Battery autonomy — is 4 hours the new 24?
Once upon a time, every off-grid dreamer wanted a full day of battery backup. In 2025, most grid-tied homes settle for 4 hours. Lithium prices have plunged, load-shedding blocks rarely exceed two hours, and smart inverters recharge batteries between cuts. Let’s see when 4 h makes sense, when 24 h still rules, and how our Solar PV Sizer wizard helps you find the sweet-spot.
1 · Where did the 4 h standard come from?
Utility-scale markets pay the most for 4 h packs, so manufacturers mass-produce 5 kWh modules aimed at that duration. Home-storage vendors simply piggy-backed on the volume curve — driving rand-per-kWh down to record lows.
4 h autonomy = 32 kWh bank on an 8 kW inverter
2 · South African reality check
Stage 2–4 load-shedding returns in two-hour blocks. A 4 h battery comfortably bridges two cuts if you top up between them. Anything bigger often sits idle, tying up capital you could spend on more panels.
3 · Cost curve: 4 h vs 24 h (LiFePO4)
| Autonomy | Battery kWh* | Typical cost | Rand/kWh |
|---|---|---|---|
| 4 h | 32 kWh | R128 000 | ≈ R4 000 |
| 24 h | 192 kWh | R768 000 | ≈ R4 000 |
*Assumes 8 kW average load at 80 % depth-of-discharge.
4 · Hybrid approach: 4 h lithium + diesel fallback
Many farms pair a modest LiFePO4 bank with a 5 kVA generator. Fuel only burns during multi-day storms, slashing your upfront spend by 75 %.
5 · How the wizard sizes autonomy
- Pick your inverter size or accept the default.
- Slide Battery Autonomy from 1 h to 24 h.
- The backend recalculates battery kWh, inverter surge rating and payback instantly.
6 · Pros & cons at a glance
- 4 h bank
✔ Lowest rand-per-kWh
✔ Charges fully on one sunny day
✔ Lighter wall/roof load
✖ Little daylight backup if skies stay grey - 24 h bank
✔ Survives multi-day outage without genset
✖ ~6× the capex
✖ May require structural sign-off
7 · Take-aways
- 4 h covers typical Stage 4 load-shedding at the lowest cost.
- Invest in excess PV or a generator before you overspend on idle lithium.
- Use our wizard’s autonomy slider to see live payback changes.
Curious how a 4 h vs 24 h choice affects your own bill? Head back to the Solar PV Sizer wizard, slide the autonomy bar and watch kWh, R/kWh and payback update in real time.