A battery stores the midday power your home can't use yet and gives it back in the evening. We ran it through our calculator in 10 cities. A small battery helps; a big one adds little.
Ranges across our 10 cities, mostly set by the price of power: a 2 kWh battery adds least in Baltimore (13.4¢ a kWh) and most in Hartford (29.2¢).
$130 a year
+$58 a year vs. no battery
360 kWh a year stored and used later
Our calculator, run Oct 11, 2026. Utility rate for each city; savings rounded to the dollar.
How fast energy moves at one moment, like the speed on a speedometer. An 800 W kit puts out at most 800 W, and the state laws cap kits in watts. A battery's power rating says how fast it can charge or give back.
How much energy, like the miles on an odometer. A battery's size is in kWh. 1 kW for 1 hour is 1 kWh; so is 100 W for 10 hours.
150 W×10 h=1.5 kWh
Power times hours gives energy. Our calculator's default home has a steady 150 W always-on load (fridge, router, standby), so overnight it uses about 1.8 kWh in 12 hours.
Drag the battery size. Hours assume the whole rated capacity is usable; many batteries keep some in reserve.
Average use per home: EIA's 2020 Residential Energy Consumption Survey, tables CE5.3a and CE5.3b.
A balcony kit has only so much left over on a typical day: 0.8 to 1.2 kWh in these cities. Past that, extra capacity sits empty. The full city-by-city study →
Energy per kilogram and sodium-ion facts: IEA, 2026. Cycles and thermal stability: National Laboratory of the Rockies, 2026. LFP fires: U.S. Department of Energy, 2024. Pack prices: BloombergNEF, Dec 2025. Read Oct 18, 2026.
BloombergNEF's 2025 survey put the average lithium-ion pack at $108/kWh. That's what manufacturers and project builders pay for packs in bulk. A finished battery in a store adds the case, charger, inverter, warranty and margin, so it costs more per kWh.
Average pack prices per kWh, 2025: BloombergNEF (Dec 9, 2025); the 93% drop is in real terms (BNEF).
Batteries for the home are tested to UL standards. UL 9540 covers whole energy storage systems, UL 1973 the batteries inside them, and UL 9540A is a fire test for how thermal runaway spreads. Check the listing, not just the box. And check the circuit too: is your outlet safe?
Of the 10 signed plug-in solar laws, only Maine's mentions batteries: it 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)).
A plug-in kit shuts off when the grid goes down, and the battery with it, unless the system is built to run on its own. The U.S. Department of Energy says solar-plus-battery systems run without the grid only "if they are designed to do so".
Free calculator
Your direction, tilt and utility rate, hour by hour, with and without a battery. Change the size under "Your assumptions" on the result page.
kW (kilowatts) is power: how fast energy flows at one moment. kWh (kilowatt-hours) is energy: how much flows over time. A 1 kW load running for 1 hour uses 1 kWh. An 800 W balcony kit tells you its top power; a 2 kWh battery tells you how much it can store.
In our calculator, $26–$55 a year for a 1 kWh battery and $37–$77 for 2 kWh, with an 800 W kit on a south-facing railing in 10 cities. Going from 2 to 4 kWh adds only $2–$8: on an average day these kits send 0.8 to 1.2 kWh to the grid with no battery, so a bigger battery mostly sits partly empty.
A typical main fridge uses about 1.7 kWh a day (EIA's 2020 Residential Energy Consumption Survey), so a full 2 kWh battery covers about 1.2 days of its average use. On our sunny early March day in Denver, the battery filled to 1.8 kWh and ran a 150 W always-on load through the night.
They trade off. LFP is more thermally stable and lasts more cycles (2,000 to more than 10,000, against 1,000 to more than 5,000 for NMC, per the National Laboratory of the Rockies), and its packs averaged $81/kWh in 2025 against $128 for NMC (BloombergNEF). NMC is lighter for the same capacity. Neither is fireproof, so look for a safety listing.
They handle cold far better (the newest cells keep about 90% of their capacity at −40 °C, per the IEA) and need no lithium, but they hold less energy per kilogram, made up under 1% of lithium-ion production in 2025, and aren't yet cheaper than LFP in most uses.
BloombergNEF's 2025 survey put the average lithium-ion battery pack at $108/kWh and stationary storage packs at $70/kWh. Those are packs bought in bulk by manufacturers and project builders, not finished batteries in a store, which cost more. Divide a battery's price by its capacity in kWh to compare.
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.
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.
Savings are our calculator's own results, run Oct 11, 2026 through the same steps as a result page: NLR PVWatts v8 hourly output, an 800 W kit upright on a south-facing railing, a flat 150 W always-on load, each city's utility rate (or the EIA state average), no credit for power sent to the grid, and a battery at 90% round-trip efficiency that carries its charge from day to day. The day chart is one day from the same kind of run for Denver. It's an estimate. Our full method.
Battery facts and prices come from the sources linked beside them, read Oct 18, 2026. We list no store prices because we don't have real ones. General information, not financial advice.