How Much Battery Runtime Do I need?
The amount of battery runtime you need from your UPS depends on whether you want to protect from just momentary power outages or longer duration blackouts. UPS manufacturers will publish runtimes on their datasheets, and these are a guide only. The amount of battery runtime will also depend on the size of UPS system chosen, the connected load (VA/Watts), the size of the battery pack installed, how charged the batteries are and even the local temperature and age/condition of the battery set.
Offline UPS have a small battery (1 or 2 blocks) built-in. They are designed to provide power backup for short duration runtimes at full load of around 3-5 minutes. If you need longer, choose a larger model than the calculated load. For example, a 1kVA offline or standby UPS with a 3minute battery will provide 6-10 minutes at 50% load (500VA).
Line Interactive UPS again have built-in battery sets and may have the facility for connection to external battery extension packs. The oversizing principle for battery runtime sizing applies. Typical runtimes with battery extension packs can run from several minutes up to 2-3hours at lower rated loads.
Online UPS have inverters rated for continuous running and plug-in battery extension packs are generally available for most models up to 10kVA. Their higher DC voltage specifications (from 36-60Vdc for single phase UPS and larger for 3phase UPS) means that more battery blocks are required for any given Ampere-hour (Ah) rating. A battery block is either 12Vdc or 6Vdc rated. A 36Vdc rail in a UPS will require 3×12Vdc blocks etc. The batteries may be housed inside the UPS cabinet (up to 20kVA) or in separate battery cabinets with installation by certified engineers. Typical runtimes are 5-10 minutes as standard with runtimes available up to 4-8 hours or longer. For larger kVA rated UPS or long runtimes of 10-12 hours or more, a local standby power generator is recommended.
How long you need to run for depends on your load an application. Most UK power outages only last a few seconds to minutes. You should select a battery runtime that either allows you to ride through power outages or close down your systems in an orderly manner (manually shutdown or using software) should the blackout be of a longer duration.
Blue Light services, hospitals, industrial manufacturing, server room and data centre facilities may require uninterrupted service provision, and this leads to installing a UPS system with sufficient battery or generator power for several hours.
Note that additional battery packs increase the overall space required for the UPS system and weight (kg) loadings. This will mean allocating more floor space, or space within a server cabinet. For large UPS systems a separate UPS room may be required, typical in mid to mega-sized datacentres.
The larger the UPS battery, the higher the total cost of ownership (TCO) as the batteries will require replacement during the life of the UPS system. For lead acid batteries the recommended working life if 3-4 years (5-year design life) or 7-8 years (10-year design life). For lithium batteries, the working life is longer, calculated at 10 years. Lead acid battery performance and working life is also dependent on room temperature. The recommended ambient is 20-25°C with the design life halving for every 1 degree rise above 30°C.