Quick answer: start with 1,200 W of solar
In a dry climate, an evaporative cooler is the cheapest cooling to run on solar; the U.S. Department of Energy puts it at about a quarter of the energy of central air conditioning. About 450 W for 8 hours at 75% on-time is 2,700 Wh a day, roughly 1,200 W of solar, with only a fan-motor start to plan for. In humid weather it stops working well, so it is a climate decision before it is a solar decision.
Planning result
At 200 W per module, the array target is 6 panels. That count assumes four peak-sun-hours, 75% end-to-end solar yield, 90% conversion efficiency, and a 20% planning margin. Evaporative coolers work best in hot, dry weather, which in the climates where they suit also brings strong sun.
How an evaporative cooler actually uses power
Planning case: an evaporative (swamp) cooler, a fan that pulls outdoor air through water-soaked pads plus a small recirculating pump, drawing about 450 W for 8 hours at 75% on-time. Published ratings range from 250 W for Hessaire's 3,100 CFM portable cooler to about 5 A at 120 V for larger portable units.
There is no compressor: the fan and the pump run steadily, and the cooling comes from water evaporating off the pads. Power depends on fan speed, while cooling depends on how dry the outdoor air is. The cooler also consumes water continuously on a hot day, which matters on a tank-fed site.
Load calculation from running watts and duty cycle
The appliance-class estimate is 450 W running for 8 h/day at a 75% duty cycle, an average of about 338 W across those hours:
Then losses and reserve raise the energy the system must collect to about 3,600 Wh/day.
Typical manufacturer nameplate/manual range for the evaporative cooler class, triangulated with U.S. Department of Energy: Energy Saver 101: everything you need to know about home cooling and Hessaire: 3100 CFM mobile evaporative cooler, then reduced to a conservative planning midpoint; this is not a product specification. Verify the exact unit's nameplate and manual before sizing.
Measure with a plug-in energy meter at the fan speed you actually use on a hot afternoon; the high-speed label figure overstates a typical day.
Solar and battery sizing table
| Design item | Planning value | Basis |
|---|---|---|
| Appliance energy | 2,700 Wh/day | Running W × hours × duty cycle |
| Solar collection target | 1,200 W | 4 peak-sun-hours, 75% yield, 20% margin |
| 200 W panel count | 6 | Rounded up; real modules must satisfy controller voltage/current limits |
| One-day battery allowance | 4,500 Wh | 90% conversion efficiency, 80% LiFePO4 usable fraction, 20% margin |
| 12.8 V LiFePO4 equivalent | 355 Ah | Nominal planning conversion |
| 25.6 V LiFePO4 equivalent | 180 Ah | Nominal planning conversion |
Cooling is mostly a daytime load, so a modest battery covers evening running.
Build the closest passing wiring-diagram preset
The Evaporative Cooler Planning System preset is an engine-passing starting point whose array, battery bank, continuous inverter rating, and surge rating each meet or exceed this page's stated planning allowances. Adjust it only from measured load data and verified equipment limits.
Inverter and startup current
Use at least 600 W continuous with 1,100 W surge for this planning case. The fan motor start is small compared with a compressor start, which is why the surge allowance here is only 1,100 W.
The 1,000 W startup estimate is not a substitute for the manual's locked-rotor, compressor-start, or motor-start requirements, and an inverter's surge rating usually lasts seconds, not minutes. A battery-to-inverter cable that is too long or too small can pull the DC input below the inverter's low-voltage threshold during that start even when the battery has plenty of energy, so use the diagram machine's real run length.
The common mistake
Ways to cut the load
- Run the fan at the lowest speed that keeps the space comfortable.
- Open a window or vent on the far side so the cooler pushes air through instead of building humidity.
- Keep the pads clean and evenly wet so the cooler delivers full cooling at a lower fan speed.
- If its water is pumped from a well or tank, count that pump's energy too.
What to verify before buying equipment
- The exact running watts, startup current, voltage, frequency, and required circuit from the appliance label and manual.
- Actual watt-hours over a representative day; seasonal heating or cooling loads need a design-day measurement.
- Local peak-sun-hours for the worst operating season, plus shade and array orientation.
- Controller maximum input voltage at the coldest expected temperature and maximum output current.
- Battery discharge current, series/parallel permission, terminal torque, low-temperature controls, and overcurrent protection.
- Conductor ampacity, voltage drop, terminal temperature ratings, disconnecting means, grounding, local amendments, and inspection requirements.
For a whole-system load list, use the load and appliance calculator. For the system context around this appliance, continue to the Off-grid heating and cooling guide.
Frequently asked questions
How much solar do I need for an evaporative cooler?
For the planning case on this page (a fan-and-pump evaporative cooler running eight hours in dry heat), allow about 1,200 watts of solar at four peak-sun-hours per day. That covers an estimated 2,700 Wh daily load plus conversion, temperature, and recharge margin. In dry heat it is a fraction of the energy of an air conditioner doing similar cooling.
What size inverter should I use for an evaporative cooler?
Plan on at least 600 W continuous and 1,100 W surge. A fan-and-pump load starts easily; continuous capacity with some headroom is enough.
What battery size keeps an evaporative cooler running for one day?
The one-day planning allowance is about 4,500 Wh nominal usable design capacity, roughly 355 Ah at 12.8 V or 180 Ah at 25.6 V for LiFePO4. Most use is in the afternoon, when the array can supply it directly.