Home Battery Bank Sizing Guide: How Many kWh of Storage Do You Actually Need?

Home Battery Bank Sizing Guide: How Many kWh of Storage Do You Actually Need?

Published by Portlandia Electric Supply — wholesale solar, storage, and standby power equipment.

Buying a home battery is a five-figure decision for most households, and the single most common mistake is sizing it wrong. Undersize the bank and the lights go out at 3 a.m. Oversize it and you've spent $4,000 on capacity you cycle twice a year. This guide walks through a practical, numbers-first method for sizing a home battery bank — whether you're backing up a whole house with a standby generator, building an off-grid solar system, or adding storage to an existing grid-tied array.

Step 1: Start From Your Actual Usage, Not a Guess

Everything in battery sizing flows from one number: how many kilowatt-hours (kWh) you use per day.

Pull your last 12 electric bills and find the kWh used each month. The U.S. average home uses roughly 29–30 kWh per day (about 880–900 kWh per month), but the range is enormous — a small efficient home might use 12 kWh/day, while a large home with electric heat, a pool pump, and an EV can exceed 60 kWh/day.

Next, decide what the battery actually has to run. There are three honest answers:

  1. Critical loads only — refrigerator, lights, internet, furnace blower, a few outlets, medical equipment. Typically 5–10 kWh/day.
  2. Critical loads plus comfort — add a mini-split or window AC, well pump, or electric water heater. Typically 12–20 kWh/day.
  3. Whole home — everything, including central AC and electric cooking. Typically 25–40+ kWh/day.

A critical-loads panel (a sub-panel fed by the battery through an automatic transfer switch) is how most well-designed backup systems keep battery costs sane. You don't need to back up the hot tub.

Quick load math for individual appliances

If you don't have bill data, estimate from nameplate watts:

Daily kWh = (watts × hours run per day) ÷ 1,000

| Appliance | Typical watts | Hours/day | Daily kWh | |---|---:|---:|---:| | Refrigerator (Energy Star) | 150 (cycles) | 24 (≈8 running) | 1.2 | | Furnace blower (½ HP) | 500 | 8 | 4.0 | | LED lighting (whole house) | 200 | 5 | 1.0 | | Internet router + modem | 20 | 24 | 0.5 | | Well pump (½ HP) | 800 | 1 | 0.8 | | Mini-split heat pump (12k BTU) | 1,000 | 8 | 8.0 | | Central AC (3 ton) | 3,500 | 6 | 21.0 | | Electric water heater | 4,500 | 2 | 9.0 |

Add up what you'd actually run during an outage. That total is your daily backup requirement.

Step 2: Convert Daily kWh Into Battery Bank Size

Your usable daily requirement isn't the battery size you buy. Three adjustments apply:

Depth of discharge (DoD). You can't use 100% of nameplate capacity without shortening battery life. Lithium iron phosphate (LiFePO4) batteries comfortably deliver 90–100% DoD; lead-acid banks should only be drawn to 50%.

Days of autonomy. For off-grid systems, size for 2–3 days of autonomy to ride out cloudy weather. For grid-backup systems paired with solar or a generator, 1 day of autonomy is usually enough because the source recharges the bank during the outage.

Inverter and round-trip losses. Budget roughly 10–15% for conversion losses.

Battery kWh needed = (daily kWh × days of autonomy) ÷ DoD × 1.1 (loss factor)

Worked example: critical loads backup

  • Daily critical loads: 8 kWh
  • Autonomy: 1 day
  • Chemistry: LiFePO4 at 90% DoD

(8 × 1) ÷ 0.9 × 1.1 ≈ 9.8 kWh of battery — comfortably covered by a single 10–14 kWh wall-mount unit.

Worked example: off-grid cabin

  • Daily usage: 6 kWh
  • Autonomy: 3 days
  • Chemistry: LiFePO4 at 90% DoD

(6 × 3) ÷ 0.9 × 1.1 ≈ 22 kWh of battery — four 5.12 kWh server-rack modules (20.5 kWh) or a 14.3 kWh plus a 5.12 kWh wall-mount pair gets you there.

