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What size UPS or inverter do I need?

Updated October 24, 2026

Power outages happen in many parts of Mexico, especially during storm season. A UPS that's too small shuts off right when you need it, and one that's too big costs more than necessary. Sizing it right is simple math. (In Mexico a UPS is usually called a no-break.)

Step 1: add up the watts of what you'll plug in

Find each device's power in watts (W) on its label or power adapter. If it only shows volts and amps, multiply them: a 12 V, 1.5 A adapter is about 18 W. Only add what you'll actually plug into the UPS.

Step 2: keep a margin

Schneider Electric (APC) recommends keeping the load around 80 % of the unit's capacity. So the minimum size is your watts ÷ 0.8. And look at the UPS's watts, not just its VA: a 900 VA unit doesn't deliver 900 W.

What you plug inWattsUPS of at least
Desktop computer (250 W) and TV (65 W)315 W394 W
Laptop (60 W) and two LED bulbs (10 W each)80 W100 W

Wattages from CFE and FIDE. Always check your devices' labels.

Step 3: minutes or hours?

A regular UPS is meant for a few minutes: just enough to save your work and shut down. If you want hours of backup, what you need is an inverter with deep-cycle batteries. The battery energy is calculated like this:

Wh = watts × hours ÷ inverter efficiency (0.9) ÷ usable battery discharge

Usable discharge is how much you should draw from a battery without shortening its life: about 50 % for lead-acid (AGM) and 80 % for lithium (LiFePO4), according to Victron's spec sheets.

Example: the laptop and two bulbs (80 W) for 4 hours need about 711 Wh of lead-acid battery (about 59 Ah at 12 V: one 100 Ah battery) or about 444 Wh of lithium.

Careful with motors

Refrigerators, water pumps and air conditioners draw several times their normal power when starting (sources range from 2 to 10 times). An office UPS isn't built for that. To back up a motor, check its starting current on the label and choose an inverter that can handle it.

List your devices and size the UPS and batteries.

Size my UPS

Frequently asked questions

How many watts does my UPS need?

Add up the watts of what you'll plug in and divide by 0.8. For example, 315 W of devices needs a UPS of at least about 394 W.

What's the difference between VA and watts?

VA (volt-amperes) is apparent power and watts is the real power the unit delivers. A UPS always delivers fewer watts than its VA rating, so look at the watts.

How long does a UPS last?

A regular UPS gives minutes of backup, enough to save and shut down. For hours you need an inverter with deep-cycle batteries sized for that load.

Can I plug my refrigerator into a UPS?

Not into an office UPS. Motors draw a lot of current when starting. A refrigerator needs an inverter that handles its startup surge; check the appliance's label.

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