Batteries are the fastest-growing add-on to residential solar, and also the most misunderstood part of a quote. Here's what the spec sheet doesn't tell you.
Usable capacity is not rated capacity
This is the single most important distinction in battery shopping. A battery's rated capacity is the number on the box. Its usable capacity — what you can actually draw — is lower, because every chemistry has a safe depth of discharge (DoD) limit.
Lithium (LiFePO4) batteries typically allow 90-100% DoD. Lead-acid batteries are usually limited to 50% to protect their lifespan — so a 10 kWh lead-acid bank delivers roughly the same usable energy as a 5 kWh lithium bank, despite the bigger number on the spec sheet. Always compare usable capacity, not rated capacity, when shopping.
How much battery do you actually need?
It depends entirely on what you're protecting. Most homeowners aren't backing up their whole house — they're backing up an "essentials" circuit: fridge, some lights, wifi, phone charging. That's typically 5-10 kWh of daily energy, not the 20-30 kWh a whole-home load would imply.
Run your actual appliance list through the Battery Size Calculator to size a bank for a target backup time, or the Battery Backup Runtime Calculator to check how long a battery you're already considering would last.
What a battery actually costs installed
Equipment price is only part of the number. Installation — new wiring, a backup circuit or panel, permitting — can add 15-40% on top depending on complexity. A simple add-on to an existing solar-ready setup is far cheaper than a full backup-panel retrofit. Get the real breakdown, not just a $/kWh sticker price, from the Home Battery Cost Calculator.
Does a battery pay for itself?
There are two separate ways a battery earns its cost back, and they stack:
- Solar self-consumption — storing excess daytime solar production to use after dark instead of exporting it at a lower credit rate (see our net metering guide).
- Time-of-use arbitrage — charging during cheap off-peak hours and discharging during expensive peak hours, which works even without solar if your utility has a meaningful rate spread.
Check the arbitrage case specifically with the Battery ROI / Time-of-Use Calculator — it's the calculation most competitor sites skip entirely, even though it's often the deciding factor in whether a standalone battery makes financial sense.
Frequently asked questions
Is a home battery worth it without solar?
It can be, if your utility has a meaningful gap between peak and off-peak electricity rates — the battery earns money by shifting when you draw from the grid, independent of any solar production.
How long do home batteries last?
Most lithium (LiFePO4) home batteries are warrantied for 10 years or a set number of cycles, often retaining 70-80% of original capacity at the end of that period.