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Triage: what the tank needs during an outage
A tank without power loses four things: circulation, filtration, heat and light. They fail on very different timescales, which is why a sensible backup plan protects one or two items and ignores the rest.
| Equipment | Typical draw | Priority on battery | Why |
|---|---|---|---|
| Air pump | 2 to 10 W | Highest | Surface agitation and bubbles drive oxygen exchange |
| Circulation pump or powerhead (DC, low speed) | 5 to 20 W at reduced speed | High | Moves water past the surface, keeps filter media oxygenated |
| Return pump (sump systems) | 20 to 100 W | Medium | Useful, but a powerhead in the display often does the job for less |
| Hang-on-back or canister filter | 5 to 30 W | Medium | Keeps bacteria fed with oxygen; restart concerns after long stops |
| Protein skimmer | 10 to 40 W | Low | Can overflow on restart; leave off battery |
| Lighting | 20 to 300 W | None | Corals and plants tolerate a dark day |
| Heater | 50 to 300 W each | None | Too much power; insulate the tank instead |
| Chiller | 100 to 500 W plus surge | None | Compressor surge and load are beyond a home UPS |
The priority is driven by physics. A home aquarium holds a lot of water, and water has a high heat capacity, so temperature changes over hours rather than minutes, especially with a lid on and a blanket around the glass. Oxygen, by contrast, is consumed continuously by fish, invertebrates and the bacteria in the filter, and replenished only at the water surface. Stop the water moving and the surface layer saturates while the rest of the tank slowly runs down.
Why heaters never belong on battery
A 200 W heater on a typical home UPS with about 200 Wh of battery would empty it in well under an hour, and much sooner in practice because lead-acid capacity drops at high discharge rates. That same energy would run a 5 W air pump for most of a day if it came from an efficient DC source. Plug heaters, lights, skimmers and chillers into the UPS's surge-only outlets (or a separate surge protector) so they resume automatically when power returns. Insulation does more for temperature than any battery a home user would buy.
Waveform: why pumps care about the sine wave
Many aquarium pumps are AC motors. Traditional magnetic-drive return pumps and powerheads use small synchronous or shaded-pole motors, and classic air pumps use an electromagnet that vibrates a diaphragm at line frequency. On a simulated (stepped) sine wave, these devices see harmonics that the motor converts to heat and noise instead of work. Common field observations include loud buzzing from air pumps, warmer pump housings, and some synchronous pumps failing to start reliably.
A few hours of that is not guaranteed to damage anything, but over a long outage the extra heat matters, and a pump that will not start defeats the purpose. Choose a pure sine wave UPS for AC pumps. DC pumps run from their own controller and power supply, which typically tolerates either waveform, but check the manufacturer's guidance.
DC pumps and their own battery backups
Many modern return pumps and powerheads are DC pumps with a controller. Several makers sell dedicated battery backup units for their DC pumps that power the pump directly at reduced speed, without an inverter. When available for your pump, these are usually more efficient than an AC UPS for the same runtime. Some controllers also detect the outage and drop to a low "battery mode" speed automatically.
Worked example: a 75-gallon reef tank
A reef tank with a sump has these loads on a normal day: DC return pump 45 W, two DC powerheads at 20 W each, heaters averaging 80 W (up to 300 W when both fire), LED lights 120 W during the photoperiod, skimmer 25 W and a controller 5 W. Normal average draw is well over 250 W. Backing that up for hours is not realistic with a UPS.
The outage plan keeps one powerhead running at about 40% speed (roughly 8 W), plus the controller (5 W), and leaves everything else off. Battery load: 13 W.
Option A: a 1500 VA pure sine line-interactive UPS
Typical battery: two 12 V 9 Ah, about 216 Wh nominal. At this light load the battery delivers close to its rating, but the inverter itself consumes roughly 10 to 25 W on battery depending on the model. Take 15 W.
216 Wh x 0.85 / (13 W + 15 W) = 183.6 / 28 = about 6.5 hours with a new battery. More than half of the energy went to running the inverter, not the pump.
Option B: the pump maker's DC battery backup, or a 12 V LiFePO4 battery
Feeding the DC powerhead directly from a 12 V lithium iron phosphate battery of about 100 Ah (roughly 1,280 Wh), with a DC converter around 90% efficient:
1,280 Wh x 0.90 / 13 W = about 88 hours, over three and a half days. Even a much smaller dedicated pump backup will typically outlast the UPS for the same weight, because no inverter overhead is involved.
Option C: a battery-powered air pump
A D-cell or rechargeable air pump with automatic outage detection costs little, switches on by itself, and runs for many hours to days depending on the model. In a reef tank it is a useful second layer rather than a replacement for circulation, since corals depend on water movement too.
