Cooling a home with solar panels

Most household loads are badly matched to a solar array. Cooking, laundry and lighting cluster around the evening, when generation has collapsed. Cooling is the exception, and it is worth understanding why before sizing anything.

A fixed south-facing array generates most around solar noon and tapers either side. Cooling demand is driven by the same sun through the same hours, lagging a couple of hours because the building takes time to heat up.

The consequence is unusual: a large proportion of the electricity your air conditioner uses is generated on your own roof at the moment it is consumed, rather than exported and bought back later. That matters because export is almost always worth less than what you pay to import.

Two practical effects follow:

• Pre-cooling works better here than anywhere. Running the unit hard in the early afternoon while generation is high, then letting the house coast into the evening, uses your own generation rather than imported power — timing, in general.
• An efficient unit needs fewer panels, not just less money. When roof area is the limit, CEER is a sizing constraint.

You cannot size anything without a number. Where the equipment is certified, you have one; where it is not, you have a nameplate and some judgement.

Note how the table splits. Room air conditioners and ceiling fans are certified and give you real numbers. Everything else — portables, evaporative coolers, whole-house fans, attic fans — has no certification program at all, so you are working from manufacturer figures.

That is not a reason to avoid them. It is a reason to be conservative when they are a significant share of your planned load — how to read what manufacturers do publish.

Every watt you do not need is a panel you do not buy, and the cheapest measures here have nothing to do with electricity.

1. Shade the windows, from outside. Solar gain is the largest single heat input in most houses. Blocking it before it enters the glass removes load from the compressor entirely — shading and film compared.
2. Vent the attic. A roof cavity at 140°F radiates into the rooms below all afternoon. On a solar house the panels themselves shade part of the roof, which helps, but ventilation does more — attic ventilation options.
3. Purge at night. Free cooling from outdoor air, using a fraction of the electricity a compressor would — whether a whole-house fan is worth it.
4. Move air over people. A fan lets you hold a higher setpoint comfortably, which reduces compressor runtime directly — what to set it to.
5. Then buy the efficient unit. Certified CEER spans 12.0 to 17.6, a 47% range — and every certified unit is now an inverter, so the label tells you nothing.

Being straightforward about the boundaries, because this is an area with a lot of enthusiastic arithmetic in it.

• A grid-tied array without a battery does nothing in an outage. Standard inverters disconnect for the safety of line workers. If outage resilience is the goal, that needs specific equipment, and a battery fan is a far cheaper answer for one room — cooling during an outage.
• Evening and overnight cooling is imported power. Generation is zero at the exact hours a bedroom needs cooling most, which is a strong argument for pre-cooling and for fans overnight rather than compressors.
• Heat waves and generation do not always coincide. Wildfire smoke, high cloud and simple haze cut output on days when demand is highest.
• Panel efficiency falls as panels get hot. The hottest afternoons are not the highest-output afternoons, which narrows the match slightly.
• We cannot tell you about tariffs or net metering. Export rates, time-of-use bands and net-metering rules vary by utility and change. Our cost figures throughout use 18.34¢/kWh, a national average, which is a comparison baseline rather than your rate.