RuntimeCalculator

UPS Runtime Calculator

Average the gear that stays on, then see how long the battery can carry it.

This is an energy estimate, not a wiring plan. Do not backfeed utility power. For medical gear, a furnace, or any load that would be dangerous if it stopped, ask an electrician.

Average from the load list. Edit the rows in the load table.

Advanced

A common conservative depth of discharge for deep-cycle AGM.

Real usable capacity still depends on the maker's BMS, age, and the temperature of the cells.

Template watts are examples. CPAP rows are power estimates only, not a medical clearance. Trolling-motor rows are estimates too: follow local water rules.

8 loads maximum. Duty is 0 to 1.

Editorial estimate

1,200 Wh nameplate, 100 Ah at 12 V

Runtime

7.5

7 h 30 min

1,200 Wh nameplate × 0.50 depth of discharge × 1.00 temperature × 0.90 efficiency = 540 Wh usable.

Usable energy
540 Wh
Average load
72 W
Battery current
6.67 A
C-rate
0.07
Conservative planning figure
6.38 h (× 0.85, editorial)
Depth of discharge
50%
Efficiency
90%
Temperature factor
1.00 at 77°F (25°C). Editorial estimate, not a lab curve.

How it works

Wh rated = Ah × V
(Skip that step if you type watt-hours.)

usable Wh = Wh rated × depth of discharge × temperature factor × inverter efficiency
hours = usable Wh / average watts

average watts = sum of (watts × duty) for each enabled row
duty is the share of time the device is actually drawing power (0 to 1)

DC amps, no inverter:
usable Ah = Ah × depth of discharge × temperature factor
hours = usable Ah / amps

Size a battery:
usable Wh needed = watts × target hours
rated Wh = usable Wh / (depth of discharge × temperature factor × efficiency)
rated Ah = rated Wh / volts

Conservative planning hours = hours × 0.85
That 0.85 factor is an editorial margin, not a second physical model.

The preset is the office template: router 15 W always on, modem 12 W always on, laptop 60 W at a 0.50 duty cycle, and monitor 30 W at a 0.50 duty cycle. The average is 72 W. The battery preset is 100 Ah at 12 V, AGM, 50% depth of discharge, 90% inverter, 77°F. That is a house-battery sized example, larger than the little battery inside a typical desktop UPS. Type the amp-hours printed on your unit.

Duty cycle matters here because a laptop and a monitor are not a flat load. If you only add the nameplate watts, the runtime looks shorter than a normal evening. If your laptop stays at full draw, set its duty to 1. If the monitor sleeps, lower it. Delete a row you do not have. The cap is 8 rows.

Small UPS batteries are often sealed lead-acid, which is why the chemistry starts at AGM and 50% depth of discharge. A lithium UPS should be switched to LiFePO4 or Li-ion so the depth of discharge default matches that chemistry. You can still move the slider. An old UPS battery often has less capacity than the label. The calculator does not know the age. The conservative line, 85% of the estimate, is only an editorial margin.

Choose DC only if the router and modem run straight from 12 V with no inverter. That sets efficiency to 100%. This page does not model transfer time, startup surge, or a generator. It will not size a panel that is tied to the utility. Do not backfeed the mains. If the load would be dangerous when it stops, ask an electrician and follow the equipment maker.

Worked examples

Office template on a 100 Ah AGM battery

12 V, 50% depth of discharge, 90% inverter, 77°F, and the four example rows. Edit the rows if your gear is different.

  1. Average watts = 15 × 1 + 12 × 1 + 60 × 0.50 + 30 × 0.50 = 72 W.
  2. Rated watt-hours = 100 × 12 = 1,200 Wh.
  3. Usable watt-hours = 1,200 × 0.50 × 1.00 × 0.90 = 540 Wh.
  4. Hours = 540 / 72 = 7.5 h.

Router and modem only, DC

Same 100 Ah AGM battery, laptop and monitor removed, DC only so there is no inverter loss. Still an example, not a measurement of your router.

  1. Average watts = 15 + 12 = 27 W.
  2. Usable watt-hours = 1,200 × 0.50 × 1.00 × 1.00 = 600 Wh.
  3. Hours = 600 / 27 = 22.22 h.

Questions

How do you estimate UPS runtime?

Add each device's watts times the fraction of time it draws power. That average is the load. Usable watt-hours come from amp-hours, voltage, depth of discharge, temperature, and inverter efficiency. Runtime is usable watt-hours divided by average watts.

What load does the office template assume?

Example rows, not a measurement of your gear: router 15 W always on, modem 12 W always on, laptop 60 W at a 0.50 duty cycle, monitor 30 W at a 0.50 duty cycle. The average is 72 W. Edit every row.

Will this size a whole-house backup?

No. The math is one battery bank and a steady average load. It does not model a generator, a transfer switch, or surge. Do not backfeed utility power. Hire an electrician for any system that ties to house wiring or that would be dangerous if it stopped.

Why is the preset an AGM battery at 50% discharge?

Many small UPS units use sealed lead-acid. The AGM preset uses 50% depth of discharge. If your unit is lithium, switch the chemistry. Type the amp-hours printed on the battery. The 100 Ah preset is a house-battery sized example, larger than the battery in a typical desktop UPS.

Can I use this for a router only?

Yes. Turn off the laptop and monitor rows, or remove them. A 15 W router plus a 12 W modem is 27 W average in the example. If the router runs on DC with no inverter, choose DC only so efficiency is 100%.

Does the calculator include battery aging?

No. An older UPS battery often has less capacity than the label. The conservative line, 85% of the estimate, is an editorial planning margin. It is not a measurement of age.

These figures are editorial estimates. The formula, chemistry defaults, and temperature table are written out on themethodology page.

Updated October 5, 2026