Rule of thumb: below about 30 W, the inverter is the load
For loads this small, a UPS's runtime is set mostly by its own idle consumption on battery, which manufacturers rarely publish. A useful bench test: plug in only the air pump, unplug the UPS from the wall, and time how long it runs (stop at the low-battery alarm). Divide the battery's Wh rating by that time to get the real total draw. If it is two or three times the pump's draw, a DC solution or a power station with DC outputs will give you several times more runtime from the same budget.
How much runtime is enough
For an aquarium, minutes of ride-through are nearly worthless and the target is hours. Most outages are short, but storms and planned shutoffs can last a day or more. A realistic plan:
- Freshwater, moderate stocking: 6 to 12 hours of aeration covers most outages. Beyond that, battery air pumps and fresh batteries.
- Reef or heavily stocked tanks: aim for a day or more of low-speed circulation via DC backup or a power station, plus a battery air pump.
- Multi-day outages: a generator or large power station, with insulation and a plan for temperature.
See how much runtime do you need for the general framework, and the runtime calculator to test specific combinations.
Setup checklist
- Split the outlets. Air pump and one circulation pump on battery outlets; everything else on surge-only outlets.
- Mount the UPS away from water. Above the splash zone and not inside a humid stand cabinet if you can avoid it; salt creep corrodes electronics. Use drip loops on every cord.
- Keep GFCI protection. Plug the UPS into a GFCI-protected outlet; see where to place a UPS.
- Set pumps to resume safely. Confirm the sump can absorb the backflow when the return pump stops, and that the skimmer will not overflow when it restarts.
- Silence the alarm if you need to. Most UPS units let you mute the on-battery beep, which matters for a tank in a bedroom.
- Test twice a year. Unplug the UPS and time the runtime on the real load.
Common mistakes
- Heater on a battery outlet. It drains the battery before the pump that mattered gets a chance.
- Buying a large UPS for a 5 W load. Inverter overhead wastes most of it. Bigger is not proportionally better.
- Assuming the UPS will keep the canister filter healthy for days. It will not; plan for the restart instead.
- Using a simulated sine unit for AC pumps and then wondering why the air pump screams. See UPS buzzing noise for related symptoms.
When a UPS is the wrong tool
An AC UPS is built to carry a few hundred watts for minutes so computers can shut down. An aquarium needs the opposite: a few watts for many hours. For freshwater tanks, a battery air pump is the cheapest effective answer. For reef tanks, a DC pump backup or a battery bank wired to DC equipment is usually the most efficient. A UPS is still a good fit for tank controllers and monitoring gear that need seamless power, and as a short bridge until a generator starts. For a comparison of the alternatives, see UPS vs portable power station and DC mini UPS vs AC UPS.
Frequently asked questions
How long can fish survive without a filter or air pump?
It varies with stocking density, temperature, tank surface area and species. A lightly stocked cool freshwater tank may be fine for many hours; a heavily stocked warm tank or a reef with high oxygen demand can get into trouble much faster. Warm water holds less oxygen. Treat a few hours as the point to have aeration running.
Should I restart my canister filter after a long power outage?
Be careful. Water sitting still in a canister for many hours can lose its oxygen, and the bacteria can begin producing harmful byproducts. Many experienced keepers drain or rinse a canister that has been off for several hours before restarting it, rather than flushing that water into the tank. Short interruptions are generally fine.
Can I run my aquarium heater on a UPS?
Technically yes for a few minutes, but it is a poor use of the battery. A 200 W heater would drain a typical home UPS quickly, leaving nothing for the air pump. Insulate the tank with blankets or foam board instead, and keep the heater on a surge-only outlet so it simply resumes when power returns.
Is a portable power station better than a UPS for an aquarium?
For long outages, often yes, because it holds more energy per dollar and some models have efficient DC outputs. Check that it can pass power through while charging and how quickly it switches over. Many power stations are not designed to act as an always-on UPS, so read the manufacturer's guidance.
Do I need a GFCI for my aquarium equipment?
It is strongly recommended. Water and mains power share the same stand, and a GFCI (with drip loops on every cord) cuts power if current leaks to ground through the water. A UPS plugged into a GFCI outlet works normally; just remember the UPS output on battery is not protected by the upstream GFCI.
Sources and further reading
- NFPA 70, National Electrical Code, Section 210.8 (GFCI protection for personnel)
- UL 1778, Standard for Uninterruptible Power Systems
- IEC 62040-3, Uninterruptible power systems: method of specifying the performance and test requirements