UPS or inverter: which one you need
A UPS (no-break) has its battery inside and is meant for the minutes it takes the power to come back, or to let you save your work and shut down: computers, modem, game consoles. An inverter with external batteries backs things up for hours: refrigerator, lights, water pump or TV during a long outage — common during storms in many parts of Mexico.
How the size is calculated
First add up the watts of everything that will be on at the same time. The unit must be bigger than that: Schneider Electric, maker of APC UPS units, recommends keeping the load around 80 % of its capacity, because at the limit it heats up, loses efficiency and the battery lasts less.
For the batteries we calculate the energy: watts × hours, divided by the inverter's efficiency (90 %) and by how deeply the battery can be discharged without shortening its life: half for lead-acid and 80 % for lithium. That energy divided by the bank voltage gives the amp-hours (Ah) you need.
Mexico's household voltage is 127 V (close to the 120 V used in the US), so US-market UPS units work here, but check the plug and the voltage range on the spec sheet.
Frequently asked questions
What's the difference between a UPS (no-break) and an inverter?
A UPS — called “no-break” in Mexico — has its battery inside and is meant to give you a few minutes when the power goes out: save your work and shut down calmly. An inverter with external batteries converts the energy of larger batteries and can back up appliances for hours.
Why should the UPS be bigger than what I plug in?
Schneider Electric, the maker of APC UPS units, recommends keeping the load around 80 % of the unit's capacity: running at the limit makes it hotter, less efficient and shortens the battery's life. That's why the calculator divides your load by 0.8.
Do I look at the VA or the watts of the UPS?
The watts. UPS units are advertised in VA, but what matters to know if it can handle your appliances is the power in watts, which is usually lower than the VA. It's on the box or the spec sheet.
Lithium or lead-acid batteries?
Lead-acid (AGM) batteries are cheaper, but to last they should only be discharged halfway; lithium (LiFePO4) batteries cost more and handle 80 % discharges with many more cycles. That's why for the same hours you need more lead-acid batteries than lithium ones.
Can I plug a refrigerator into an inverter?
Yes, but motors start with a surge several times their normal draw. The inverter must handle that surge (peak or surge power on its spec sheet) and it's best if it's pure sine wave. Check the appliance's label or ask the manufacturer for its starting current.
Sources: Schneider Electric (APC), 80 % load recommendation; Victron Energy spec sheets (Phoenix inverter efficiency, AGM and LiFePO4 battery cycles). Appliance watts: CFE, FIDE and Midea México (current refrigerator).
Want to know how much your appliances cost on your bill? Use the appliance energy calculator.
Learn more: What size UPS or inverter do I need?