Sizing an array for a single appliance is the clearest way to understand solar, because it strips the question down to one load, one number of hours, and one amount of sunlight. Here is that calculation for a window air conditioner, with the assumptions stated so you can change them.
A window air conditioner drawing 900 W for 8 hours a day uses [hed_daily_energy appliance=”window-air-conditioner”] of electricity. At 4.5 peak sun hours and a 20 percent system derate, that needs about [hed_panels_needed appliance=”window-air-conditioner”] standard 400 W panels, an array of roughly 2.00 kW. Fewer in Arizona, more in Seattle.
The arithmetic, with this appliance in it
Four steps, and none of them are difficult:
- Daily energy. 900 W multiplied by 8 hours, divided by 1,000, gives [hed_daily_energy appliance=”window-air-conditioner”] per day.
- Panel output. A 400 W panel at 4.5 peak sun hours produces 1.8 kWh on paper.
- Apply the derate. Inverter conversion, wiring, heat, dust and imperfect orientation take roughly 20 percent, leaving about 1.44 kWh per panel per day.
- Divide. [hed_daily_energy appliance=”window-air-conditioner”] divided by 1.44 kWh gives [hed_panels_needed appliance=”window-air-conditioner”] panels, rounded up.
Peak sun hours is not the same as daylight hours. It is the number of hours of full-strength sun the day is equivalent to, which is why a 14-hour summer day might only count as five.
Panels needed by panel size
Larger panels mean fewer of them, not less roof area. Output per square metre has barely changed; the modules simply got bigger.
[hed_panel_matrix appliance=”window-air-conditioner”]How many panels where you live
Location changes this more than any other input. The same window air conditioner needs roughly twice the array in the Pacific Northwest that it needs in southern Arizona.
[hed_sunhours_matrix appliance=”window-air-conditioner”]For a figure based on your actual coordinates rather than a regional band, use the location-based solar production calculator.
Panels alone will not run it
[hed_solar_note appliance=”window-air-conditioner” field=”note_solar”]
Solar produces during the day. If the load runs when the sun is down, that energy has to come out of a battery, and the battery has to be larger than the energy it delivers because you never discharge it fully.
[hed_battery_size appliance=”window-air-conditioner”]Inverter sizing for a window air conditioner
[hed_solar_note appliance=”window-air-conditioner” field=”note_inverter”]
[hed_inverter_size appliance=”window-air-conditioner”]Inverters are rated for continuous output and, separately, for a brief surge. An inverter that can run the load but not start it is a common and frustrating way to undersize a system.
Cooling and solar are unusually well matched
Cooling load and solar output peak within an hour or two of each other. The hottest part of the afternoon is also the brightest, which means a grid-tied array can cover a large share of cooling directly with very little storage involved.
That changes the sizing question. If you only need cooling during daylight, size for the daytime load and skip the battery. If you run it overnight through summer, the battery table below is the part that matters.
What it costs to run on the grid
For comparison, running the same window air conditioner from the grid costs [hed_cost appliance=”window-air-conditioner” period=”day”] a day and [hed_cost appliance=”window-air-conditioner” period=”year”] a year at current average rates. The full breakdown is on our window air conditioner running cost page.
Questions people actually ask
[hed_faq]How many solar panels does it take to run a window air conditioner?About [hed_panels_needed appliance=”window-air-conditioner”] standard 400 W panels, based on 900 W running for 8 hours a day and 4.5 peak sun hours. In a sunnier location it is fewer; in a cloudier one, more.
How much electricity does a window air conditioner use per day?Roughly [hed_daily_energy appliance=”window-air-conditioner”] per day at 900 W for 8 hours. Change either number and the panel count changes with it.
What size inverter do I need for a window air conditioner?[hed_solar_note appliance=”window-air-conditioner” field=”note_inverter”] The sizing table above gives the continuous and surge figures.
Can I run a window air conditioner on solar without a battery?Only while the sun is producing. A grid-tied system with no battery runs the load during daylight and draws from the grid otherwise. Off-grid, or during an outage, the battery table above is what determines whether it keeps running.[/hed_faq]
Size the whole system, not one appliance
- Solar System Size Calculator for the whole house rather than one load.
- Solar Panel Count Calculator to work backwards from a target array size.
- Usable Battery Capacity Calculator for depth of discharge and real usable kWh.
- Inverter Size Calculator for combined loads and surge.
- Off-Grid Solar System Sizing Calculator if there is no grid connection at all.