Short answer
In our calculator, most of what a battery adds comes from the first kilowatt-hour or two. For an 800 W kit on a south-facing railing in 10 cities, a 1 kWh battery adds $26–$55 a year and a 2 kWh battery $37–$77, about 1.6 to 1.8 times the savings without one. Going from 2 to 4 kWh adds only $2–$8. It doesn't make the kit backup power.
What a battery adds, city by city
The same 800 W kit on a south-facing railing, in one big city per state with a plug-in solar law, run through our calculator with no battery and with three battery sizes. Each city uses its utility's rate, or the state average where we couldn't look one up. The home has a steady 150 W always-on load, and power sent to the grid earns nothing.
| City | Rate | No battery | 1 kWh | 2 kWh | 4 kWh | Made |
|---|---|---|---|---|---|---|
| Hartford, CT | 29.2¢ | $11655% used | $17182% used | $19393% used | $19995% used | 710 kWh |
| Newark, NJ | 27.1¢ | $10856% used | $15983% used | $18093% used | $18495% used | 710 kWh |
| Los Angeles, CA | 24.5¢ | $10654% used | $15781% used | $18293% used | $18695% used | 790 kWh |
| Portland, ME | 25.6¢ | $10252% used | $15478% used | $17790% used | $18594% used | 770 kWh |
| Burlington, VT | 23.9¢ | $9454% used | $14282% used | $16092% used | $16595% used | 730 kWh |
| Manchester, NH | 22.5¢ | $9153% used | $13779% used | $15791% used | $16395% used | 770 kWh |
| Denver, CO | 16.1¢ | $7250% used | $11178% used | $13091% used | $13695% used | 890 kWh |
| Richmond, VA | 16.2¢ | $6558% used | $9483% used | $10694% used | $10896% used | 700 kWh |
| Baltimore, MD | 13.4¢ | $5555% used | $8182% used | $9293% used | $9596% used | 740 kWh |
| Salt Lake City, UT | 12.3¢ | $5354% used | $8182% used | $9293% used | $9495% used | 800 kWh |
"Used" is the share of what the kit makes that your home uses, directly or later from the battery. Each city name opens its result page, where you can change the battery size.
Why a bigger battery adds so little
A battery can only store what the kit makes and your home doesn't use at that moment. With no battery, the kit sends 0.8–1.2 kWh a day to the grid on average in these cities, more on sunny summer days and less in winter. A 1 to 2 kWh battery catches most of that and gives it back in the evening. A 4 kWh battery is rarely full, so the extra capacity mostly goes unused: from 2 to 4 kWh, savings rise by only $2–$8 a year.
In Denver, for example, the kit makes about 890 kWh a year. Without a battery your home uses 50% of it; with 2 kWh, 91%. Savings go from $72 to $130 a year, and to $136 with 4 kWh.
Your home's always-on load changes the answer
The less your home uses while the sun is up, the more there is to store, and the more a battery adds. Here is Denver with three always-on loads: a quiet apartment, our default, and a home with more running all day.
| Always-on load | No battery | 2 kWh battery | Battery adds |
|---|---|---|---|
| 100 W | $53 | $114 | +$61 (116%) |
| 150 W | $72 | $130 | +$58 (80%) |
| 300 W | $113 | $140 | +$27 (24%) |
It isn't backup power
A plug-in kit shuts off when the grid goes down, so the power doesn't flow into lines that are supposed to be dead. Adding a battery doesn't change that by itself. The U.S. Department of Energy puts it this way: Solar-plus–battery storage systems rely on advanced inverters to operate without any support from the grid in case of outages, if they are designed to do so.
(energy.gov). If you want power in an outage, check that the battery is built to run without the grid.
What the laws say about batteries
Of the ten signed plug-in solar laws, only Maine's mentions batteries. Its definition reads: "Eligible system" means a plug-in photovoltaic system or plug-in battery system with an export capacity of 1,200 watts or less
(35-A M.R.S. § 3475(1)(A)). So in Maine a battery counts toward the same limit as the panels. The plug-in sections of the other nine laws don't mention batteries. Your state's law.
What these numbers leave out
- Battery prices. We don't list any. Divide the price you're quoted by the extra savings a year above to get a simple payback, then compare it with the battery's warranty.
- Time-of-use rates. We value every kWh at one price. If your evening power costs more than your midday power, a battery is worth more than shown here.
- Battery wear. Our model assumes 90% round-trip efficiency and doesn't model capacity loss over the years.
- Your real usage. A steady 150 W load is an assumption. Upload your utility's meter file on a result page to use your home's own hourly use.
Try your own address with a battery
Your direction, tilt, rate and battery size, hour by hour.
Batteries and balcony solar: questions
Is a battery worth adding to a balcony solar kit?
It depends on the battery's price, which we don't track. What it adds is easier to say: in our calculator, $26–$55 a year for 1 kWh and $37–$77 for 2 kWh with an 800 W kit on a south-facing railing, across 10 cities. Divide the price by the extra savings a year for a simple payback, and compare that with the battery's warranty.
What size battery for a balcony solar kit?
In our runs, about 1 to 2 kWh. On an average day the kit sends 0.8 to 1.2 kWh to the grid with no battery, so a bigger battery mostly sits partly empty: going from 2 to 4 kWh adds $2–$8 a year.
Does a battery help more if my home uses little power during the day?
Yes. In Denver, with a 100 W always-on load a 2 kWh battery takes savings from $53 to $114 a year; with 300 W, from $113 to $140. The less your home uses while the sun is up, the more surplus there is to store.
Will a battery keep my lights on in a power outage?
Not on its own. A plug-in kit shuts off when the grid goes down. The U.S. Department of Energy says solar-plus-battery systems run without the grid only "if they are designed to do so", so check that before you count on one.
Do state plug-in solar laws cover batteries?
Maine's does: its law counts a "plug-in battery system" as an eligible system within the same 1,200 W limit (35-A M.R.S. § 3475(1)(A)). The plug-in sections of the other nine signed laws don't mention batteries.
How we calculated this
Every figure is our calculator's own result, run Oct 11, 2026 through the same steps as a result page: NLR PVWatts v8 hourly output for the city, the 800 W inverter limit, a flat 150 W always-on load, the utility's rate (or the EIA state average), and no credit for power sent to the grid. The battery charges from each hour's surplus and covers the load when the panels fall short, at 90% round-trip efficiency, carrying its charge from day to day. It's an estimate. Our full method.
Maine's law and the Department of Energy page were read on Oct 5, 2026. General information, not financial advice.