Key takeaways
- Without net metering, surplus solar is given away — a battery captures it for the evening.
- A battery raises self-consumption from roughly 80% to 95% or more.
- The extra savings are modest: often $50–$150 a year, so payback depends on battery price and your rate.
- Time-of-use rates make batteries much more valuable, especially in California.
- Grid-tied plug-in systems shut off in a blackout; backup requires a system with an isolated outlet.
Plug-in solar has one built-in inefficiency: every state law exempts it from net metering, so any electricity your home doesn't use at the moment it's produced flows out to the grid for free. A battery fixes that by storing the surplus and releasing it later.
The question is whether the extra savings justify the extra cost.
How a balcony battery works
Most plug-in batteries are AC-coupled or DC-coupled units that sit between the panels and the outlet:
- Panels charge the battery and power your home at the same time.
- When your home uses less than the panels produce, the surplus goes into the battery instead of the grid.
- In the evening, the battery discharges through the outlet — still within your state's AC output cap.
Many units let you set a constant output (for example, 200 W to match your base load) or follow a smart meter or app schedule.
The math: how much more do you save?
Take an 800 W kit producing about 1,130 kWh a year (US-average sunshine, south-facing, tilted).
- Without a battery, about 80% is used at home: ~904 kWh.
- With a battery, 95% or more: ~1,074 kWh.
The battery recovers roughly 170 kWh a year. Its value depends on your rate:
| Rate | Extra savings per year | Payback on a $700 battery |
|---|---|---|
| 18.3¢ (US average) | ~$31 | 20+ years |
| 25.2¢ (New Jersey) | ~$43 | 16 years |
| 33.6¢ (California) | ~$57 | 12 years |
On a flat rate, then, a battery mostly doesn't pay for itself through recovered surplus alone. Two situations change that.
1. Oversized systems or low daytime use
If you're out all day and your base load is small, your self-consumption without a battery might be 50–60% rather than 80%. Then a battery can recover two or three times as much energy, and the payback shortens dramatically. The same applies if you install a 1,200 W system in a small apartment.
2. Time-of-use rates
On TOU plans, the battery isn't just recovering surplus — it's moving cheap midday solar into expensive evening hours. In California, where peak rates (typically 4–9 pm) can be far above off-peak, shifting 1–1.5 kWh into the peak every day can be worth $100–$200+ a year for many customers. That's where balcony batteries make the most financial sense.
Backup power: read the fine print
People often buy batteries for blackouts. With plug-in solar, there's a catch: a certified grid-tied system must stop feeding your wall outlets when the grid fails. That anti-islanding behavior is what keeps line workers safe, and it's required by every state law.
So a plug-in battery won't keep your home's circuits running in an outage. What some units can do is power devices plugged directly into the battery's own outlets — a fridge, a router, a phone charger — while isolated from the grid. If backup matters to you, look for that feature explicitly, and never try to backfeed your home's wiring during an outage.
Choosing a battery
- Certification: the battery and inverter combination should be certified for grid-tied plug-in use (UL 3700 listing where available; at minimum UL 1741-SB for the inverter and UL 9540 for the storage system).
- Capacity: 1–2 kWh for an 800 W kit; 2–3 kWh for 1,200 W.
- Output control: programmable or meter-following output lets you match your base load.
- Chemistry: lithium iron phosphate (LFP) batteries are the norm for safety and cycle life.
- Placement: indoors or in a sheltered spot within the manufacturer's temperature range; many batteries won't charge below freezing.
- Warranty: look for 10 years or a stated cycle count.
Should you add a battery?
Yes, probably, if you're on a time-of-use rate with expensive evenings, you're away during the day with low base load, or you value device-level backup.
Probably not yet, if you're on a flat rate near the national average and someone is home during the day — spend the money on better panel placement instead.
Either way, you can start without one: many systems let you add a battery later. Model both scenarios in the calculator by changing the "share used at home" slider from 80% to 95%.
Frequently asked questions
Can a plug-in solar battery power my home during an outage?
Not through your wall outlets. Certified plug-in systems must stop feeding your home's wiring when the grid goes down. Some battery units have their own built-in AC outlets that can power devices plugged directly into them during an outage — check for that feature specifically.
What size battery do I need for balcony solar?
For an 800 W kit, 1–2 kWh usually captures most of the midday surplus in a typical home. Bigger batteries mostly sit partially empty unless you also charge them from the grid at off-peak rates.
Does the 1,200 W limit apply with a battery?
Yes. State laws cap the AC output delivered through the outlet. A battery doesn't change that ceiling; it changes when the energy is delivered.