Plug-in batteries for balcony solar
A plug-in battery is the second half of the answer to plug-in solar's weakness: it produces most at midday, when a house often draws the least. This page covers which laws address a battery, the two certifications that apply to it, what is actually for sale in the US right now, and the arithmetic behind whether one pays.
What a plug-in battery is
A plug-in battery is a cord-connected storage unit, either built into the same housing as a plug-in solar system's microinverter or added alongside one. It stores whatever surplus the panel makes beyond what the house is drawing at that moment, then discharges it back through the same outlet later, typically timed to the evening when usage and prices both tend to be higher. Some units ship stackable, so a single base model can be scaled up by adding more of the same battery.
Which laws cover it
Of the ten states with a plug-in solar law, only Maine names a plug-in battery system and covers it directly. California names one too, specifically to exclude it. The other eight define a photovoltaic device and say nothing about storage.
| State | Battery coverage |
|---|---|
| Utah | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| Maine | Covered: the law names a plug-in battery system directly. |
| Virginia | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| Colorado | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| Maryland | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| New Hampshire | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| Connecticut | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| Vermont | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| New Jersey | Silent: the law defines a photovoltaic device; a battery inside one isn't addressed either way. |
| California | Excluded: the law covers solar only, with no storage provisions. |
A battery that is "not addressed" is not the same as one that is forbidden. It means the law's exemption from interconnection and fees applies to the solar device it describes, and a court or utility has not yet been asked whether an integrated battery under the same export cap gets the same treatment. Ask before you buy, the same way you would about the panel.
Two standards
UL 3700 covers the plug-in solar device: the inverter that backfeeds a branch circuit, its overcurrent protection, and its outage-isolation feature. It says nothing about a battery's storage function. UL 9540 is the separate standard for an energy storage system: cell safety, thermal-runaway protection, and how the unit manages its own charge and discharge. A product that both generates and stores power needs a listing under each standard for its own half of the job; a UL 3700 listing by itself has addressed the solar side, not the battery.
What is sold in the US today
EcoFlow's STREAM Ultra pairs a 1.92 kWh LiFePO4 battery with a built-in microinverter, listed at $2,399 and stackable to 11.52 kWh across six units. As of mid-2026, EcoFlow sold it for use in Utah only, the one state whose law has been in force long enough to build a market around, with expansion tied to more states passing their own. Anker's SOLIX balcony and Solarbank products were not sold in the US as of July 28, 2026, and were not UL certified for this market. Treat both as dated examples of a market still forming behind the laws, not a current catalog, and check certification and availability again before buying anything.
When a battery pays
Exporting a surplus watt from a plug-in system usually pays nothing (seedoes it pay), so the choice for a watt you are not using right now comes down to two options: shift a load to meet it, or store it for later. A battery is the second option. CESA's guide to plug-in solar puts a rough size on what that buys: a 1–2 kWh battery can run a refrigerator and a CPAP machine for 24 hours (CESA, "Plug-In Solar," June 2026).
Run that against a single STREAM Ultra: 1.92 kWh, discharged into an evening base load of 200–400 W (the same range the load-shifting guide already uses for a typical house), covers roughly five to ten hours before it is empty. That is an evening's worth of lights, a refrigerator, and standby draw, not a whole night of HVAC or EV charging, and not whole-home backup. Whether it is worth $2,399 depends on what those kilowatt-hours would otherwise have cost at your most expensive hour, and on how many evenings a year the panel actually fills it.
How the optimizer uses one
Register a plug-in battery in Hungry Machines and it is optimized the same way a whole-home battery is, scaled down to its size: a charge target for how full to keep it, a reserve it will not discharge below, and a plan that charges it from solar surplus or a cheap grid hour and discharges it against the price curve into the evening. See the "Adding a battery" section ofplug-in solar and load shifting for how that fits alongside the rest of a home's schedule, EV charger, HVAC, water heater, and robot included.
Frequently asked questions
Is a plug-in battery legal everywhere plug-in solar is?
Not automatically. Of the ten state laws in this guide, Maine is the only one that names a plug-in battery and covers it the same as the solar device. California names one too, to exclude it from its law entirely. The other eight define a photovoltaic device and say nothing about storage, so a battery built into an NRTL-certified device that stays under the export cap isn't forbidden, but it isn't addressed either. Ask your utility before adding one, the same way you would before adding the panel.
Does a plug-in battery need UL 9540, UL 3700, or both?
Both, because they cover different functions. UL 3700 is the plug-in solar standard: the inverter that backfeeds your circuit and shuts it off in an outage. UL 9540 is the energy-storage standard: cell safety and how the battery manages charge and discharge. A kit that generates and stores power needs a listing for each half; a UL 3700 listing on its own says nothing about the battery inside it.
What can I actually buy today?
As of mid-2026, EcoFlow's STREAM Ultra, a 1.92 kWh battery with a built-in microinverter, was sold for use in Utah only, the state whose law has been in force longest. Anker's SOLIX balcony products were not sold in the US as of July 28, 2026, and were not UL certified for this market. Both are dated examples of a market that is moving with the laws, not a current catalog, so re-check availability and certification before you buy.
Is a plug-in battery worth the cost?
It depends on the price of the battery against the spread between your cheapest and most expensive hours, the same arithmetic as any battery. A plug-in unit adds one more variable: it can only ever store what the panel it's paired with produces, so its payback is capped by a small system's output, not by how big a battery you could afford. See the does-it-pay page for the worked numbers behind a plug-in system on its own; the battery is an addition on top of that, not a separate investment to size independently.
About Hungry Machines
Outsmart your electric bill - Hungry Machines Energy provides an energy management system that shifts HVAC, EV charging, home batteries, and other electric loads to cheaper hours for homes and buildings with solar power or variable electricity rates.