Chapter 5 of 5 · How do batteries move energy in time?
Battery Storage Field Guide
Solar and wind generate on nature's schedule, not ours. Batteries don't make energy; they move it in time, from cheap midday sun to the expensive evening. Follow the same town as Chapter 1 through a home, a grocery store and a small factory to see what's inside a battery, how long it lasts, how to size one and what it really costs.
How a battery cell charges and discharges
Every battery, from a phone to a grid container, is built from cells. Here is one LFP cell, the kind most home and grid batteries now use. Press Charge or Discharge and watch where the lithium goes.
State of charge25%
lithium ion (Li⁺), moves inside the cellelectron (e⁻), moves through the wire
Charging
The charger (rooftop solar or the grid) pulls electrons out of the cathode and pushes them through the wire into the anode.
Lithium ions follow on the inside. They leave the iron phosphate, swim through the electrolyte and slip through the separator.
They tuck in between the graphite layers of the anode. The cell is full when the graphite holds all the lithium it can.
Discharging
Lithium would rather be back in the cathode, so as soon as there's a path, ions leave the graphite and cross back through the electrolyte.
Electrons can't cross the separator, so they take the long way round, through your lamp, fridge or the grid. That flow is the electric current you use.
Ions and electrons meet again in the cathode. The cell is empty when the graphite has given up its lithium.
Ions inside, electrons outside
The separator is the trick. It lets lithium ions through but blocks electrons, so every ion that crosses inside forces one electron to travel through the outside wire. Nothing is burned or used up: the same lithium shuttles back and forth thousands of times.
Why about 3.2 volts?
Charging pushes lithium "uphill" into the graphite, like pumping water up into a tank. The height of that hill sets the voltage: about 3.2 V for LFP and 3.6–3.7 V for NMC. A little energy turns into heat each way, which is why you get back roughly 85–90% of what you put in.
Why cells wear out
Each cycle, a little lithium gets trapped in a thin film that grows on the anode and in other side reactions. Less lithium shuttling back and forth means less capacity. Heat speeds this up, which is why cooling matters. Try it in Wear it out.
From one cell to a battery. One cell is small. Wiring cells in a chain (series) adds up their voltage; wiring them side by side (parallel) adds up their capacity. A home battery holds dozens of large cells; a grid container holds thousands. See how they stack up in the Diagram. Battery makers name the electrodes by what they do while discharging, so the cathode stays the "cathode" even while charging. LFP stands for lithium iron phosphate (LiFePO₄); the anode is graphite, written C₆.
Inside the box: from cell to grid
DC side of the inverterAC, site voltageUtility primary (~12 kV)Monitoring (telemetry)Control (setpoints)Safety layer
Click any part to see what it does, what happens if it fails, and roughly what it costs.
Start at the cell on the left and work right: each step is a bigger, more capable piece built from the one before.
Playground
Four small tools, one question each. Numbers are realistic in scale but simplified; change one thing at a time and watch what moves.
Who
Day
Where the home's power comes from, hour by hour
Point at the chart to read the values
solar used directlybattery dischargingfrom the gridbattery chargingsolar left overdemand
Battery state of charge
Batteries earn money from the gap between cheap midday power and expensive evening power. The more batteries a grid builds, the smaller that gap gets. In California, evening peak prices fell by about a third from mid-2025 to mid-2026, and most batteries there now earn more from being available (resource adequacy) than from the price gap. That's also why grid batteries there are mostly 4 hours: long enough to cover the whole evening after the sun drops.
Chemistry
Most home and grid batteries cycle about once a day. Demand-charge batteries may cycle less.
Extra capacity so the battery still meets the goal after years of fade.
