The short answer: runtime in hours = usable watt-hours ÷ load in watts. Usable watt-hours is the rated capacity multiplied by depth of discharge (about 0.8) and inverter efficiency (about 0.85). Two numbers and one division — everything else is derating.
Why published runtime figures disagree with your site
Every supplier quotes runtime at 25 °C on a new pack, at a load that flatters the product. That is not dishonesty so much as a convention nobody has agreed to stop using. The figures diverge when three things change:
- Temperature. Above 40 °C, lithium capacity falls and the inverter gets less efficient. A sealed outdoor cabinet in summer is 15 °C hotter than the air outside it.
- Depth of discharge. A pack cycled to 100 % every night loses roughly half its cycle life compared with one cycled to 80 %. Most reputable BMS units stop at 80–90 % for that reason — which is 10–20 % of the runtime you thought you bought.
- Power factor and surge. A motor that draws 300 W running may pull 900 W for two seconds on start. The energy is small, the current is not, and a marginal inverter trips.
The calculation, step by step
Take a 12 V / 1 A router — 12 W — on a unit with 100 Wh of rated capacity:
| Step | Calculation | Result |
|---|---|---|
| Rated energy | given | 100 Wh |
| Usable at 80 % DoD | 100 × 0.8 | 80 Wh |
| After inverter losses | 80 × 0.85 | 68 Wh |
| Runtime at 12 W | 68 ÷ 12 | 5.7 h |
| Derate for 45 °C cabinet | 5.7 × 0.8 | 4.5 h |
That last line is the number to quote to a customer. The first line is the number on the box.
Runtime table: what a given load actually gets
The table below uses 100 Wh rated, 80 % DoD, 85 % inverter efficiency at 25 °C — the same basis as the worked example. Read across, then apply your own temperature factor.
| Load | Typical device | Runtime at 25 °C | At 45 °C |
|---|---|---|---|
| 5 W | ONU alone | 13.6 h | 10.9 h |
| 12 W | Router + ONU | 5.7 h | 4.5 h |
| 25 W | 4 cameras + PoE switch | 2.7 h | 2.2 h |
| 50 W | NVR + 2 cameras | 1.4 h | 1.1 h |
| 120 W | Small switch stack | 34 min | 27 min |
| 500 W | Workstation + lighting | 8 min | 6 min |
| 2000 W | Heater, full load | 2 min | 1.6 min |
Three mistakes that cost a site its runtime
1. Sizing on the device label
A router labelled 9 V / 0.6 A is 5.4 W at the device, but the adapter behind it may draw 12 V / 1 A from the wall. Size on the adapter, not the label — the difference is often 40 %.
2. Ignoring the switch
Four cameras at 6 W is 24 W. The PoE switch feeding them adds 3–5 W of overhead plus its own conversion losses, so the real figure is nearer 32 W. Backing up the cameras but not the switch protects nothing.
3. Assuming the pack stays new
LiFePO4 loses roughly 2–3 % of capacity a year at 25 °C, faster at 45 °C. A four-year-old pack in a hot cabinet delivers about three quarters of its first-year runtime. If the site must hold for eight hours in year five, specify for year five.
What to send a supplier instead of a product name
- The adapter's output voltage and current, or the device model
- How many hours must be covered, and whether that is an outage or a scheduled disconnect
- Where the unit sits: indoor, sealed cabinet, or in sun
- Whether the load has a motor, compressor or heater in it
Those four answers are enough to size almost any backup requirement. The calculator applies the same method automatically, and a written answer takes about a working day.