Autonomy requirements are usually stated casually — “we want a week of backup” — without anyone costing it. This worked example shows what seven days actually means in capacity, weight and money, so the trade-off can be decided deliberately rather than discovered at installation.
The site
- Remote monitoring station: camera, logger and 4G router
- Measured daily consumption: 840 Wh/day
- Duty: continuous, no scheduled downtime
- Access: 4×4 required, roughly monthly
- Climate: warm, 25-38 °C ambient in enclosure
The arithmetic
Required stored energy = daily Wh × autonomy days ÷ usable depth of discharge
Seven days at 840 Wh/day is 5,880 Wh of delivered energy — before accounting for what the battery can actually give up.
| Chemistry | Usable DoD | Nominal capacity needed | Approx. weight |
|---|---|---|---|
| Gel lead-acid | 50% | 11,760 Wh (~920 Ah at 12.8 V) | ~300 kg |
| AGM lead-acid | 50% | 11,760 Wh | ~290 kg |
| LiFePO4 | 80% | 7,350 Wh (~575 Ah at 12.8 V) | ~60 kg |
| LiFePO4 | 90% | 6,533 Wh (~510 Ah) | ~55 kg |
What that means in practice
The gel option is 300 kg of battery. On a pole or a wall-mounted cabinet that is not a design detail — it is a structural problem, and it needs a ventilated enclosure, a lifting plan and probably a concrete pad. The lithium option is 60 kg, can be carried by two people, and fits the same cabinet the rest of the equipment sits in.
Over ten years on a daily cycle, the gel bank will likely be replaced twice. Each replacement means a site visit, a lifting exercise and a temporary outage. Lithium, at 3,000-6,000 cycles, is generally replaced once or not at all within the same period.
Where seven days is the wrong answer
Autonomy is not free, and it is not always necessary:
- If the site can be visited weekly, three to four days is usually sufficient and much cheaper
- If the array is oversized, the battery rarely reaches deep discharge and a smaller bank survives — but this only works where cloud is intermittent, not seasonal
- If the load can shed, a smart controller that drops the camera or reduces reporting frequency can turn seven days of full-load autonomy into five days of reduced-load autonomy for a fraction of the cost
The honest trade-off
| Approach | Pros | Cons |
|---|---|---|
| Big gel bank, 7 days | Lowest unit price per kWh, proven technology | Heavy, bulky, needs ventilation, two replacements in ten years |
| LiFePO4, 7 days | Light, compact, long life, flat voltage | Higher upfront cost, needs low-temperature charge protection in cold sites |
| 4-day bank + load shedding | Cheapest overall, smaller enclosure | Reduced data during extended cloud — acceptable only if the data user agrees |
FAQ
How do I calculate battery size for 7 days of autonomy?
Multiply daily consumption by seven, then divide by the usable depth of discharge — 0.5 for gel, 0.8 for LiFePO4. For 840 Wh/day that is about 11.8 kWh nominal of gel or 7.4 kWh of lithium.
Is 7 days of autonomy always necessary?
No. It depends on access difficulty and on how much data loss the application tolerates. Many monitoring sites run well on 3-4 days.
Does temperature affect autonomy?
Yes, significantly for lead-acid — a cold battery delivers less of its rated capacity, and a hot one ages faster. Both effects should be reflected in the sizing margin.
We size battery banks against real load, access interval and climate. Send us your load profile or see LiFePO4 vs gel.