First check: does the unit need one?
Some commercial units are designed to run across a wide voltage range. Check the operating voltage range on the spec sheet and compare it with what your site actually sees. A simple voltage logger left for a week, covering evenings and weekends, tells you far more than a single reading.
Step 1: running VA
Running VA = 230 V × nameplate running current. If only watts are given, VA = W ÷ power factor. A 500 W reach-in at PF 0.85 is about 588 VA; a 400 W visi cooler at PF 0.85 is about 471 VA.
Step 2: allow for the starting surge
The stabiliser must pass the compressor's starting current without its own protection tripping and without its output collapsing. Starting VA = 230 × locked-rotor current. Stabiliser makers usually state which appliance sizes a model is suited to, and that guidance is the best basis.
Where no such guidance exists, a common rule of thumb in the trade is to pick a stabiliser rated at least two to three times the running VA of a single compressor appliance. It is a rule of thumb, not a standard, and it does not replace the maker's appliance rating.
Worked example: single units and a mainline
Assumptions: four units with running VA of 313 (undercounter), 588 (reach-in), 375 (chest freezer) and 471 (visi cooler); the reach-in's starting current is 15.3 A (assumed); controllers stagger restarts after a power cut.
| Case | Working | Result |
|---|---|---|
| Reach-in, individual stabiliser | 588 VA × 2.5 (rule of thumb) | about 1.5 kVA |
| Visi cooler, individual stabiliser | 471 VA × 2.5 (rule of thumb) | about 1.2 kVA |
| All four running | 313 + 588 + 375 + 471 | 1,747 VA |
| Worst single start while others run | 1,747 − 588 + (230 × 15.3 = 3,519) | about 4.7 kVA |
| All four starting together | 230 × 45.6 A | about 10.5 kVA |
What the example shows
A mainline stabiliser for the shop needs to handle about 4.7 kVA of momentary demand if restarts are staggered, plus any other loads on that line, such as lights and fans. Without staggering, the simultaneous restart demand more than doubles. Staggered restart delays are much cheaper than a bigger stabiliser.
What to check on the spec sheet
- Input voltage range: it must cover your lowest and highest logged voltage, with margin.
- Capacity at low input: at low input voltage, input current rises (I = output VA ÷ (input V × efficiency)), and some stabilisers derate. At 160 V input, a 500 VA load at 95% efficiency draws about 3.3 A instead of about 2.3 A at 230 V.
- Output accuracy and correction speed: relay-type models switch in steps quickly; servo types correct smoothly but more slowly. Either can suit refrigeration if range and rating fit.
- Time delay on power return: a delay of a few minutes lets compressor pressures equalise before restart and protects against rapid on-off cycling.
- High and low voltage cut-off: the stabiliser should disconnect the load rather than pass dangerous voltage through.
Wiring and placement
Place the stabiliser where it stays dry and ventilated, not on the floor of a wet kitchen. The circuit feeding it must be sized for the higher input current at low voltage. Earth the stabiliser and the appliance properly.
Frequently asked questions
- Is a stabiliser rated for an AC suitable for a fridge?
- It can be if its VA rating, input range and time delay suit the fridge's starting surge. Compare the fridge's running and starting current with the stabiliser maker's appliance guidance.
- Should I use one big stabiliser or one per appliance?
- One per appliance limits a failure to one unit and can be matched to each load. A mainline stabiliser protects everything and can cost less overall, but it must carry the combined running load plus starting surges, and if it fails everything is exposed.
- Why is a stabiliser rated in kVA rather than kW?
- Because the current it must carry depends on volts × amps, whatever the power factor. A compressor drawing 500 W at a power factor of 0.85 still pushes about 588 VA of current through the stabiliser's windings and switches, so kVA is the fair rating to compare against.
- Do stabilisers protect against power cuts?
- No. A stabiliser corrects voltage while there is supply. For outages you need an inverter, UPS or generator. Its time delay does help when power returns, by holding the compressor off for a few minutes.