Worked example: whole-home backup

  • Daily usage: 30 kWh
  • Autonomy: 1 day (solar or generator recharges daily)

(30 × 1) ÷ 0.9 × 1.1 ≈ 36.7 kWh — a 13.5 kWh all-in-one unit won't cut it alone; plan on 2–3 stacked units or a 14–16 kWh wall-mount pair plus generator support.

Lithium vs. Lead-Acid: The Real Comparison

LiFePO4 (lithium iron phosphate) is the default choice in 2026 for good reason:

  • 90–100% usable capacity vs. 50% for lead-acid — a 10 kWh lithium bank does the work of a 20 kWh lead-acid bank.
  • 6,000+ cycle life (15–20 years of daily cycling) vs. 500–1,200 cycles for flooded lead-acid and 1,000–2,000 for AGM.
  • No maintenance — no watering, no equalization charges, no venting requirements.
  • ~95% round-trip efficiency vs. 80–85% for lead-acid — your solar array goes further.
  • Half the weight and footprint per usable kWh.

Lead-acid still makes sense in narrow cases: infrequently cycled standby applications on a tight upfront budget, extreme cold installations where lithium's low-temperature charge limits are a problem, and legacy systems with existing lead-acid charging infrastructure. On a cost-per-cycle-kWh basis, lithium won years ago — a $1,500 LiFePO4 module delivering 5 kWh × 6,000 cycles costs about 5¢ per kWh-cycle; lead-acid at half the price but a quarter of the throughput costs roughly double that.

Product Picks by System Size

We stock the batteries below at wholesale pricing. Inventory moves fast — current stock status is on each product page.

Small banks (5–10 kWh): critical loads, cabins, RVs

Medium banks (10–20 kWh): whole critical-loads panels, partial home backup

Large banks (20+ kWh): whole-home backup and off-grid homes

On BYD: the Battery-Box Premium line (HVS/HVM/LVS) remains an excellent modular high-voltage option — BYD cells also sit inside many third-party systems. Our BYD stock fluctuates; contact us for current availability and freight quotes on the LVS and LVL configurations.

Don't Forget the Other Half of the Equation: Power (kW)

A battery bank has two ratings: energy (kWh) — how long it runs — and power (kW) — how much it can run at once. A 10 kWh battery with a 3 kW inverter can't start your well pump and run the AC simultaneously, no matter how much energy is stored.

  • Sum the running watts of simultaneous loads, then check surge watts — motors (pumps, compressors) draw 3–5× their running watts for a few seconds at startup.
  • Match the battery's continuous and peak discharge ratings to your inverter. As a rule of thumb, you want at least 0.5C of discharge capability (a 10 kWh bank delivering 5 kW continuously).
  • For generator-backed systems, an automatic transfer switch handles the changeover so the battery bridges the 10–30 seconds while the generator starts — see our automatic transfer switch collection for Generac, Cummins, and Briggs & Stratton ATS units from 50A to 600A.

Sizing Cheat Sheet

| Your situation | Daily kWh to back up | Recommended bank | |---|---:|---| | Fridge, lights, internet, furnace blower | 5–8 kWh | 1× 5.12 kWh EG4 (expandable) | | Critical loads + well pump + mini-split | 10–15 kWh | 1× EG4 14.3/16 kWh or 2× 5.12 kWh rack | | Whole home minus central AC | 20–25 kWh | 2× 13.5 kWh class units or 14.3 + 5.12 kWh | | Whole home with central AC | 30–40 kWh | 3× stacked units, or battery + generator hybrid | | Off-grid (3-day autonomy) | daily use × 3.3 | Scale with server-rack modules |

The Bottom Line

Size from your bills, not from a brochure. Decide honestly which loads matter during an outage, convert that to kWh, adjust for depth of discharge and autonomy, then choose lithium for anything that cycles more than occasionally. When in doubt, pick a modular system — EG4's rack and wall-mount lines let you start at 5 kWh and grow to 30+ kWh on the same inverter.

Need help running the numbers on a real project? Portlandia Electric Supply sells all of the batteries above at wholesale pricing with freight quotes nationwide — call us or request a quote and we'll size the bank with you.

Related reading: The 20/80 Battery Rule in 2026

Related charts & calculators

Keep these quick-reference charts handy while you plan:

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