Transformer capacity is rated in Volt-Amps (VA) which is generally the same as wattage (Watts). Most loads are labeled with their proper operational voltage (Volts), current (Amps), frequency (Hz), and wattage (Watts or VA).

To compute a load’s VA requirements, multiply its rated voltage (Volts) by its rated current (Amps). This information is usually found on the back of the load - on its nameplate. If you cannot locate the labeled voltage and current (to calculate the VA), use the wattage (Watts), if shown. Exceptions are fluorescent lights, neon signs, gas discharge lamps, specialized electronic controls, etc which require a transformer with a VA capacity 1.5 times the wattage rating, if the volts and amps are not shown. Select a transformer with a VA rating equal to or larger than the VA found on the nameplate of the load.

Using a transformer with a smaller VA capacity than that required by the load will cause the transformer to overheat and eventually burn out.

You can always use a transformer with a larger VA rating than the load requires.
Example 1
You wish to use a 120 V 60 Hz electric drill on 230 V 50 Hz. The nameplate reads:
INPUT: 120V     50/60Hz     2.4 A

Since they will be used one at a time, choose a 1500 VA transformer (SD-14) that will be sufficient for the largest item, the vacuum cleaner. Of course, the other 2 loads, the sewing machine and printer, may be operated at other times, singly, or even at the same time since their total VA is only 252.

Example 3
It is necessary to operate several appliances at the same time...

Choose a transformer of 1000 VA capacity (SD-13, which is closest to, but not smaller than, the total VA of all appliances used). This will supply all of the above loads all at once or one at a time if you so choose.

GROUNDED VS UNGROUNDED

If the load has a third pin ground (washing machine, microwave, etc.), use a transformer with a grounding power cord and receptacle. If the load has only 2 prongs and no grounding pin (lamp, toaster, blender, charger, etc.), choose a less expensive transformer with a 2 pin format and no grounding pin. To accommodate both grounded or ungrounded plugs, choose a transformer with a third pin ground which is usable for both types. TODD SYSTEMS manufactures many transformers with different styles of mounting and electrical connectors (ungrounded, grounded, wiring leads, junction boxes for permanent wiring, etc.). Choose a transformer with the most convenient style as well as sufficient VA rating.

Transformers should be allowed to have access to free air around them. They should not be operated when placed next to a radiator, in a box, under furniture, surrounded in a bookshelf, etc. Transformers are designed to operate and feel quite warm to the touch.
For large appliances whose nameplate shows both "running amps" and "starting amps," use the running amps to figure the wattage (VA) and, therefore, the transformer capacity needed. It is generally less costly (but less convenient) to use one larger transformer for several loads than to have one transformer for each. However, transformers intended for use with a permanently placed, frequently run load, such as a refrigerator, washing machine, etc., should be left attached and used for that load only.