Battery storage has transformed from a niche product for off-grid enthusiasts into a mainstream addition to residential solar systems, driven by falling lithium-ion costs, favorable federal incentives, and increasing grid unreliability in some regions. But battery storage economics depend heavily on local utility rates, incentive structures, and how you intend to use the system — backup, arbitrage, or maximizing self-consumption.
Choosing the Right Battery Size
The two most common sizing approaches are backup-based and self-consumption-based, and they often yield different results. For backup sizing, identify your critical loads — refrigerator (1.5–2 kWh/day), lighting (0.5–1 kWh), phone/laptop charging (0.3 kWh), medical devices — and multiply by desired days of autonomy, then divide by depth of discharge. A typical US household targeting 1–2 days of critical-load backup needs 15–30 kWh of usable storage. For self-consumption optimization, model your solar generation profile against your consumption profile: a 10 kW solar system in Phoenix generates roughly 50 kWh on a peak summer day, with most generation between 9 AM and 4 PM. If your home uses 8 kWh during those hours and exports the rest, a 13.5–20 kWh battery can capture the midday surplus for evening use, raising self-consumption from ~35% to ~75–85%. The 'right' size depends on your priorities — backup security or bill optimization — and on whether your utility still offers full retail net metering or has shifted to reduced export rates.
Battery Chemistry: Lithium-Ion vs. Lead-Acid
Virtually all new residential storage installations use lithium-ion chemistry, specifically LFP (lithium iron phosphate) for its superior safety, cycle life, and ability to hold a 100% state of charge without degradation. LFP batteries achieve 3,000–6,000 cycles to 80% capacity, round-trip efficiency of 90–95%, and usable depth of discharge of 80–90%. Tesla Powerwall, Enphase IQ Battery, SolarEdge Home Battery, and Franklin WH all use LFP. Lead-acid batteries (flooded or AGM) are still used in some off-grid and RV applications due to lower upfront cost per kWh ($150–$300/kWh vs. $800–$1,200/kWh installed for lithium). However, lead-acid's 50% usable DoD, 70–85% round-trip efficiency, 500–1,000 cycle life, and maintenance requirements make it a poor choice for daily-cycling grid-tied residential applications. For most homeowners, the total lifecycle cost of lithium is lower despite the higher upfront price. Flow batteries (vanadium redox, zinc-bromine) are emerging commercial-scale options offering essentially unlimited cycle life and 100% DoD, but remain too expensive and space-intensive for residential use as of 2024.