Interactive lesson

Batteries and inverters, step by step

Interactive lesson: Wh instead of Ah, lead-acid or LiFePO4, capacity and system voltage, charge controller, inverter, cables, fuses and a safe battery room.

Free sample Chapters 1 and 2 (8 steps) are free. The full lesson comes with the Batteries and inverters module. Already bought? Unlock

This lesson is general guidance, not professional training. Local regulations and expert advice always take precedence. Read our disclaimer.

The whole lesson as text

1. Wh and depth of discharge

The battery is the tank, the inverter is the tap.

Wh, not just Ah — Wh = Ah × voltage. A 12 V 100 Ah battery holds 1,200 Wh and a 48 V 100 Ah battery 4,800 Wh, although both say "100 Ah".
Lead-acid: only half — A lead-acid battery run completely flat every day lasts about a hundred cycles. Of 1,200 Wh, 600 Wh is usable.
LiFePO4: 80–90 % — LiFePO4 takes 80–90 % discharge thousands of times. The same capacity gives roughly 1,000 Wh.
Discharge rate — A 100 Ah lead-acid battery at 5 A (C/20) gives its full 100 Ah, but at 50 A (C/2) only 60–70 Ah. With LiFePO4 the maximum continuous current is set by the BMS.

Why: A panel makes electricity only while the sun shines; the light in the evening and the fridge at night run off the battery. The battery is the most expensive part of the system and the only one that wears out: chosen wrongly or charged badly, it dies in two or three years.

A LiFePO4 battery is labelled 12.8 V, 25.6 V or 51.2 V instead of 12, 24 or 48 V.
You can't tell the state of charge of a LiFePO4 battery from its voltage: read it on a shunt battery monitor or the BMS display.

2. Lead-acid or LiFePO4

Don't compare batteries by price, compare them by the cost of each kWh used.

No car batteries — A car battery is built to give a big current for a few seconds and be topped up straight away. Discharging it every day kills it in a season or two.
AGM and GEL — Deep-cycle AGM or GEL is for small systems and camping: 300–800 cycles at 50 %, a life of 3–6 years, no maintenance.
Solar lead-acid — OPzS and OPzV give 1,200–2,500 cycles and 10–15 years. They need distilled water and give off hydrogen when charging.
LiFePO4 — 3,000–6,000 cycles at 80 %, 8–12 kg per kWh, no gas. Almost every new system today.

Why: Cost per kWh = battery price ÷ (kWh × usable share × number of cycles). Lithium costs more at the start, but roughly 5–7 times less per kWh used.

In the module's example AGM comes out at £0.49 and LiFePO4 at £0.07 per kWh. The prices are for illustration only; work it out with your own.
For anything from 1 kWh up, compare AGM or GEL with LiFePO4.