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UPS & Battery Backup Runtime Calculator

Add what needs to stay on in a power cut, from the broadband router to the NAS and the CCTV. We'll work out how long each UPS and power station would last now and as its battery ages, warn you about switchover times and sine waves that can reset computers, and show the cost per hour of backup.

What needs to stay on

Watts are typical figures while working, not the maximum on the power adapter. PoE devices (cameras, access points, phones) are counted with 10% extra for the switch's power conversion. Measure with a plug-in power meter for the best answer.

Load 0 WAbout 0 VAStart-up peak none

How long, and where it lives

Your own UPS or battery (optional)

Recommendation

Runtime

Compare them all for your load

Select a column heading to sort.

£ per hour is the price divided by the hours of runtime it gives your load when new. £ per year is what the battery costs you each year it lasts: for a UPS, the replacement battery divided by its life here; for a power station, its price divided by its battery life (at most 10 years). The 5-year cost adds the replacement batteries you'd need in 5 years (or a whole new unit where we don't have a battery price).

How long do UPS batteries really last?

BatteryWhat makers quoteWhat to expect
Sealed lead-acid (VRLA, AGM) in a UPS3 to 5 yearsOften 2 to 3 years, and as little as 1 to 2 in a warm cupboard or after several long power cuts. Runtime fades well before the UPS reports a failed battery, so test it.
Lithium iron phosphate (LiFePO4)3,000 to 6,000 full cycles to 70 or 80%In backup use it's rarely cycled, so age and heat set its life: typically 10 years or more before it falls to 80%.
Lithium-ion (NMC)500 to 1,000 cycles to 80%Ages faster when kept full and warm, typically 5 to 8 years in backup use.
  • Heat is the big one. APC says most of its batteries last three to five years, and that every 8 °C above 25 °C halves that: a battery good for 4 years at 25 °C lasts about 2 at 33 °C. A UPS in a cupboard full of routers and switches is often that warm, and the battery inside the UPS runs a few degrees hotter than the air around it.
  • Long power cuts use them up. A sealed lead-acid battery manages only a few hundred full discharges, and one left flat for days after a long cut may never recover. Let it recharge fully as soon as the power's back.
  • Runtime fades first. A battery counts as worn out at 80% of its original capacity, but the UPS usually only warns you when it fails a self-test. In our experience supporting small offices, many UPS batteries need replacing after 1 to 3 years, and a UPS that has never been tested often lasts seconds, not minutes, when the power finally goes.
  • Lithium iron phosphate lasts far longer. It's rated for thousands of cycles and copes better with being kept full, but heat still shortens its life. Keep power stations somewhere cool and, if the app allows, cap the charge at 80 to 90% when you don't need every minute.
  • Test it. Every few months, unplug the UPS from the wall and time how long it lasts with your real load, and run its own self-test. Replace the battery when the runtime falls below what you need.

UPS or power station?

  • A UPS is built to keep computers running through blips and give them time to shut down safely, and it tells them when to (over USB or the network). Most give 5 to 30 minutes at a typical office load. Lead-acid batteries are cheap to replace but wear out in a few years.
  • A power station holds far more energy for the money and lasts hours, which suits routers, phones and cameras through a long cut. Most switch over more slowly (10 to 20 ms) and few can tell a NAS to shut down.
  • For a NAS or server, use a UPS (or a power station with a USB shutdown signal and a 10 ms switchover) and set the NAS to shut down after a few minutes on battery.
  • For broadband and phones in a long cut, a power station is usually the better buy. The street cabinet or exchange may lose power too, and mobile masts often have only a short battery backup.

