How to use the calculator
- Set the quantity of each device you want to keep powered during an outage, and check its watts against the label or power adapter. The watts shown are editable starting values for typical devices, not measurements of your equipment.
- Add any other load in watts.
- Enter the battery or UPS capacity in Wh (or Ah and volts), plus usable capacity and inverter efficiency.
- Read the runtime. It updates immediately whenever you change a value.
Formula
Load (W) = Σ quantity × watts + other load
Energy (Wh) = capacity (Wh) or capacity (Ah) × voltage (V)
Usable energy (Wh) = Energy × usable % ÷ 100 × inverter efficiency % ÷ 100
Runtime (h) = Usable energy ÷ Load
Battery needed (Wh) = Load × wanted hours ÷ (usable % × efficiency %)
Minimum UPS output (W) = Load × 1.25
These are the relations E = P × t and P = V × I. The 25% headroom is a planning assumption (loading a UPS to about 80%), not a standard; use the maker’s rating.
Units
Power in watts (W), energy in watt-hours (Wh) or amp-hours (Ah) at a stated voltage (V), time in hours (h), and efficiency and usable capacity in percent (%). Many UPS units also state apparent power in VA; the load a UPS can carry is limited by its rated watts, which is lower than the VA number.
Affiliate product searches by load range
Source
The calculation uses the standard relations E = P × t and P = V × I and only the numbers you enter. No sourced device measurements are used: the device watts are editable examples that you should replace with the label value. The tool does not measure any real UPS, battery or device.
Limits and safety
- Devices draw different power over time (a router is steady, a fridge compressor or fan motor cycles and surges). Use the label’s maximum, or measure with a plug-in power meter, for a safer estimate.
- UPS electronics draw some power themselves, and battery capacity falls with age and in the cold. Real runtime is usually shorter than the estimate.
- A UPS or battery can only supply up to its rated output in watts; the runtime result does not check that.
- Do not connect a battery system to home wiring yourself. That needs a qualified electrician and approved equipment.
Frequently asked questions
How do I calculate how long a UPS will run my router and cameras?
Add up the watts of every device, then divide the UPS battery energy in watt-hours by that total, after allowing for usable capacity and inverter efficiency. For example, 40 W of load on a 500 Wh battery at 80% usable and 85% efficiency gives 500 × 0.8 × 0.85 ÷ 40 ≈ 8.5 hours.
How do I convert Ah to Wh for a backup battery?
Multiply amp-hours by the nominal battery voltage. A 12 V 100 Ah battery holds about 1,200 Wh before usable capacity and inverter losses.
What is the difference between VA and watts on a UPS?
VA is apparent power and watts is real power. A UPS can only carry loads up to its rated watts, which is lower than its VA rating, so compare your total load with the watt rating.
Why is my real UPS runtime shorter than the estimate?
Batteries lose capacity as they age and in the cold, UPS electronics use power themselves, and devices can draw more than the typical watts entered. Use label values and keep headroom.
Which load range does each affiliate link cover?
The links open Amazon searches for a UPS or power station class that matches your total load: up to 100 W, 100–400 W, 400–1,200 W and over 1,200 W. They are searches, not tested recommendations, so check the specifications on the merchant page.
Related guides
- Watts to Lumens: Practical Energy-Saving LED Lighting Guide
- LED Fixture Lumens Guide: Energy-Efficient Home Lighting