Quick answer: start with 3,650 W of solar
A 1.5-ton unit is where cooling stops being a small add-on. At about 1,700 W with the compressor on 60% of 8 hours it uses 8,160 Wh a day, which needs roughly 3,650 W of solar, a 48 V battery bank in this site's preset, and an inverter that can start a compressor several times its running size.
Planning result
At 200 W per module, the array target is 19 panels. That count assumes four peak-sun-hours, 75% end-to-end solar yield, 90% conversion efficiency, and a 20% planning margin. Cooling peaks with solar output, but in humid climates the compressor keeps running into the night to remove moisture, which moves energy onto the battery.
How a 1.5-ton air conditioner actually uses power
Planning case: a 1.5-ton (18,000 BTU/h) air conditioner, either a large window unit or a single-zone split system and normally on a 240 V circuit, drawing about 1,700 W with the compressor running at about 60% on-time over 8 hours. A unit sold as 1.5 HP is often much smaller, roughly 12,000 BTU/h, so check the BTU/h figure on the label. Goodman's 1.5-ton GSXN4 condenser uses about 1.5 kW of total system power at 95 °F outdoors; this case plans a little higher to cover window units and older systems.
At this size the difference between fixed-speed and inverter-driven compressors matters. A fixed-speed unit cycles between full power and off. An inverter (variable-speed) split system ramps its compressor down to a fraction of full output once the space is cool and runs longer at that low output, which usually cuts the daily watt-hours and nearly removes the start surge.
Load calculation from running watts and duty cycle
The appliance-class estimate is 1,700 W running for 8 h/day at a 60% duty cycle, an average of about 1,020 W across those hours:
Then losses and reserve raise the energy the system must collect to about 10,880 Wh/day.
Typical manufacturer EnergyGuide/nameplate/manual range for the 1.5-ton air conditioner class, triangulated with Goodman: GSXN4 split system air conditioner specification sheet and U.S. Department of Energy FEMP: Purchasing energy-efficient residential central air conditioners, then reduced to a conservative planning midpoint; this is not a product specification. Verify the exact unit's EnergyGuide label, nameplate, and manual before sizing.
Measure at the breaker with a whole-circuit energy monitor on a hot day; a 240 V unit cannot use a plug-in meter. Record daily kWh, not just the running watts.
Solar and battery sizing table
| Design item | Planning value | Basis |
|---|---|---|
| Appliance energy | 8,160 Wh/day | Running W × hours × duty cycle |
| Solar collection target | 3,650 W | 4 peak-sun-hours, 75% yield, 20% margin |
| 200 W panel count | 19 | Rounded up; real modules must satisfy controller voltage/current limits |
| One-day battery allowance | 13,600 Wh | 90% conversion efficiency, 80% LiFePO4 usable fraction, 20% margin |
| 12.8 V LiFePO4 equivalent | 1,065 Ah | Nominal planning conversion |
| 25.6 V LiFePO4 equivalent | 535 Ah | Nominal planning conversion |
Overnight cooling is what makes this load expensive. If the cabin can be pre-cooled in the afternoon and closed up, much of the energy comes from the array directly.
Build the closest passing wiring-diagram preset
The 1.5-Ton Air Conditioner 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 2,200 W continuous with 9,000 W surge for this planning case. A fixed-speed compressor draws its locked-rotor current for a fraction of a second at every start: Goodman lists 35.1 locked-rotor amps for its 1.5-ton compressor, about 8,100 W at 230 V, which is why the surge allowance here is 9,000 W. A soft starter or a variable-speed split system starts far more gently.
The 8,100 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
- Choose an inverter-driven (variable-speed) unit; it cuts both the start surge and the daily kWh.
- Insulate and shade the roof first; in a cabin it is usually the biggest summer heat gain.
- Cool only the rooms in use with a zoned split rather than the whole building.
- Pre-cool during peak production and raise the set point at night.
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 a 1.5-ton air conditioner?
For the planning case on this page (an 18,000 BTU/h unit cooling a small cabin for eight hours), allow about 3,650 watts of solar at four peak-sun-hours per day. That covers an estimated 8,160 Wh daily load plus conversion, temperature, and recharge margin. A variable-speed unit running at part load usually uses less than this planning case.
What size inverter should I use for a 1.5-ton air conditioner?
Plan on at least 2,200 W continuous and 9,000 W surge. Check that the inverter supplies the voltage the unit needs, often 240 V, and can ride through its compressor start.
What battery size keeps a 1.5-ton air conditioner running for one day?
The one-day planning allowance is about 13,600 Wh nominal usable design capacity, roughly 1,065 Ah at 12.8 V or 535 Ah at 25.6 V for LiFePO4. Running this unit overnight is what makes storage large; daytime-only cooling needs much less.