Buck-boost transformers offer an economical solution to the adjustment of line voltages that are slightly above or below normal (usually within a 20% difference). When a buck-boost transformer is connected as an autotransformer, only a portion of the load kVA is actually transformed. The majority of the load kVA is passed directly through to the source. For this reason a buck-boost transformer may be used to supply a much larger kVA load than is indicated on the nameplate.
Buck-boost transformers can be used to adjust stable voltages only.