Decide what actually needs backup
A full, closed chest freezer holds temperature for some time without power because the frozen product stores cold. A frequently opened visi cooler does not. Medicines fridges and stock that is expensive or hard to replace come first. Check how your own units behave in a cut with a thermometer before deciding; it will tell you whether you need hours of backup or just enough to cover the longest normal cut.
Step 1: energy over the outage
Average power = running power × fraction of time the compressor runs. In summer, a well-loaded freezer may run a large share of each hour. Energy for the outage (Wh) = average power (W) × outage hours.
Battery energy needed (Wh) = load energy ÷ (inverter efficiency × usable depth of discharge). Battery capacity (Ah) = battery Wh ÷ battery bank voltage.
Worked example: one chest freezer
Assumptions: freezer runs at 300 W; runs 40% of the time in summer, so average 120 W; outage to cover 4 h; inverter efficiency 85%; lead-acid tubular battery used to 50% depth of discharge; 12 V bank; starting current 9.8 A.
| Step | Working | Result |
|---|---|---|
| Energy over outage | 120 W × 4 h | 480 Wh |
| Battery energy | 480 ÷ (0.85 × 0.5) | about 1,130 Wh |
| Battery capacity at 12 V | 1,130 ÷ 12 | about 94 Ah, so 100 Ah or more |
| Running VA | 300 W ÷ PF 0.8 | 375 VA |
| Starting VA | 230 V × 9.8 A | about 2,250 VA for a moment |
Step 2: inverter rating and waveform
The inverter must deliver the starting surge without shutting down. Check its surge or peak rating and how long it can sustain it, not just the continuous VA. In the example, the continuous load is under 400 VA but the start needs about 2,250 VA for a moment, so an inverter rated around 1 kVA continuous may or may not start it depending on its surge capability. Ask the inverter maker, giving them the compressor's LRA.
Use a pure sine wave inverter for compressors. Modified or stepped waveforms make motors run hotter and noisier, and some controllers misbehave on them.
Step 3: recharge time
After a 4-hour cut, the battery needs to recharge before the next one. If cuts come close together, check the charger current: a 100 Ah bank discharged by half needs roughly 50 Ah back plus charging losses. A small charger can take most of a day, leaving you exposed to the next cut. Lithium batteries allow deeper use of their capacity and faster charging, at higher cost.
Generator (DG) sizing
For a generator, add up the running kVA of everything it will carry, then check the largest motor start on top. Using the four-unit shop from our stabiliser guide: 1.75 kVA running, about 4.7 kVA if the largest compressor starts while the others run, and about 10.5 kVA if all start together. Generators dip in voltage and frequency when a motor starts; the maker can advise how much motor starting a given set handles.
Staggered restarts again make the biggest difference. Add a transfer delay so the generator stabilises before the refrigeration loads connect, and stagger the units after that.
Backup checklist
- Know each unit's running watts, power factor and starting current.
- Size battery Wh for the outage you plan to cover, not the longest one imaginable.
- Choose inverter and generator by their surge capability, not only continuous rating.
- Use restart delays so compressors do not start together.
- Keep batteries ventilated and away from the hot condenser exhaust.
Frequently asked questions
- Can a home inverter run a commercial freezer?
- Sometimes, if its surge rating covers the freezer's starting current and its battery bank covers the outage. Many home inverters are sized for fans and lights and cannot start a compressor reliably. Check with the inverter maker using the freezer's nameplate data.
- Is an online UPS better than an inverter for refrigeration?
- An online UPS gives a clean, continuous output and suits sensitive loads such as monitoring equipment. For compressors, what matters most is surge capability, waveform and battery energy. Either can work if correctly sized; compare them on those points.
