Understanding battery storage sizing
How much capacity a backup scenario actually requires
A business needing to back up a 45 kW critical load for 8 hours needs 360 kWh of raw energy delivered, but after accounting for 92% round-trip efficiency and keeping to 90% depth of discharge, the battery needs about 434.8 kWh of nameplate capacity, noticeably more than the raw energy figure alone would suggest. Run that nameplate figure through the battery demand charge payback calculator if the same battery will also be used for peak shaving, or the battery storage size calculator in our battery storage section for the same sizing math at a residential and small scale.
Why nameplate capacity has to exceed the raw energy figure
Round-trip efficiency losses mean some energy is lost converting to and from storage, so more has to be stored than will actually be delivered. Depth of discharge limits add further margin, since most lithium battery systems are designed to avoid full 0-100% cycling to preserve battery life and warranty terms. Together, these two factors typically add 15% to 25% on top of the raw energy calculation. Sandia National Laboratories hosts the DOE's peer-reviewed Energy Storage Handbook, with dedicated chapters on exactly these round-trip efficiency and depth of discharge fundamentals, and the depth of discharge impact calculator isolates that one variable if you want to see its effect on its own.
Deciding what counts as a critical load
Critical loads are typically refrigeration, life-safety systems, essential IT and communications, and whatever equipment would cause the most damage or lost revenue if it lost power. Sizing for every load in the building is usually far more expensive than necessary, so most commercial backup systems are designed around a critical subset, not the whole facility. The demand charge calculator can help identify which loads drive your peak demand in the first place, which often overlaps heavily with what you'd want backed up. The Department of Energy's end-use load profile data is a useful reference if you want to sanity-check typical commercial load patterns against your own utility bills.
How backup duration relates to battery size
Roughly yes for capacity, since kWh requirements scale directly with hours of backup needed. The discharge power rating in kW is a separate consideration and doesn't need to scale with duration, since it's set by your peak simultaneous critical load rather than how long you need to run it. That distinction matters most when the same battery is also doing double duty for demand charge peak shaving, where the power rating, not the energy capacity, is what limits the reduction achievable, a tradeoff the solar-plus-storage payback tool weighs against total project cost. The depth of discharge impact calculator covers the battery & storage angle of the same decision. If demand charges are a big part of your bill, the battery charge time from solar calculator in our Battery & Storage section can show whether a battery would shave your peak enough to pay for itself.