How Much Battery Storage Do You Actually Need for Whole-Home Backup

Key Takeaways

• Battery capacity is measured in kilowatt-hours, which determines how much energy you can store

• Power rating determines what loads you can run simultaneously

• Minnesota homes often have unique backup needs including sump pumps, heating, and humidity control

• Smart panels like SPAN can extend battery life by prioritizing essential loads during outages

• One Powerwall provides about 13.5 kWh of usable capacity, which may or may not be enough depending on your goals

Understanding Battery Capacity vs. Power Rating

Battery specifications include two numbers that matter most for backup planning.

Capacity is how much total energy the battery stores, measured in kilowatt-hours (kWh). A 13.5 kWh battery can deliver 13.5 kilowatt-hours of energy before it needs recharging.

Power rating is how much energy the battery can deliver at once, measured in kilowatts (kW). This determines what you can run simultaneously. A battery with a 5-kW continuous rating can power loads totaling 5 kilowatts at the same time.

Both matter. A battery with high capacity but low power rating stores a lot of energy but cannot run heavy loads like air conditioning. A battery with high power but low capacity can run big loads but only for a short time.

Sizing for Your Goals

Different homeowners have different backup goals.

Short-term bridge (4-8 hours): You want to keep essentials running through a typical outage until the grid comes back. One Powerwall often handles this if you are not trying to run air conditioning.

Overnight backup (12-18 hours): You want to make it through the night comfortably and wake up with some capacity remaining. One to two Powerwalls depending on loads.

Multi-day resilience (24-72 hours): You want to ride out an extended outage with minimal lifestyle disruption. Two or more Powerwalls plus adequate solar production to recharge during the day.

True whole-home backup: You want everything to run as if the grid never went down. This typically requires two to four Powerwalls depending on home size and air conditioning needs.

What Do Your Essential Loads Actually Use?

Before sizing a battery, you need to know what you want to power during an outage.

Refrigerator and freezer typically use 100-400 watts when running, with a surge when the compressor kicks on.

Sump pump uses 500-1,500 watts when active. In Minnesota basements, this is often a critical load during storms.

Lighting depends on your bulbs. LED lighting throughout a home might use only 100-300 watts total.

Internet and electronics including your router, phone chargers, and a laptop might total 100-200 watts.

Heating is where things get complicated. A natural gas furnace needs electricity for the blower and controls, typically 300-500 watts when running. Electric heat or heat pumps require significantly more power.

Air conditioning is the biggest load in most homes, often 2,000-5,000 watts depending on the system size.

Add up your essential loads. This tells you the minimum power rating you need. Then estimate how many hours per day those loads run to calculate capacity requirements.

Minnesota-Specific Considerations

Minnesota homes have backup needs that differ from homes in other climates.

Sump pumps are critical in many Minnesota basements, especially during spring thaws and heavy rain. If your basement floods when the sump pump stops, this is a non-negotiable backup load.

Heating in winter requires careful planning. Even gas furnaces need electricity. If you want to backup heat during a winter outage, you need enough battery capacity and solar production to keep the system running for potentially several days.

Humidity control matters in summer. Some Minnesota homes run dehumidifiers continuously. Depending on your basement, this might be essential during extended outages.

Garage door openers are easy to overlook. If your vehicles are in the garage during an outage, you may want the ability to open the door.

How Many Powerwalls Do You Need?

A single Tesla Powerwall 3 provides 13.5 kWh of usable capacity with 11.5 kW continuous power output.

For essentials only (lights, refrigerator, internet, phone charging), one Powerwall can typically provide 12-24 hours of backup depending on usage patterns.

For essentials plus sump pump and furnace, one Powerwall provides backup for a typical overnight outage but may not last through a multi-day event without solar production to recharge.

For whole-home backup including air conditioning, most homes need two or more Powerwalls to handle the combined power draw and provide meaningful duration.

The right number depends on your specific loads, your tolerance for managing energy during outages, and whether you have solar to recharge batteries during the day.

