How to Size a Home Battery System: Solar + Storage Calculator (2026)

The most common question homeowners ask before buying a battery: "What size do I actually need?"

The answer depends on three things — which loads you want to back up, how many hours of backup you need, and whether you have solar to recharge the battery during an extended outage. This guide walks through the sizing formula, breaks it down by real home scenarios, and explains where one battery ends and two batteries begin.

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The Battery Sizing Formula

Here's the core calculation:

Battery kWh needed = (Critical load watts × Backup hours) ÷ 1,000 ÷ 0.85

The 0.85 efficiency factor accounts for two real-world losses:

• Inverter conversion losses (~5%)
• Depth of discharge limit (~10% reserve to protect battery life)

Example: You want to run 2,000W of critical loads for 12 hours.

(2,000W × 12 hours) ÷ 1,000 ÷ 0.85 = 28.2 kWh

Two Powerwall 3 units (27 kWh total) would cover this scenario.

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Step 1: Identify Your Critical Loads

Not everything in your home needs battery backup. Prioritize in layers:

Tier 1 — Life-Safety (Always include)

Load	Typical Wattage
Medical equipment (CPAP, oxygen concentrator)	150–600W
Refrigerator	100–200W
Freezer	100–200W
LED lighting (whole house)	200–400W
Internet router + modem	20–50W
Phone and device charging	50–150W

Tier 1 subtotal: ~700–1,600W continuous

Tier 2 — Comfort (Add if budget allows)

Load	Typical Wattage
Mini-split (1 zone, 9,000 BTU)	700–900W
Well pump (½ HP)	900–1,500W (running) / 3,000–5,000W (surge)
Sump pump	400–800W

Tier 2 subtotal: ~1,000–2,500W (intermittent, not continuous)

Tier 3 — High-Draw (Usually excluded or on separate panel)

Load	Typical Wattage
Central HVAC (3-ton)	3,500–5,000W
Electric range	3,000–8,000W
Electric water heater	4,500–5,500W
EV charger (Level 2)	7,200–11,500W

Key insight: Most essential-load panels exclude Tier 3. A 10 kWh battery that tries to run a central HVAC unit and EV charger will drain in under 2 hours. Size for what you actually need backed up.

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Step 2: Choose Your Backup Duration

How long do outages typically last where you live?

Scenario	Recommended Backup Duration
Brief grid events (< 4 hrs)	4–8 hours
Typical storm outage	12–24 hours
Extended outage / wildfire season	24–72 hours
Off-grid / full independence	3–7 days (with solar recharge)

If you have solar panels, you only need to survive the first night — then solar recharges the battery each day and the system runs indefinitely. This is the key reason solar + storage beats battery-only systems for extended outages.

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Step 3: Run the Numbers for Your Home

Small Home / Apartment (< 1,500 sq ft)

Typical critical loads: Fridge, lights, router, phone charging = ~600–900W Backup needed (24 hours):

(900W × 24 hrs) ÷ 1,000 ÷ 0.85 = 25.4 kWh

Or more realistically (loads run intermittently, ~40% duty cycle):

(900W × 24 hrs × 0.4) ÷ 1,000 ÷ 0.85 = 10.2 kWh

Recommendation: 1 battery (10–14 kWh) — one Powerwall 3 or two Enphase IQ Battery 5P units comfortably covers a small home for 24+ hours on essential loads.

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Medium Home (1,500–2,500 sq ft)

Typical critical loads: Fridge + freezer, lights, router, mini-split (1 zone), phone charging = ~1,500–2,200W Backup needed (24 hours at 40% duty cycle):

(2,000W × 24 hrs × 0.4) ÷ 1,000 ÷ 0.85 = 22.6 kWh

With solar recharge: One 13.5 kWh battery + 6 kW solar = sustainable indefinitely. The battery covers overnight; solar recharges by midday.

Recommendation:

• With solar: 1 battery (13.5 kWh) is sufficient for most medium homes
• Without solar: 2 batteries (27 kWh) for multi-day backup without recharge

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Large Home (2,500–4,000 sq ft)

Typical critical loads: All of the above + second HVAC zone or larger mini-split = ~2,500–4,000W Backup needed (24 hours at 35% duty cycle):

(3,500W × 24 hrs × 0.35) ÷ 1,000 ÷ 0.85 = 34.6 kWh

Recommendation: 2–3 batteries (27–40 kWh) — two Powerwall 3 units (27 kWh) or three Enphase IQ Battery 5P units (15 kWh + higher-capacity additions) depending on your solar setup.

