Resource
How to turn a load and a target backup time into the right battery bank — the formula, the derating that trips people up, and a full worked example.
Battery sizing answers one question: for a given load, how much stored energy do I need to keep running for a set time? Everything else — number of blocks, strings, cabinet size — follows from that. Our engineers do this for you on every quote, but here is the method so you can sanity-check the numbers.
Take the UPS load in kW (or kVA × power factor). Size batteries to the load you must ride through, not the UPS nameplate, unless you want full-rated runtime.
Decide the backup you need — a few minutes for a graceful shutdown, or longer to bridge until a generator takes over. Runtime is the biggest driver of battery cost.
Energy needed (Wh) = Load (W) × Runtime (h), then divide by the combined inverter and battery-discharge efficiency and the usable depth of discharge.
Ah = Wh ÷ DC bus voltage. The UPS DC bus (e.g. 240 V, 384 V, 480 V) fixes how many series blocks each string needs.
Blocks per string = DC voltage ÷ block voltage. Add parallel strings to increase Ah (and therefore runtime) without changing the string voltage.
Reduce usable capacity for low temperature, end-of-life (80%) and ageing. Add headroom so the bank still meets runtime years later.
The usable energy a battery bank must deliver is larger than the raw load × time, because some energy is lost in conversion and you never fully discharge the cells:
Energy (Wh) = Load (W) × Runtime (h) ÷ ( Inverter efficiency × Battery discharge efficiency × Depth of discharge )
Then convert energy to amp-hours at your UPS DC bus voltage, and work out the string layout:
Ah = Wh ÷ DC bus voltage · Blocks per string = DC bus voltage ÷ block voltage
Typical planning values: inverter efficiency ~0.94, battery discharge efficiency ~0.95, and a usable depth of discharge around 0.8–0.9 for VRLA at UPS discharge rates. Always confirm against the specific battery's discharge tables.
Suppose a 20 kVA UPS at unity power factor supplies a 20 kW (20,000 W) load, and you want 15 minutes (0.25 h) of backup on a 384 V DC bus using 12 V VRLA blocks.
This is a planning estimate. Final battery selection uses the manufacturer's constant-power discharge tables at the exact backup time, temperature and end-of-life condition. Share your load and runtime and we'll size it precisely.
Chemistry choice
Lower upfront cost and widely used. Expect a 3–5 year service life, a larger footprint, more weight and sensitivity to heat. A sound choice for shorter runtimes and tighter budgets.
Longer life (often 10+ years), much faster recharge, a smaller and lighter footprint, and better tolerance of higher temperatures. Higher initial cost, lower total cost of ownership. Vertiv HPL cabinets provide scalable lithium storage.
FAQ
Start from the load in watts and the energy the batteries can usefully deliver. Runtime rises with more battery capacity (Ah) or a higher DC bus voltage, and falls as the load, ageing and temperature derating increase. The worked example on this page shows the full calculation.
kVA/kW describe the UPS load. Watt-hours (Wh) describe the energy a battery bank stores; amp-hours (Ah) describe that same energy at a given DC voltage (Wh = Ah × V). You size the bank in Wh from the load and runtime, then convert to Ah at your UPS's DC bus voltage.
Divide the UPS DC bus voltage by the block voltage. For example, a 384 V DC bus with 12 V VRLA blocks needs 32 blocks per string; a longer runtime is achieved by adding parallel strings, not more blocks per string.
VRLA has a lower upfront cost but a shorter life (typically 3–5 years) and a larger footprint. Lithium-ion lasts far longer (often 10+ years), recharges faster, tolerates higher temperatures and takes less space, at a higher initial cost but usually a lower total cost of ownership.
Rated capacity assumes ideal conditions. Real installations lose usable capacity to temperature (below 25 °C), ageing (end-of-life is defined at 80% capacity), and inverter/battery discharge inefficiency. Sizing with a design margin ensures the required runtime is met throughout the battery's life.
Talk to an engineer
Give us your load, runtime, DC bus and battery type — we'll return an exact block/string layout, runtime table and price.