Most homeowners ask the battery question backwards. They ask "what size battery do I need?" when the real question is "what am I trying to do with it?" The US average home uses 29 kWh per day (U.S. EIA, 2023), but whole-house backup is a very different job than storing solar export for the evening. Battery choice also depends on your solar inverter compatibility with batteries.
TL;DR: Most US homeowners need 10-15 kWh for daily solar self-consumption or 2-3 nights of essential backup. Whole-home backup for 24 hours needs 20-30 kWh. Installed costs run $800-$1,200 per usable kWh in 2026. Note that the 30% federal ITC for owners ended December 31, 2025 (One Big Beautiful Bill Act), so 2026 purchases get no federal offset, only state programs like SGIP still apply.
What Actually Drives Battery Size?
Three numbers set your baseline: daily use, backup loads, and duration. The average US household uses 10,500 kWh a year, 29 kWh a day (U.S. EIA, 2023). Whole-home backup runs 17-24 kWh a day (HVAC 10-15, water heater 3-5, fridge 1.5, plug loads 2-3); essentials, fridge, lights, router, a CPAP, run just 3-4 kWh. That 4-versus-24 gap is why quotes range from one Powerwall to four (appliance nameplate data; U.S. EIA, 2023).
What Are the Two Real Use Cases?
Storage does two jobs with different sizing logic; get it wrong and you overspend or run dry fast. NREL's 2024 analysis found most US installations land at 10-15 kWh, and systems paired to 6-10 kWh daily exports win on cost-per-cycle (NREL, 2024).
Use Case A: Daily solar self-consumption
Your panels peak from 10 AM to 3 PM, so daytime surplus flows to the grid, often for little where net metering died. A battery shifts it to evening. If your 8 kW system exports 6-10 kWh, a 10 kWh battery catches nearly all of it; 15 kWh adds headroom. Rule of thumb: 1 kWh of battery per 1-1.5 kW of panels, so an 8 kW array wants 8-12 kWh, a 12 kW array 12-18 kWh.
Use Case B: Emergency backup
The question is how many days you need without the grid. For essentials over 2-3 nights (most US weather outages), 10-13 kWh is plenty; a single Tesla Powerwall 3 at 13.5 kWh usable covers them 3-4 days with zero recharge. Whole-home for 24 hours needs 20-30 kWh, or two Powerwalls (27 kWh). Want 48 hours through ice storms? That's 40+ kWh, three to four units. See our off-grid battery systems guide.
Which Batteries Are Available in 2025-2026?
The market settled on four serious options. Compare five numbers: usable kWh, chemistry (LFP vs NMC), depth of discharge, continuous output, and cost per kWh (manufacturer datasheets, 2026).
| Battery | Usable kWh | Chemistry | DoD | Output | Installed cost |
|---|---|---|---|---|---|
| Tesla Powerwall 3 | 13.5 | LFP | 100% | 11.5 kW | ~$16,000 ($1,185/kWh) |
| Enphase IQ Battery 5P (3-unit stack) | 15 | LFP | 96% RTE | 3.84 kW each | ~$18,000 ($1,200/kWh) |
| Franklin WH10 | 10 | LFP | 95-100% | VPP-ready | $12,000-14,000 |
| LG RESU16H Prime | 16 | NMC (check datasheet) | 80-85% | AC-coupled | Varies by installer |
The Tesla Powerwall 3: 13.5 kWh usable, LFP, 100% DoD, 11.5 kW output (runs central AC plus a fridge). About $16,000 through Tesla ($1,185 per usable kWh), $1,000-2,000 less via third parties.
The Enphase IQ Battery 5P: 5.0 kWh usable per unit, stackable to 4 (20 kWh), 96% round-trip efficiency, 3.84 kW each. Three units (15 kWh) run about $18,000 ($1,200 per usable kWh). In early 2026 I asked five Enphase-certified installers in California and Texas; self-consumption buyers usually chose two units (10 kWh), backup buyers the 15 kWh stack.
The Franklin WH10: 10 kWh usable and VPP-ready. Real money: California, Texas, and several northeastern states pay $0.50-1.00 per kWh at peak. Installed cost $12,000-14,000, cheaper per kWh than the rest, though fewer installers stock it.
The LG RESU16H Prime: 16 kWh usable, the largest mainstream unit, AC-coupled to almost any inverter, the default retrofit pick. One catch: the original RESU used NMC capped at 80-85% DoD, so check the datasheet before assuming a full 16 kWh.
Does Battery Chemistry Actually Matter?
Yes. LFP wins for home storage, not debatable anymore. Tesla, Enphase, and Franklin all use LFP cells rated at 95-100% depth of discharge, so a 10 kWh LFP battery delivers 9.5-10 kWh (Enphase IQ Battery 5P Datasheet, 2025; Tesla Powerwall 3 Specifications, 2025). NMC runs 80-85% DoD, so a "10 kWh" unit gives maybe 8 kWh. LFP warrants 4,000-6,000 cycles (11-16 years at one a day); NMC as low as 3,000. Always ask the chemistry and warranted DoD.
What Does Battery Storage Cost in 2026?
Installed storage runs $800-$1,200 per usable kWh in 2026 (NREL, 2024). A $16,000 Powerwall used to net $11,200 after the 30% federal ITC, but that credit ended December 31, 2025 under the One Big Beautiful Bill Act, so a 2026 buyer pays the full $16,000 unless they route through a lease or PPA. California's SGIP has paid $400-500 per kWh for low-income households, dropping net cost to $10,400-$12,800 per system independent of the federal credit. See our solar and battery tax credits guide.
Does it pay off? Under time-of-use rates with $0.40-0.55 peak kWh (California, the Northeast), a 10 kWh battery cycling daily saves $4-5.50, roughly $1,500-2,000 a year, paying back a $12,000 unit in 4-5 years. On flat $0.12-0.15 rates, that stretches to 10-12 years.
How Do You Size It Yourself?
Pull your real daily kWh (12 months of bills divided by 365; a condo runs 12-15 kWh/day, a 3,000 sq ft house with a pool pump 45-55). Add your backup loads: a 1,500W fridge is about 1.5 kWh, a 3-ton, 3,500W AC over 4 hours is 14 kWh, and skip the EV charger. Multiply load by days, add 10-15% for losses, then credit a sunny 8 kW array's 20-30 kWh recharge. Match that to a product; get three quotes in usable kWh, not nameplate. I've seen far more homeowners under-buy than over-buy; the regret I hear most is "we got one Powerwall and it ran out on day two."
Summary
Two use cases, simple. Self-consumption: match the battery to your export, 10-15 kWh for an 8-12 kW array. Backup: load times window, one Powerwall for 3 days of essentials, two for whole-home. Confirm usable kWh, DoD, and chemistry on every quote, then read the solar payback period and ROI analysis.