Firstly, get the basic facts lined up.

What input variations are you having to cope with?
You may well know the answer to this from past experience. If not, it must be ascertained by monitoring the supply over a period of time. Where there is a good mains supply, a stabiliser covering input variations of -15% to +15% will usually be adequate, but variations of -25% to +25% or more can be encountered in a location with poor mains supply.

Choosing a stabiliser with the right input variation capability is important because if its input voltage range is exceeded, then its output will increase or decrease by the same amount by which it has gone "out of limit".

How about supply frequency?
Most supplies are 50 or 60 Hz but we can meet other frequencies to order. Your normal mains supply frequency is unlikely to vary more than ±2%, and this is well within the capability of our stabilisers. But if the supply is from a local generator it is possible that the frequency could deviate beyond this range and suitable protection should be considered to protect both the load and the stabiliser.

The rating of your equipment
You need to check the rating of the equipment to be supplied, whether quoted in amps or kVA, and whether single or three phase. We also need the nominal voltage line to neutral and line-to-line if three phase, plus the frequency and power factor.
This information will usually be shown on the rating plate attached to the equipment, otherwise you will need to consult the manufacturers of the equipment or take measurements. It is important to measure its true RMS value.