Solar Batteries: Types, Costs & How Many You Need

The short answer

A solar battery does one of two jobs: backup power (keep the lights and fridge on during outages) or bill saving through time-shifting (use your daytime solar in the expensive evening hours). For most new systems in 2026 the right chemistry is LFP lithium — higher upfront cost per kWh than lead-acid, but roughly 80–90% usable capacity and several times the cycle life. The sizing formula is simple: battery kWh = daily kWh you want from batteries × days of autonomy ÷ usable depth of discharge. Indicative 2026 installed costs run around US$600–1,200 per usable kWh for residential lithium in many markets, and far less for lead-acid — every figure below is an indicative range, not a quote, because prices vary widely by market.

The two jobs: backup power vs time-shifting

Before touching a spec sheet, decide which job you're hiring the battery for — because it changes everything about sizing. Backup power is about outages: load shedding, storms, grid failures. The battery sits mostly full and discharges only when the grid drops, so you size for your essential loads (lights, fans, fridge, Wi-Fi, a few sockets) and for autonomy — how many hours or days the longest typical outage lasts. Time-shifting is about money: your panels overproduce at midday when nobody is home, and your evening peak lands when the sun is gone. The battery soaks up the midday surplus and discharges it every single evening, so you size for your whole evening load and for daily throughput — this bank cycles every day, which is why cycle life matters so much more here. Some households want both, and a single bank can do both jobs — but the dominant job drives the sizing, as the strategy table below shows. One more note: if your utility offers fair net metering, the grid already stores your surplus as credits, which weakens the time-shifting case; where net metering is weak or absent, time-shifting with batteries is often what makes a solar system pay.

Battery chemistries: LFP lithium vs lead-acid

Two families dominate residential solar storage. LFP (lithium iron phosphate) is the modern default: 80–90% of its nameplate capacity is usable every cycle, indicative cycle life is 4,000–6,000 cycles at 80% depth of discharge, round-trip efficiency is around 90–95%, and it needs no maintenance. Lead-acid (flooded, AGM, or gel) is the old workhorse: cheaper per nameplate kWh, but only about 50% usable depth of discharge without punishing its lifespan, indicative cycle life of 500–1,500 cycles, 75–85% efficiency, and flooded types need watering and ventilation. The comparison table makes the trade-off concrete — and for the full chemistry deep dive, including NMC lithium and cold-weather behavior, see our LiFePO4 vs lead-acid for solar guide.

Specification	LFP lithium	Lead-acid (flooded / AGM)
Usable depth of discharge	80–90%	~50%
Indicative cycle life	4,000–6,000 cycles	500–1,500 cycles
Round-trip efficiency	90–95%	75–85%
Indicative lifespan (daily cycling)	10–15 years	3–6 years
Maintenance	None (sealed)	Watering + ventilation (flooded)
Upfront cost per kWh (indicative)	Higher	Lower

The honest math: lead-acid looks cheaper on the price tag, but because you can only use half of it and you replace it two to four times over a 25-year panel life, LFP usually wins on lifetime cost for daily-cycling systems. Lead-acid still earns its place for small backup banks that cycle rarely, tight budgets, and off-grid cabins where a battery can be checked and watered.

Indicative cost per kWh of storage (2026)

Battery prices are quoted two ways, and mixing them up is the classic buying mistake: nameplate kWh (what the label says) vs usable kWh (what you can actually draw). A 10 kWh lead-acid bank at 50% usable depth gives you 5 usable kWh — so its cost per usable kWh is roughly double the sticker figure. Indicative 2026 market ranges, battery-only vs fully installed:

Battery type	Indicative cost, battery only (per kWh nameplate)	Indicative installed residential system (per usable kWh)
Flooded lead-acid	US$100–200	US$250–500
AGM / gel lead-acid	US$150–250	US$350–650
LFP lithium	US$300–600	US$600–1,200

These bands vary widely by market — import duties, brand tier, and installer margin move them substantially, and installed figures can include or exclude inverter upgrades, wiring, and permitting. Two rules of thumb: always compare quotes on cost per usable kWh, and for lithium ask whether the quote includes the battery management system and hybrid inverter — the battery is often only half the installed project.

Sizing: how many batteries you need

The formula is the same everywhere: bank kWh (nameplate) = daily kWh to cover × autonomy days ÷ usable depth of discharge. "Daily kWh to cover" is not your whole bill — it's the energy you want the battery to supply (night-time loads for time-shifting; essential loads for backup). "Autonomy days" is how long the bank must last without sun: 1 for time-shifting (the sun comes back tomorrow), 1–2+ for backup depending on your outage pattern. "Usable depth" is 0.8–0.9 for LFP, ~0.5 for lead-acid. The worked example below assumes a household wanting 8 kWh per night — for a full step-by-step walkthrough on a common system size, see our 5 kW solar system battery sizing guide, and model your own loads in the solar system calculator.

Chemistry	Usable depth	8 kWh/night, 1-day autonomy	8 kWh/night, 2-day autonomy
LFP lithium	80%	10 kWh bank	20 kWh bank
Lead-acid	50%	16 kWh bank	32 kWh bank

Two sizing traps: averages lie — size for your heaviest-use season (summer cooling or winter heating), not the yearly average; and kWh is not kW — the bank must also deliver enough power (kW) to start your biggest loads. A 10 kWh bank with a 3 kW inverter will not start a 1.5-ton air conditioner. Inverter sizing is part of the same exercise — the calculator above covers both.

Which battery strategy fits you

Match the strategy to your situation, not to your neighbor's:

Your situation	Best strategy	Sizing emphasis
Frequent outages / load shedding	Backup-first bank	Critical loads + 1–2 days autonomy
High evening tariffs, weak or no net metering	Time-shifting bank	Full evening load, daily cycling — lithium's cycle life pays here
Fair net metering, rare outages	Batteries optional	Small backup bank or skip storage entirely
Off-grid / no utility connection	Large autonomy bank	2–3 days autonomy, sized for worst-weather weeks

The common thread: backup favors autonomy days, time-shifting favors daily throughput. A time-shifting bank cycles roughly 300+ times a year, which is where lithium's 4,000+ cycle life earns its premium; a backup bank in a stable-grid area may cycle only a few dozen times a year, which is where lead-acid's lower price can still make sense.

Lifespan and maintenance basics

What kills batteries is heat, deep discharge, and neglect — in that order. Keep banks in a cool, ventilated space (every sustained 10°C above ~25°C roughly halves lead-acid life; lithium tolerates heat better but still ages faster hot). Don't routinely drain past the usable depth figures above. Flooded lead-acid needs distilled-water top-ups and terminal checks; sealed lithium needs nothing but an occasional glance at the battery management system readings. Budget one lead-acid replacement over the panels' 25-year life; a quality LFP bank installed well may never need replacing.