Lithium-ion vs. Lead-acid: a homeowner’s cheat-sheet
Every solar quote seems to toss around battery buzzwords—LiFePO4, AGM, autonomy hours, DoD. Below is a plain-English guide so you can compare apples with apples when you size storage with our Solar PV Sizer wizard.
1. Chemistry 101
- Lithium-ion (LiFePO4) – lightweight, 90–95 % round-trip efficiency, up to 6 000 cycles at 80 % depth-of-discharge (DoD).
- Sealed Lead-acid (AGM / Gel) – cheaper upfront, 75–85 % efficiency, ~1 000 cycles at 50 % DoD.
- Flooded Lead-acid – the classic wet-cell, demands maintenance, lowest cost per kWh if space isn’t an issue.
2. Why autonomy matters
Autonomy hours tell you how long your home keeps running when the sun and grid both disappear. Our wizard lets you pick 2–48 h. The maths:
Battery kWh = Daily load × (Autonomy h ÷ 24) ÷ Usable DoD
Example—25 kWh daily load, 8 h autonomy, lithium battery (90 % DoD):
25 × (8 ÷ 24) ÷ 0.9 ≈ 7.4 kWh
The same autonomy on lead-acid at 50 % DoD doubles the required capacity.
3. Temperature & cycle life
Lead-acid capacity drops 20 % in winter below 10 °C. Lithium loses cycle life if baked above 40 °C. Stick batteries in a garage or insulated box, not the attic.
4. Cost per warranted kWh
| Chemistry | Typical $/kWh | Cycles (@80 % DoD) |
|---|---|---|
| Li-ion (rack-mount) | $450–550 | 6 000 |
| Li-ion (all-in-one) | $600–800 | 6 000 |
| AGM lead-acid | $250–350 | 1 000 |
| Flooded lead-acid | $150–250 | 800 |
Divide dollars by cycles to see long-term value: lithium often wins despite the sticker shock.
5. Should you oversize?
If you regularly cycle the battery to 80 % daily, oversizing by 20 % can extend lifespan by 30 %. Our MVP sizing rounds to the nearest kWh; in real projects, installers often add a module or two for headroom.
6 · Take-away checklist
- Pick autonomy to match actual outage patterns—4 h covers most urban cuts.
- Use lithium if space is tight or discharge cycles exceed 1 per day.
- Factor usable DoD (90 % lithium, 50 % lead-acid) into kWh maths.
- Temperature kills batteries—site them in the 10–30 °C sweet spot.
Ready to crunch your own numbers? Jump back to the Solar PV Sizer wizard and slide the autonomy bar to see how lithium vs. lead-acid affects total system size.