In this guide
Short answer: usually not yet, on savings alone. A battery for a plug-in kit costs about $800 to $1,500 extra and typically saves $30 to $90 a year more, so payback often takes 15 years or longer. It makes sense if you're away all day, have a 1,200 W kit, pay high evening rates or want backup power from a model with a separate outlet.
What a battery does in a plug-in system
A plug-in kit sends its power into your home through an outlet. Whatever your appliances use at that moment, you save. Whatever they don't use flows out to the grid, and in nearly every US state with a plug-in law, exports aren't paid. A battery catches that surplus at midday and releases it in the evening, when the panels are idle and you're cooking, watching TV or charging things.
Most plug-in batteries sit between the panels and the outlet. The inverter output into your wall stays under your state's cap (usually 1,200 W) whether the power comes from the panels or the battery.
When the extra savings are big enough
| Your situation | Self-use without battery | With battery | Worth it? |
|---|---|---|---|
| 400 W kit, someone home in the day | ~95% | ~98% | No |
| 800 W kit, someone home in the day | ~80% | ~95% | Rarely |
| 800 W kit, home empty 8 am–6 pm | ~50–60% | ~90% | Possibly |
| 1,200 W kit, home empty in the day | ~40–50% | ~85–90% | Often |
Example: an 800 W kit in a high-price state (30¢/kWh) producing 1,000 kWh a year, in a home that's empty during the day. Without a battery you use about 550 kWh ($165). With a battery you use about 900 kWh ($270). The battery adds about $105 a year. At $1,000 extra, that's roughly a 10-year payback, which is close to the battery's expected life. In a 15¢ state, it's twice as long.
Before buying a battery, try the free fix: run the dishwasher, washing machine and dehumidifier at midday on timers. Shifting 1 kWh of daily use into sunny hours does what a small battery does.
What the laws say about batteries
- Maine expressly covers plug-in "photovoltaic or battery" systems, one per customer.
- Most other state laws cap output to the outlet and require certification; a battery inside a certified system is generally treated as part of the device. Check how your state counts the cap.
- Certification: look for the battery's own listing (commonly UL 1973 or UL 9540) as well as the plug-in system's (UL 3700 or UL 1741). See UL 3700 explained.
- Great Britain: the government's plug-in specification excludes battery storage. See plug-in solar in the UK.
Products on the US market
EcoFlow's STREAM Ultra combines a 1.92 kWh LFP battery with up to 1,200 W of AC output and up to 2,000 W of solar input. It launched in the US at $1,899 in May 2026, initially for Utah, with UL 3700 and UL 1973 certifications reported as pending in September 2026. More all-in-one batteries are expected as UL 3700 certifications come through. Our kit guide lists what's available.
Backup power is a separate question
A plug-in system must stop feeding the wall outlet during an outage. A battery only gives you backup if it has a separate outlet designed to run on its own (an "off-grid" or "backup" socket). Check the US spec sheet for that feature before you buy; see does balcony solar work in an outage?
How to size it
- Estimate surplus: a sunny-day output of 800 W × about 5 hours is 4 kWh; subtract what your home uses in those hours (often 1.5 to 3 kWh).
- Match the battery to the surplus: for most 800 W kits, 1 to 2 kWh is enough.
- Check the output cap and certifications for your state.
- Run the numbers in the calculator with a higher self-use percentage (try 90%) and compare.
Frequently asked questions
Do I need a battery for balcony solar?
No. Most plug-in kits work without one, and for a 400 to 800 W kit in a home with a fridge, router and other daytime loads, most output is used anyway. A battery helps if you're out all day or have a 1,200 W kit.
How much extra does a battery save?
It saves the value of the power it shifts from midday to evening. If a battery shifts 1 kWh a day for 250 days a year at 20 cents per kWh, that's about $50 a year, which is slow payback on a battery that adds $800 to $1,500.
Is a plug-in battery legal?
In several US states, yes: Maine's law covers plug-in solar or battery systems, and most laws cap the output that flows into the outlet rather than the battery size. The battery must be certified (for example to UL 1973 or UL 9540) as part of a qualifying device. In Great Britain, the plug-in route excludes batteries.
Will a plug-in battery give me backup power?
Only if it has a separate backup outlet that works when the grid is down. The part connected to your wall outlet must shut off during an outage, like any plug-in kit.
What size battery should I get with a plug-in kit?
Roughly 1 to 2 kWh for an 800 W kit. That's about what such a kit produces beyond your daytime use on a sunny day. Larger batteries sit half-empty most of the year.
Update log
- Guide published.
Sources and official texts