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1 Battery vs 2 Batteries: The Decision Matrix

Situation	Recommendation
Essential loads only, < 1,500 sq ft	1 battery
Medium home + solar recharge	1 battery
Medium home, no solar	2 batteries
Large home or central HVAC backup	2 batteries
EV charging during outage	2+ batteries
72-hour backup without solar	2–3 batteries
Whole-home backup (any load, any time)	2+ batteries

The most common mistake homeowners make: sizing for the loads they want to run rather than the loads they need to run. A second battery adds ~$10,000–$15,000 all-in. Make sure you actually need it before adding it.

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Solar Pairing: How Panels Change the Math

When you pair batteries with solar, the sizing question shifts from "how much storage" to "how much storage to bridge overnight."

Rule of thumb for solar + storage sizing:

Battery kWh = Solar system kW × 2

A 6 kW solar system generates ~24–30 kWh on a sunny day. A 13.5 kWh battery holds enough for overnight use and is fully recharged by midday. This 2:1 ratio works well for most climates.

What this means in practice:

Solar System Size	Recommended Battery	Coverage
4 kW	8–10 kWh (2× IQ Battery 5P)	Essential loads, indefinite with sun
6 kW	13.5 kWh (1× Powerwall 3)	Essential + 1 AC zone, indefinite with sun
8–10 kW	20–27 kWh (2× Powerwall 3 or 4× IQ 5P)	Whole-home, indefinite with sun
10–15 kW	27–40 kWh	Large home whole-home, multi-day cloudy

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Which Battery Best Fits Your Sizing Needs?

Once you know how much kWh you need, the battery choice often becomes clearer:

Need 5–10 kWh and want to expand later? → Enphase IQ Battery 5P — modular, buy 1–2 units now, add more later

Need 13.5 kWh, high power output, starting fresh with solar? → Tesla Powerwall 3 — best continuous power per unit, integrates solar inverter

Need 27 kWh but want flexibility with your existing inverter? → Two Franklin Home Power units or two Powerwall 3 units depending on your existing solar brand

Need 20–30 kWh with maximum longevity? → Sonnen Eco series — 15,000 cycle warranty, LFP chemistry

For a full head-to-head comparison of the two most popular options, see our Powerwall 3 vs Enphase IQ Battery 5P guide →

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Incentives That Affect How Many Batteries You Buy

California SGIP

California's SGIP rebate applies per kWh of storage installed. In 2026:

• Standard tier: $150–$200/kWh
• Equity tier (income-qualified): $850–$1,000/kWh
• Equity Resiliency (fire zone + income-qualified): up to $1,000/kWh

On a 13.5 kWh Powerwall 3, that's $2,025–$13,500 back depending on eligibility. For a two-battery system (27 kWh), standard-tier SGIP alone can offset $4,050–$5,400.

The SGIP math often makes a second battery more cost-effective than it looks at face value — especially for income-qualified households.

Federal Tax Credit

The 30% federal ITC for standalone storage was eliminated in July 2025. However, if you're pairing batteries with a new solar installation, the solar portion of the project still qualifies for the 30% ITC on solar hardware.

Check your state's current battery incentives →

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What to Tell Your Installer

Once you've done this sizing exercise, bring these numbers to your installer conversation:

1. Critical load list with wattages (or ask your installer to audit your panel)
2. Desired backup hours — 12 hours, 24 hours, or multi-day?
3. Whether you have or are adding solar — this changes battery sizing significantly
4. Your SGIP eligibility — income-qualified status unlocks dramatically higher rebates

A good installer will do a load analysis as part of their site visit, but arriving informed gets you a more accurate proposal faster.

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Get a Free Battery Assessment

Battery sizing is home-specific — your panel configuration, existing solar inverter, and critical load list all affect the right answer.

Get a free EnergyScout assessment →

Our vetted California installer network reviews your home specs, identifies SGIP rebate eligibility, and provides competitive quotes for the right-sized system. No spam, no sales pressure — just accurate numbers.