The Recharge Advantage

A typical 10 kW solar array in Minnesota produces roughly 35-45 kWh on a sunny summer day. Even on a cloudy day, you might generate 15-25 kWh. Compare that to a single Powerwall's 13.5 kWh capacity – on a good day, your solar system can fully recharge your battery and then some.

This changes everything about how you plan for outages.

Example: 10 kW Solar + Two Powerwall

Consider a setup with 10 kW of solar panels and two Powerwalls (27 kWh total capacity):

Sunny summer day scenario:

• Morning: Batteries start at 50% (13.5 kWh remaining from overnight)

• 9 AM - 4 PM: Solar produces 40 kWh while home uses 15 kWh

• Result: Batteries fully recharged plus 11.5 kWh exported or used directly

• Evening/overnight: 27 kWh available for 12+ hours of whole-home power

Cloudy day scenario:

• Solar produces 20 kWh while home uses 18 kWh during daylight

• Result: Batteries gain 2 kWh net, maintaining capacity for another night

The bottom line: With adequate solar production, you can potentially run indefinitely during an extended outage as long as you manage loads appropriately and get reasonable sun.

Sizing Solar for True Resilience

For genuine energy independence during outages, your daily solar production should meet or exceed your daily consumption. In Minnesota, this typically means:

• Essential loads only (10-15 kWh/day): A 5-6 kW system can keep up even on moderate days

• Essentials plus AC (25-35 kWh/day): A 10+ kW system provides solid coverage on most days

• Full home operation: Size solar to match your typical daily usage, usually 8-12 kW for most homes

The key insight is that solar does not just extend your backup; it can make your backup essentially unlimited in the right conditions.

How Smart Panels Change the Equation

Traditional backup systems are all-or-nothing. Either a load is on the backup circuit or it is not.

Smart panels like SPAN allow dynamic load management. During an outage, you can prioritize which circuits receive power and turn off non-essential loads remotely through an app.

This flexibility means a smaller battery can go further. Instead of sizing for worst-case simultaneous usage, you can manage loads intelligently and stretch your backup capacity.

For example, you might run the air conditioner during the hottest afternoon hours, then switch to essential-only mode overnight to preserve battery for the next day.

What Greenway Recommends

At Greenway Solar, we help homeowners think through backup goals before recommending battery capacity.

We start by identifying your essential loads and understanding what outages typically look like in your area. Then we model different scenarios to show how various battery configurations would perform.

For most Minnesota homeowners who want reliable backup for essentials plus sump pump and furnace, one to two Powerwall paired with solar provides solid protection. Homeowners who want whole-home backup including air conditioning typically need two or more units.

Smart panel integration through SPAN can extend the value of whatever battery capacity you choose by giving you active control during outages.

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FAQs

Can I add more batteries later if one is not enough?

Yes. Powerwall systems can be expanded by adding additional units. Installing the wiring and infrastructure for future expansion during your initial installation can reduce the cost of adding capacity later.

Will my battery recharge during an outage if I have solar?

Yes, if your solar system is designed for backup operation. During a grid outage, your solar panels can recharge your battery during daylight hours, potentially providing indefinite backup as long as you manage loads appropriately.

How long do batteries last before needing replacement?

Tesla warranties the Powerwall for 10 years with guaranteed capacity retention. Most lithium batteries last well beyond the warranty period, though capacity gradually decreases over time.

Do I need a smart panel to use battery backup?

No. Batteries work with traditional electrical panels. However, smart panels provide significantly more control and flexibility during outages, allowing you to maximize the value of your battery capacity.

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Right-Size Your Backup System

Battery sizing is not about buying as much capacity as possible. It is about matching your backup goals with the right amount of storage.

At Greenway Solar, we walk through your specific situation, model different scenarios, and help you choose a configuration that provides the protection you want at a price that makes sense.

Ready to explore battery storage for your home? Contact us through www.greenwaysolar.org or call us to discuss your backup power goals.

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