Solar Battery Backup for Power Outages: What Size Do You Need

Alain Karatepeyan, CEO- Vantage Point Solar
June 3rd, 2026
8 min read

Battery size depends on your essential load in kilowatt-hours and your desired runtime during an outage. A typical home needs 10-15 kWh of storage to run critical appliances for 24 hours; larger homes or longer outage windows require 20-30 kWh or more. The calculation starts with measuring what you actually need to power, not what you'd like to power.[1]

The framework for thinking about battery sizing

Three dimensions determine the right battery capacity: essential load (which appliances must stay on), runtime (how many hours the battery must sustain that load), and usable capacity (what the battery can actually discharge). These three dimensions interact. A larger runtime reduces the required kWh per hour, but increases total capacity costs. A smaller essential load cuts both runtime and total capacity needs. Understanding these trade-offs prevents oversizing and undersizing in equal measure.

Dimension 1: Defining your essential load

Your essential load is the simultaneous power draw of appliances that must run during an outage. Most homes cannot run everything. A refrigerator draws 600-800 watts when the compressor cycles on. A well pump runs at 1,000-2,000 watts. A sump pump at 1,000 watts. LED lighting at 10-20 watts per room. Internet equipment at 50-100 watts. Running all of these at once requires 3-4 kW of instantaneous capacity and continuous supply.[2]

Calculate this by listing appliances you consider non-negotiable, finding their nameplate wattage, and determining how many run simultaneously. A refrigerator, internet router, lights in three rooms, and a sump pump running together equals roughly 1,500-2,000 watts continuous draw. Multiply that by the hours you want the system to sustain that load (say, 24 hours) to get 36-48 kWh. Most homes then choose a subset: perhaps refrigeration, lights, and internet only, bringing the number down to 8-10 kWh for 24 hours.[1]

Dimension 2: Runtime expectations and battery chemistry

Runtime is how many hours the battery must power your essential load. A 24-hour runtime assumes no solar recharge during the outage window. A 12-hour runtime assumes day-length outages when the sun can begin recharging by afternoon. A 72-hour runtime (3 days) assumes a severe weather event or grid failure lasting multiple days with minimal sunlight.

Chemistry matters here. Lithium iron phosphate (LFP) batteries discharge at rates of 80-95 percent usable capacity, meaning a 15 kWh LFP battery delivers roughly 12-14 kWh of usable power.[3] Lead-acid batteries deliver 50 percent usable capacity; a 30 kWh lead-acid system provides only 15 kWh usable. As of Q1 2026, LFP has become the dominant chemistry for residential backup because the higher usable capacity reduces the physical footprint and cost per kilowatt-hour delivered during an outage.[2]

Dimension 3: How solar recharge affects total capacity

A battery paired with solar panels needs less total capacity than a battery-only system because the solar array recharges during daylight outages. If a 5 kW solar array generates 20 kWh on a clear day, and your essential load consumes 8 kWh during that same day, the battery only needs to bridge the 4-6 kWh shortfall plus cover nighttime hours. This reduces required battery size by 30-50 percent compared to a battery-only backup.[1]

However, solar recharge depends on weather. A three-day cloudy outage in winter generates far less solar power than three days in June. Battery sizing must account for worst-case seasonal conditions if you want true reliability, not just backup on sunny days. This is why homes in regions with frequent winter storms often size for larger batteries and accept the cost premium for peace of mind.

Case in point: A home in California's fire-prone regions

A homeowner in Sonoma County with a 3,000 sq ft home, electric heat pump, and well pump identified critical loads: refrigerator, well pump, lighting, and internet. Simultaneous draw was 2,500 watts. For a 36-hour outage (assuming one night plus a day of potential recharge on day two), she needed 90 kWh theoretically. With a 6 kW solar array and LFP chemistry delivering 85 percent usable capacity, she sized a 15 kWh system. The solar array recharged 12-15 kWh on a clear day during the outage window, making 15 kWh usable battery sufficient to sustain critical loads.[3]

She initially considered oversizing to 25 kWh but found the cost increase (roughly $8,000 more) unjustifiable for a scenario where solar would contribute significantly. Instead, she paired the 15 kWh system with load shedding rules: if the battery dropped below 30 percent and the sun wasn't rising, the well pump would shut off to prioritize refrigeration and lights.

