How to Size a Home Battery System: The Math Behind Getting It Right
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Three Different Problems Requiring Three Different Answers
Most sizing guides for home batteries treat the problem as a single question: how many kilowatt-hours do I need? The real question is: what are you trying to accomplish? Three distinct use cases require different sizing logic, and conflating them leads to either an expensive oversized system or an underpowered one that doesn't do what you bought it for.
The three use cases are backup-only (no solar), solar self-consumption (maximize daytime solar use and minimize grid exports), and time-of-use arbitrage (charge cheap, discharge expensive). Each has a different optimal battery size for the same home.
Use Case 1: Backup-Only Sizing
If you're buying a battery specifically for power outage coverage, the sizing question is: how many days of essential loads do you want to cover, and which loads are essential?
Start by listing what you actually need to run during an outage. Not everything in the house — just the critical loads.
A representative essential load profile for an average US home:
- Refrigerator: 150W average draw × 24 hours = 3.6 kWh/day
- Lighting: 40W average across needed fixtures × 10 hours = 0.4 kWh/day
- Device charging (phones, laptop): 100W × 5 hours = 0.5 kWh/day
- WiFi router and modem: 20W × 24 hours = 0.5 kWh/day
- Small TV: 80W × 4 hours = 0.3 kWh/day
Total: roughly 5.3 kWh per day in essential loads. This excludes central HVAC (3–5 kW draw), electric water heater (4–5 kW), and electric range (8–10 kW) — none of which should be expected from a backup battery system unless you specifically size for them.
One Tesla Powerwall 3 at 13.5 kWh usable covers approximately 2.5 days of the essential load profile above. Two units (27 kWh) covers 5 days. For solar recharging during an outage: a 5 kW array in summer produces roughly 25 kWh on a clear day, which would fully recharge a single Powerwall and then some. In winter, expect 10–15 kWh from the same array.
A key nuance: battery capacity rating is the total stored energy, but usable capacity accounts for the depth of discharge limit. Most lithium batteries are rated at 90–95% depth of discharge. A battery rated at 13.5 kWh is often already stated as usable (after DOD limits), but verify with the manufacturer's spec sheet.
Use Case 2: Solar Self-Consumption Sizing
If your goal is to maximize the use of your own solar energy and minimize selling power back to the grid at unfavorable net metering rates — or if your utility has eliminated net metering — the sizing logic is different.
The target is storing excess daytime solar generation to cover evening demand. The key number is your "solar surplus" — the difference between what your panels produce and what you consume during daylight hours.
Example: a 7 kW solar array on a typical summer day produces 35 kWh. If the home consumes 15 kWh during daylight hours, the surplus is 20 kWh. A battery sized at 20 kWh would theoretically capture all of that surplus for overnight use. In practice, round-trip efficiency of 92–95% for lithium batteries means you recover about 18.5–19 kWh of the 20 kWh stored.
The practical limit: battery size beyond the typical daily solar surplus produces diminishing returns. A 40 kWh battery paired with a 7 kW array still only captures the 20 kWh of daily surplus in summer — you're paying for 20 kWh of capacity that sits idle every day. Size the battery at 80–100% of your average solar surplus, not your total daily consumption.
Use Case 3: Time-of-Use Arbitrage Sizing
With TOU pricing (covered in depth in our guide to time-of-use electricity rates), the battery charges from the grid during cheap off-peak hours and discharges during expensive on-peak hours. The sizing goal is matching the battery's discharge capacity to your peak-period consumption.
In California's PG&E territory, peak hours run roughly 4–9pm. The average US home uses about 2–3 kW during this 5-hour window, meaning 10–15 kWh of peak-period consumption. A 13.5 kWh battery (one Powerwall 3) covers that window with a modest buffer.
The arbitrage math: if you charge 13.5 kWh at $0.28/kWh ($3.78) and discharge 12.5 kWh (after 92% round-trip efficiency) at $0.55/kWh (avoiding $6.88 of peak-rate purchases), you save $3.10 per day. Over 180 peak-rate days per year, that's $558 annually. At that rate, a $10,700 net-cost Powerwall 3 takes about 19 years to pay back on arbitrage alone — which means TOU arbitrage is best viewed as a bonus benefit on top of backup value, not a primary financial justification.
Power Output: The Often-Ignored Constraint
Capacity (kWh) gets most of the attention in battery sizing discussions. Power output (kW) is equally important and often where systems underperform expectations.
A battery's continuous power output rating tells you what loads it can simultaneously support. The Enphase IQ Battery 10C has a 6 kW continuous output rating. A single Enphase unit can't start a central air conditioner (which draws 3–5 kW at startup, often 5–7.5 kW at startup surge) while also running a refrigerator, lights, and other loads. The Tesla Powerwall 3's 11.5 kW continuous output handles the same load combination comfortably.
Check the power output specification against your critical load panel's expected simultaneous draw, not just the total kWh capacity. Starting surge currents for motors (AC units, well pumps, sump pumps) can be 3–6× their running draw for the first few seconds.
