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 we see is sizing it wrong. Undersize the bank and the lights go out at 3 a.m. on night two. Oversize it and you've spent $4,000 on capacity you cycle twice a year. We've specced hundreds of banks — cabins, critical-loads panels, whole-home backup, full off-grid — and the jobs that work all start the same way: with the customer's real kWh, not a guess and not a sales pitch.

This guide walks the exact method: measure your daily usage, convert it to battery capacity with honest derates, pick a chemistry, and then check the other half of the equation — power in kW — that everyone forgets until the well pump won't start. Every table below is one you can spec from.

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:

• Critical loads only — refrigerator, lights, internet, furnace blower, a few outlets, medical equipment. Typically 5–10 kWh/day.
• Critical loads plus comfort — add a mini-split or window AC, well pump, or electric water heater. Typically 12–20 kWh/day.
• 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. I talked a customer out of a second 16 kWh battery last spring by moving six circuits to a critical-loads panel; the $900 of electrical work saved him $3,700 of battery he'd have charged twice a year.

Add up what you'd actually run during an outage. That total is your daily backup requirement — and look how the table explains the market: a refrigerator, lights, internet, and a furnace blower total about 6.7 kWh/day, which is why 10 kWh class batteries dominate critical-loads backup. The 3-ton central AC alone wants 21 kWh/day. That's the whole ballgame in one row.

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)

The math checks against the formula: 5 kWh × 1 ÷ 0.9 × 1.1 = 6.1 kWh; 10 × 2 ÷ 0.9 × 1.1 = 24.4 kWh; 40 × 3 ÷ 0.9 × 1.1 = 146.7 kWh. Same numbers, no hand-waving.

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 no heated enclosure is planned, 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.

One cold-weather note from the field: LiFePO4 cells can't be charged below freezing without damage, which is why the heated battery versions exist. We learned to spec heated or indoor-rated batteries on every unconditioned-space install after a January service call in which a beautiful new bank sat at 40% state of charge, refusing a charge at 18°F, doing its BMS-mandated job perfectly while the customer froze. Browse the chemistry options in our LiFePO4 battery collection, or the full 48V battery aisle if you're building around a hybrid inverter.

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

A battery bank has two ratings, and kWh is only one of them. The other is power — how many kilowatts the bank and inverter can deliver at once. A 20 kWh bank behind a 5 kW inverter still can't start a 3-ton air conditioner.

Check three power numbers against your load list:

• Continuous inverter output must exceed your largest simultaneous load combination. Add the running watts of everything you expect to run at once.
• Surge rating must cover motor starts — well pumps, compressors, and AC units pull 2–3× running watts for a second or two. A ½ HP well pump drawing 800 W running can demand 2,000–2,400 W to start.
• Battery discharge rate (C-rate) must support the inverter's draw. A 5.12 kWh module rated at 100A continuous delivers about 5 kW at 48V — one module can't feed a 12 kW inverter at full tilt no matter how big the nameplate kWh looks.

A soft starter on the central AC — cutting locked-rotor amps by 60–70% — is the cheapest kW you'll ever buy. It's a $300–400 part that routinely lets a 12 kW hybrid inverter carry a house that "needed" 18 kW. Our hybrid inverter lineup lists continuous and surge ratings on every product page; match them to this table before you match the kWh.

Measure your daily kWh, decide honestly what the battery must run, apply the DoD and autonomy math once on paper, and check the kW side before you buy. Do that and you'll buy a bank that actually carries the house — not one that dies at 3 a.m. and not one with $4,000 of shelf-ware capacity. The formula is four operations long; the product tables above turn the answer into a SKU. When you're ready, the energy storage collection has the stock, and our battery backup runtime calculator sanity-checks whatever you pick against your load list.

• Battery Backup Runtime Calculator — plug in your load list and bank size, get honest hours.
• Solar System Size Calculator — size the array that recharges the bank.
• Wire Ampacity Chart (NEC) — size the conductors between bank and inverter.
• NEC Wire Sizing Guide — full ampacity and derating workflow for battery and generator circuits.