Synthesis: What this means for different households

For rural or off-grid homes: Size for 48-72 hours of runtime because grid restoration is slower. A 5 kW essential load with 72-hour runtime requires 15-25 kWh of usable capacity. Budget $30,000-$50,000 for a LFP system of this scale, including panels.

For suburban homes with grid backup: Size for 24 hours and 2-3 kW essential loads. A 10-15 kWh LFP system costs $15,000-$25,000 installed. Most outages last less than 8 hours, so this provides comfortable margin.

For homes with backup generators: A smaller battery (5-8 kWh) covers a 2-4 hour window while the generator auto-starts. This hybrid approach costs 40 percent less than a large battery-only system and handles silent outages where the generator fails to start immediately.

What the data shows

Load Profile	24-Hour Runtime	48-Hour Runtime	Cost Per kWh Delivered
2 kW continuous (essential only)	8 kWh LFP	16 kWh LFP	$1,500-$1,800
3 kW continuous (essential + one comfort)	12 kWh LFP	24 kWh LFP	$1,400-$1,700
5 kW continuous (most circuits on)	20 kWh LFP	40 kWh LFP	$1,300-$1,600

The cost-per-kWh-delivered improves as systems scale, but the law of diminishing returns applies after 15-20 kWh because you're paying for capacity you'll rarely use simultaneously. Most installers report customers oversizing by 20-30 percent due to fear of undersizing; right-sizing requires honest load measurement.[2]

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Quick answers

How do I measure my essential load? Go through each appliance's rating plate, list what runs simultaneously during an outage, and sum the wattages. Refrigerators, internet, and lighting are almost universal; water heating, air conditioning, and EVs are load-shedding candidates.

What's the difference between battery capacity and usable capacity? A 20 kWh battery rated for 90 percent usable capacity delivers 18 kWh during an outage. The extra 10 percent protects the battery's lifespan. LFP batteries have higher usable percentages (80-95 percent) than older chemistries.

Can I use my solar panels to recharge during an outage? Yes, if your system includes a battery and a charge controller rated for islanding (operating independent of the grid). A 5 kW solar array generates 15-20 kWh on a clear day but zero on cloudy days.

How long does a 10 kWh battery back up a typical home? At 1 kW average load (refrigerator, lights, internet), roughly 10 hours. At 2 kW, five hours. Your actual runtime depends on which appliances you prioritize.

What size battery do I need for a heat pump? Heat pumps draw 2-4 kW during startup and 1-2 kW running. A 24-hour runtime requires 24-48 kWh depending on temperature. Most homes choose generators for heat pump backup rather than batteries alone because the cost is prohibitive.

Should I get a bigger battery to avoid topping it off frequently? Partial cycling extends battery life, but most modern LFP systems warrant 70-80 percent depth-of-discharge regularly. A 15 kWh system is not damaged by using 12 kWh daily. Oversizing purely for charge cycles costs more than occasional replacement.

What happens if my battery runs empty during an outage? The battery disconnects and shuts down connected circuits. You lose power to backed-up loads until the grid returns or the battery recharges from solar. This is why sizing matters; an undersized system forces early load-shedding or complete power loss.

Can I add more battery capacity later? Depends on your inverter's stacking capacity and electrical code. Most modern systems allow modular expansion up to a point. Plan for at least one additional 5-10 kWh unit if future expansion is likely.

References

[1] Sunrun. "The Homeowner's Guide to Energy Storage and Backup Power." Sunrun Research, 2025.

[2] U.S. Department of Energy. "Battery Storage for Residential Resilience: Cost and Performance Analysis." Office of Energy Efficiency and Renewable Energy, 2026.

[3] EnergySage. "Solar + Battery Sizing for Outage Protection." EnergySage Consumer Report, Q1 2026. https://www.energysage.com/guides/