Hi everyone,
when dimensioning a 24 V DC UPS, the question we hear most often is: “How many Ah do I need for X minutes of backup?” Here is a simple way to estimate it.
Required energy (Wh) = load power (W) x required runtime (h)
Battery capacity (Ah) = energy (Wh) / 24 V / converter efficiency / usable depth of discharge
Example: a 200 W load with 1 hour of runtime, 90 % efficiency and 50 % usable depth of discharge:
200 Wh / 24 V / 0.9 / 0.5 ≈ 18.5 Ah
Add a reserve for aging and temperature, often 20–30 %. The usable depth of discharge depends strongly on the battery chemistry, so check the battery datasheet before you finalize the calculation.
Before sizing the battery, make sure the UPS power rating covers your load. For example, the UGS2440 is rated at 480 W at 24 V:
UGS2440 Wöhrle | Uninterruptible Power Supply (UPS) Systems | DigiKey Marketplace
How do you usually choose the design runtime: a fixed Ah value or a fixed runtime?
The method of estimation given may be useful for an informal approximation, but additional considerations are strongly recommended for actual selection purposes.
First, the discharge rate at which the batteries’ capacity is specified must be accounted for. It is not uncommon for AGM lead-acid batteries to have nameplate capacity ratings specified for a 20-hour discharge period, and for their effective capacity at a 1-hour rate to be roughly half of this figure. At the higher discharge rates implied by the available timer settings on the mentioned UGS2440 device, effective capacity would be lesser still to a degree that could exceed the margin available in the example figures used, and result in a failure to meet requirements upon initial commissioning of the system.
Accounting more closely for battery aging is also recommended, if system function at a time beyond the date of initial installation is of interest. Service lives of the same battery type are commonly quoted at a 25°C temperature, with a footnote that it is reduced by half for every 8-10°C above that figure. For high-rate UPS applications, the loss-of-capacity criteria often used to characterize service life may not be suitable, but needful instead of being considered along with the associated increase of internal resistance. Similar considerations apply also under conditions of low-temperature operation.
Hi Rick,
thank you for the detailed feedback. The calculation in the tip is meant as a rough estimate. Many users find it difficult to work out the backup time for a given load, so this approach is intended as a first orientation for them.
The points you mention are absolutely correct. Discharge rate, temperature, aging and internal resistance have to be taken into account to define the exact backup time and to select the right battery for the application.
Best regards,
Luca
