for bills or for outages.

If your rate has a meaningful peak-to-off-peak spread, the arithmetic on all of this is in our load shifting guide, and the anatomy of a time-of-use rate is in the time-of-use entry. One honest caveat that belongs in any such calculation: energy does not survive the round trip intact. Every stored kilowatt-hour comes back smaller, by an amount that varies by model and by the basis the maker publishes it on, which we compare in our round-trip efficiency roundup.

Does sitting at full hurt it?

We would like to answer this with a curve. There is no curve to quote, from any manufacturer in our tracked set. Nobody publishes degradation as a function of state of charge, which means anyone showing you one drew it themselves.

What manufacturers do publish is the opposite warning, in two different registers. Tesla's Powerwall 3 datasheet gives a storage state of energy of 25 percent initial, which is shipping and storage guidance rather than a warranty term. Three makers put the same idea in binding language. Anker's warranty notes state that "When the SOC of the battery decreases to 0%, charge the battery within seven days," and that permanent damage from failing to do so is not covered. Qcells excludes faults or damage arising from the system "not being charged for more than twenty five (25) consecutive calendar days for any reason." BYD excludes deterioration resulting from "the Product not being operated for any period of 6 months or more."

So the documented hazard in the guidance and in these contracts is a battery left flat or left idle, not a battery left full. That is not the same as an endorsement of sitting at 100 percent indefinitely, and it is not a claim that state of charge does not matter. It is a report of where the published warnings actually point. What each maker does commit to on capacity over time is set out in our degradation guide.

What to check on your own system

Find out whether you are permitted to export, and get the answer from the utility document rather than the app. That single fact determines whether a full battery means a credit or a curtailment.

Find out what your export credits are worth by hour, from your utility's published schedule. If your battery reliably fills before the high-value window opens, you are exporting at the cheap end and buying at the expensive end.

And find out what the system is actually doing at 1 p.m. in June. Most monitoring apps will show whether production is flat-topping against your house load, which is what curtailment looks like on a graph.

Every figure and quotation on this page comes from a published manufacturer document or from the CPUC, read August 19, 2026. Tariffs and firmware behavior both change, so confirm the current documents for your own equipment and utility before acting on any of it.