Round-Trip Efficiency and What It Means for Sizing
Lithium iron phosphate (LFP) and NMC lithium batteries used in home storage achieve 92–95% round-trip efficiency. This means for every 100 kWh you put in, you get 92–95 kWh back out. Lead-acid batteries (less common in residential storage now) achieve 70–80% round-trip efficiency.
The efficiency gap matters when sizing for TOU arbitrage or solar self-consumption. If you're counting on 20 kWh of solar surplus to cover 20 kWh of overnight load, you need at least 21–22 kWh of battery capacity to account for losses. This is a 5–10% upward adjustment in sizing calculations.
Putting It Together: Sizing for a Real Home
Consider a home with 15 kWh average daily consumption, a 6 kW solar array, PG&E TOU pricing, and a desire for 2-day outage coverage.
Backup-only sizing: 5.3 kWh essential loads × 2 days = 10.6 kWh minimum, so one 13.5 kWh battery works.
Solar self-consumption sizing: 6 kW array produces ~30 kWh on summer days; home uses 10 kWh during daylight; surplus of 20 kWh — so 20 kWh battery capacity is optimal (two Enphase IQ 10C units at 20.16 kWh, or one Powerwall 3 at 13.5 kWh which captures 67% of surplus).
TOU arbitrage sizing: peak period (4–9pm) consumption is approximately 10 kWh — one Powerwall 3 handles this with buffer.
The answer for this home: one to two batteries, depending on how much you value complete solar self-consumption vs. acceptable partial self-consumption. Two Enphase IQ 10C units at ~$14,000 installed (before tax credit) cover all three use cases at 20 kWh capacity. One Powerwall 3 at ~$15,300 covers backup and arbitrage, with partial self-consumption.
Before making a final decision, cross-reference battery options in the home battery storage comparison guide, and read our ROI calculator guide for a more detailed financial analysis framework.
Frequently Asked Questions
How many kWh do I need for a home battery backup?
Essential loads in a typical US home — refrigerator, basic lighting, device charging, WiFi — total about 5.3 kWh per day. One 13.5 kWh battery (like a Tesla Powerwall 3) covers approximately 2.5 days of essential loads without solar recharging. If you want 2 days of backup including minimal HVAC use, plan for 15–20 kWh of storage capacity.
What is the difference between battery capacity (kWh) and power output (kW)?
Capacity (kWh) is how much total energy the battery stores — it determines how long you can run loads. Power output (kW) is the maximum rate at which the battery can deliver energy — it determines which loads you can run simultaneously. A battery with 13.5 kWh capacity but only 6 kW output can't run a central air conditioner (3–5 kW) plus other loads at the same time. The Tesla Powerwall 3's 11.5 kW output handles most whole-home loads; the Enphase IQ 10C at 6 kW per unit may need stacking.
How do I size a battery for solar self-consumption?
Calculate your average daily solar surplus — the difference between what your panels produce and what you consume during daylight hours. If a 7 kW array produces 35 kWh and you consume 15 kWh during daylight, the surplus is 20 kWh. Size the battery at 80–100% of that daily surplus. Going much larger produces diminishing returns because you can only store as much as the panels generate.
Can I size my battery for time-of-use electricity savings?
Yes. For TOU arbitrage, match battery discharge capacity to your peak-period consumption. In PG&E territory with 4–9pm peak hours, the average home uses 10–15 kWh during that window. One 13.5 kWh battery covers this. The financial return is roughly $500–$600/year in markets with a large spread between off-peak ($0.28/kWh) and on-peak ($0.55/kWh) rates — meaningful but not sufficient as the sole financial justification for a $10,000+ battery.
What is round-trip efficiency and does it affect battery sizing?
Round-trip efficiency is the percentage of energy you recover from storage versus what you put in. Lithium batteries achieve 92–95%; lead-acid achieves 70–80%. For sizing, this means if you want to recover 20 kWh overnight, you need to store 21–22 kWh during the day. Factor in a 5–10% upward adjustment to your capacity target when sizing for solar self-consumption or TOU arbitrage.
How much battery do I need to run my air conditioner during an outage?
Central HVAC systems draw 3–5 kW running and 5–7.5 kW at startup surge. Running a 3.5 kW AC unit for 8 hours uses 28 kWh — more than two Powerwall 3 units. In practice, battery backup for whole-home HVAC requires 3+ batteries (40+ kWh) or solar recharging during the day. Most backup battery systems are designed for essential loads only; HVAC backup requires explicit, intentional oversizing.
Is it worth oversizing a home battery system?
Not significantly. Once you've covered your essential daily load and solar surplus, additional battery capacity sits unused most days and adds cost without proportional benefit. The exception is if you want extended outage coverage (5+ days) or have plans to add more solar later. The 30% ITC lowers the marginal cost of each additional kWh, but the law of diminishing returns still applies after your primary use case is covered.
