6.6kW Solar System Backup Battery: Sizing, Options and Tips

Assuming a 6.6kW solar system and asking yourself, what battery will you need? Relax, you're not the only one. Lots of solar-pondering homeowners have asked the same question: How do I choose the right battery size for my solar system? Want power backup during an outage or slash your electricity bill in half? Having the right-sized battery with your solar system is the difference.

Why Install a Backup Battery in a 6.6kW Solar System?

A 6.6kW solar system yields 24–28kWh of electricity during good sun. But what happens to excess solar power without storage? It feeds back to the grid — sometimes for pennies. A backup battery allows you to store that power and use it later when the sun is not out or during a blackout.

Benefits:

• Energy independence: Power loads during outages
• Lower bills: Utilize stored solar during the night or high-rate times
• Maximize solar use: Minimize wastage and grid dependence

Ever had your power go out while cooking dinner? With a battery, your lights, fridge, and even your Wi-Fi will just keep on going like nothing occurred.

What Size Battery Is Best for a 6.6kW Solar System?

Generally speaking, the sweet spot is a 10–13.5kWh battery. It's large enough to store a day's surplus solar energy but not so massive that it sits idle most of the time.

Factors to consider:

• Daily energy use: Look at your electricity bills — if you use 20kWh/day, a 10–12kWh battery might be enough to supply evening loads.
• Backup length: Do you need 4 hours or 24? The longer the duration, the bigger the battery.
• Depth of Discharge (DoD): 80–100% DoD are what most lithium batteries yield. A 13.5kWh battery with 90% DoD yields ~12kWh usable.
• System losses: Account for 5–15% inverter and wiring inefficiencies in calculation.

Tip: Some users found that oversizing the battery doesn’t always pay off—especially if the solar panels can’t fully recharge it daily.

Recommended Battery Choices

Two batteries are used frequently with 6.6kW systems:

Tesla Powerwall 2 – 13.5kWh, 90% DoD, ~10,000 cycles

LG Chem RESU 10H – 9.8kWh, 95% DoD, ~6,000 cycles

They are lithium-ion, compact, and hybrid inverter-compatible.

Installation & Inverter Considerations

Fitting a battery is not plug-and-play:

• If you have microinverters, you might need a hybrid inverter for battery compatibility.
• A 48V lithium battery bank is standard for residential installations.
• Batteries need mounting in temperature-proof, well-ventilated areas — garages are perfect.

If this sounds complex, don’t worry — a qualified solar installer will size and configure the setup for you.

Should You Invest in a Battery?

Others say their battery will have paid itself off in 7–10 years. Others say it's worth it just for the peace of mind of a backup. As one homeowner put it: "I installed a 10kWh battery not to save money, but so my internet and fridge stay on through outages — completely worth it."

So ask yourself: Are you looking to save money, gain energy independence, or both?

Whatever the reason, choosing the correct size battery means your 6.6kW solar system works overtime — even when the sun doesn't shine.