Site (loads the worked example)
Demand charge savings
Time-of-use savingsiOn a time-of-use rate, the same kWh costs more at peak times (usually late afternoon and evening) than off-peak (overnight, and often midday). The battery fills up when power is cheap, or from your own solar, and runs the house during the peak, so you buy less of the expensive kind.Yearly saving = price gap × kWh moved a day × 365
i
Cumulative savings against the upfront cost
savings so farnet cost
Battery glossary
Common mix-ups
What people say
What's actually true
Quiz
What batteries cost
The same kilowatt-hour of storage costs very different amounts depending on what's included and how big the project is. Figures as of October 2026, mostly from 2025 surveys.
The cost ladder, $ per kWh
Going from about $70 to about $1,000 per kWh is mostly not the battery. It's design, permits, electricians, interconnection and sales, which cost roughly the same whether the battery is small or large, spread over very few kWh at a home. Solid bars show a single average; striped bars show a range. The ink tick on the home bar is the Powerwall 3 average (~$998/kWh).
Three worked examples
Round, illustrative numbers. Common assumptions: 90% usable depth, 95% efficiency, 20% aging margin, $15/kW-month demand charge. Plug in your own utility's rates with the Pay it back tool.
Incentives in 2026 as of October 2026
Homeowners buying outright get no federal credit. The 30% residential credit (Section 25D) ended December 31, 2025. Many websites still list it.
Businesses and leased or PPA home systems can still use the commercial credit (Section 48E). Storage that starts construction before 2033 remains eligible.
New foreign-sourcing (FEOC) limits apply to projects claiming the credit, and they tighten each year through 2030.
State and utility programs now matter more. Examples: Oregon's battery storage credit and California's SGIP, which pays per kWh. Check your own state and utility.
Tariffs on imported cells and packs can move US prices a lot; BNEF estimated a 60% tariff would raise US turnkey costs by about 60%. Check current status before relying on any figure here.
How this squares with Chapter 3. Chapter 3 prices its 750 kW / 1,500 kWh battery at $350–$500/kWh, above the ~$219/kWh US turnkey average shown here. They don't conflict: "turnkey" covers the battery, inverter, enclosure and integration, but not construction or the grid connection. Chapter 3's figure is an installed price for a small 2-hour project, which carries those extra costs over fewer kWh.
EIA, US electric generating capacity expected to reach a record high in 2026 — eia.gov
Industrial Info Resources on EIA data, US battery storage growth (43.6 GW at end of 2025) — industrialinfo.com
Modo Energy, how a battery energy storage system makes money (ERCOT and CAISO revenue stacks) — modoenergy.com
Modo Energy, CAISO battery storage in 2026: 8 things to watch (resource adequacy) — modoenergy.com
Global Infrastructure Hub, Hornsdale Power Reserve project — gihub.org
Energy-Storage.news, Moss Landing: what we know so far — energy-storage.news
Commercial and industrial installed ranges via mgrid.org citing IDTechEx (to verify) — mgrid.org
Built for learning, not for engineering sign-off. Numbers are realistic in scale, not quotes.
Safety: layers, and the record so far
Layers of protection
Battery systems are protected in layers, fastest first. The BMS trips in milliseconds when one cell drifts out of range. Thermal management keeps cells in their comfortable range. Gas and heat detection, explosion venting, suppression and spacing between units are there so that if a cell does go into thermal runaway, the failure stays in one unit.
The fire most people have heard of, at Moss Landing, California in January 2025, was in a large indoor facility of an older design. It's a big part of why outdoor, spaced, containerized LFP systems are now the norm. Industry groups point to the code framework and the improving record; community groups raise questions about siting, emergency response, and smoke and runoff. Both are part of how projects get approved.
Standards to know by name: UL 9540 (system safety listing) · UL 9540A (thermal-runaway fire test) · NFPA 855 (installation code) · UL 1973 (stationary batteries) · IEEE 1547 (grid interconnection, from Chapter 3)
≈ 99% lower
Failure incidents per unit of deployed capacity, 2018 to 2025, from EPRI's incident database. EPRI notes the database may not capture every incident.
≈ 72%
Share of incidents that happened during construction, commissioning or the first two years of operation. The early months are when problems show up.