Switchover time and computers

  • When the mains fails, a standby or line-interactive UPS switches to battery. APC quotes 6 to 12 ms for its Back-UPS models and 2 to 10 ms for Smart-UPS (line-interactive) models. An online (double conversion) UPS runs from its inverter all the time, so there's no switchover at all.
  • A computer's power supply has to ride through that gap. Intel's ATX12V design guide asks for at least 16 to 17 ms at full load; the newer ATX 3.1 relaxed it to 12 ms at full load. Cheap supplies may manage less, and NAS boxes and routers on small power adapters vary.
  • Power stations quote 10 to 20 ms. At 10 ms most computers are fine. At 20 ms many will be, especially when lightly loaded, but some will restart. Jackery says its units aren't for servers or equipment that can't tolerate any interruption, and Anker warns that pass-through may interrupt a computer or NAS.
  • Whatever you choose, test it with your real equipment: unplug the UPS or power station from the wall while everything is running.

Pure sine wave matters

  • Cheaper UPSs make a stepped (simulated) sine wave on battery. CyberPower explains that the brief zero-voltage gap in each cycle can make active-PFC power supplies, now standard in desktop PCs, servers and many NAS boxes, think the power has failed and shut down.
  • Choose a pure sine wave UPS for computers. Every power station in our list makes a pure sine wave.
  • Rate a UPS by watts, not just VA: a 1,500 VA UPS may supply only 865 to 1,000 W.
  • Keep the load under about 80% of the rating so it copes with start-up surges and things you add later.

Shutting down safely

  • Connect the UPS's USB cable to the NAS or server. Synology, QNAP, TrueNAS and Unraid all support standard USB UPSs, and can pass the signal on to other machines using NUT (Network UPS Tools).
  • APC PowerChute, Eaton's Intelligent Power Manager and CyberPower PowerPanel shut down Windows PCs and servers. Larger UPSs take a network card so many machines can be told at once.
  • Set the shutdown to start after a few minutes on battery, not when the battery is nearly flat: an aged battery can drop out suddenly.
  • Most power stations have no shutdown signal. The EcoFlow RIVER 3 Plus and DELTA 3 Plus are exceptions in our list.

Safety and good practice

  • Never rely on a power station or office UPS for life-support or medical equipment. EcoFlow's manuals say its power stations aren't for life-sustaining medical equipment. Use equipment designed and certified for that job.
  • Don't daisy-chain: never plug a UPS into another UPS or a power station, or a power station into a UPS.
  • Keep laser printers, heaters and kettles off the battery sockets. Laser printers draw huge spikes while warming up.
  • Power stations often switch their AC sockets off when the load is small (Jackery after 12 hours below 25 W; EcoFlow has an AC timeout). Turn these timers off when using one as a UPS. It can only act as a UPS while its mains input can supply more than the load.
  • Give UPSs and power stations space to breathe, away from heaters and sunny windows. Replace batteries with the maker's own cartridge, and take old ones to a battery recycling point: never put them in the bin.

How we work it out

  • Lithium: runtime = capacity × 95% (what the battery management lets you use) ÷ (load ÷ 92% inverter efficiency + the inverter's own draw). The inverter's draw is the maker's figure where published, otherwise 5 to 30 W depending on its size. This matched independent measurements of EcoFlow, Jackery and Anker units within a few percent, erring slightly low.
  • Lead-acid UPS: we use the maker's runtime figures, interpolated for your load. They already include the UPS's losses and Peukert's effect (a battery gives less at high loads). For your own UPS we use Peukert's law with an exponent of 1.2.
  • Ageing: lead-acid reaches 80% capacity after 3.5 years at a battery temperature of 25 °C (the lower half of the 3 to 5 years makers quote), halving for every 8 °C hotter. The battery inside a UPS runs about 5 °C warmer than the room because of the charger and electronics next to it, so in a 25 °C office that's roughly 2 years, and about 1.5 in a warm comms cupboard. Each full discharge uses a further 1/250th of its life. It fades slowly at first, then quickly. Lithium loses about 2% a year (3% for NMC), doubling for every 10 °C above 25 °C, plus wear from each discharge against its rated cycles.
  • These are estimates. Batteries vary a lot, so test yours.

Sources

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