Air Conditioning and Solar Power

One of the basic principles of planning a retreat is to minimize your energy needs, and a key part of that is the design of the retreat so as to make it as well insulated as possible.  This will cut down on your heating energy requirements in the colder months, and should also cut down on your cooling energy requirements in the warmer months.

Well, that’s the theory of it, anyway.  The reality is a bit different.

The thing is that while a well insulated house will slow down the rate at which outside heat comes in to your house, it also traps the heat inside and, well, keeps it there, which can mean that inside temperatures will rise to match the outside temperatures, no matter how extreme it may be outside, and you’ll be forced to ‘give in’ and open up all the doors and windows in the summer months, just to get some air flow, even if of hot ambient air.

You’ll also try to also flush out the hot air in the coolest hours of the night, so you start off each day with as low an indoor temperature as possible, and for the first part of the day, as it inexorably rises, you’ll be moderately comfortable, then when inside and outside temperatures approach the same point, you switch from an all shut up to an all open strategy for the rest of the day.

A related issue may be humidity control, depending on if you’re blessed with a relatively dry climate or cursed with a humid one.

This heating effect is of course more pronounced in summer than winter.  In winter, it is a good thing, but in summer, not so good.  Our bodies are radiating heat all the time (100W – 150W for a typical moderately active adult, less while we sleep, more when doing strenuous physical activity), and all the energy we use indoors eventually ends up as heat, too.  So, depending on your energy consumption each day, you probably have the equivalent of a one bar heater on all day every day, which is why, all year round, your indoors temperature is warmer than outdoors, even before you start adding specific additional heating.

We, ourselves, hate being hot, and productivity studies have shown people become materially less productive whenever temperatures start to climb above 70°.  We also hate trying to sleep in a hot stuffy room, and can confirm from personal experience the additional studies that correlate good or bad sleeping with the ambient room temperature.  We love air conditioning.

On the other hand, air conditioning can consume large amounts of energy.  A typical 110V a/c window unit will run at about 1 watt for every 10 BTU of cooling – a 10,000 BTU unit would draw 1000 watts, although note that its duty cycle – that is, the amount of time it will be on – will be maybe 25% – 50%, so you’re getting an hour of cooling for maybe only 250 – 500 watt hours of energy.  Larger a/c systems, and using higher voltages and/or three-phase power, can be more efficient than this and give you more cooling per Watt hour.

As an interesting additional comment, did you know that because a/c units simply shift heat rather than create cold, they move more heat than the energy they consume.  This has implications for both winter and summer – if you have a heat pump, it will create probably two to three times as much heat per kWh of energy as would a normal resistance heater, depending on the temperature of the outside air.  Cooling units typically ‘suck out’ three or four times as much energy as they consume.

Energy Efficiency Issues

Needless to say, if you are installing a/c at your retreat, you want it to be as energy-efficient as possible.

In the US, a/c systems are given a SEER rating or sometimes an EER rating.  Both are a measure of their energy efficiency – the higher the number, the better.  SEER numbers are higher than EER numbers for the same unit by about 15% (ie something with a 14 EER rating would be the closely similar to something else with a 17 SEER rating.

Normally, when a person buys an a/c unit, they give some passing thought to the SEER rating, but pay more attention to other issues like the cost, the noise level, and so on.  However, for a grid-down situation, where energy is never plentiful and always ‘expensive’ in some form or another, you’ll want to make the SEER rating one of your primary focuses.

Generally, split systems, with a unit outside and a separate unit inside are more efficient than all-in-one units such as are typically installed in window frames.  Split systems can give you SEER ratings into the mid to high 20s; all-in-one units struggle to reach 15.

Heating Too?

As we hinted at obliquely above, if you’ll be needing to use electrical heating in the winter, do consider a heat pump rather than just basic simple resistance heaters, because you’ll get two or three times as much heat from each unit of electrical energy with a heat pump than a regular resistance heater.

The efficiency of a heat pump, for heating purposes, depends on its design and the outside temperatures you’ll likely encounter.  The colder it is outside, the less efficient the heat pump becomes.

There are heat pumps specifically designed to work better in very low outside temperatures, and beyond that, you can also switch from an air-exchange heat pump to one with underground piping, transferring the heat from the warmer ground rather than from the cooler air.  Underground piped systems can become quite a lot more complicated and expensive, so we’d consider those with caution, unless you really need an electrically powered heating solution for your retreat.

In general, we’d hesitate to recommend relying primarily on a solar based electrical heating system, unless you’re so overflowing with solar power that you have plenty spare, even on the coldest and least sunny winter days.  If, for whatever reason, you have no other sources of energy from which to create heat (such as firewood), then maybe you have to use solar, and in such a case, it might be a better and more direct approach to simply install a solar heating system, directly transferring what heat there might be from the sun from outside to inside.

Whereas with cooling, the more sun there is, the more you need cooling, and the more solar power you have available to meet that need, with heating, the equation is the opposite.  The less there is sun, the more you need heating, but the less solar energy you have available, in any form, to use for heat.

But, having said that, we’d probably look at the cost difference between getting a cooling-only a/c system and a dual heat/cool system.  If there’s not a lot of difference in cost, we’d get the dual purpose system, because on the days when we do have surplus solar power, why not save our firewood or other energy sources and use the solar power for our heating needs.

Your A/C Needs are Matched by Your Solar Power Outputs

So, as mentioned in the preceding paragraph, there’s a wonderful thing about solar power that makes it sensible to consider about using your solar power to drive an a/c unit.  The stronger the sun, the higher the temperatures, and, at the same time, the greater the power output from your solar panels.  Okay, so that’s a bit of a simplification – in some areas, it can be hot, humid and horrible, even if there’s little or no sun at all, but in other areas, if the sun is obscured, the temperatures drop.

Our point is simply this.  You’ve probably tailored your solar power system to provide you all the power you need in the winter months with little sunlight.  So, now you’re in the summer months, with more and stronger sun each day, you’ll be getting a lot more energy from your solar setup – maybe even more than you need.  Because of the close relationship between your solar panels generating more ‘bonus’ energy for your use, and the times when you’d most benefit from a/c, it becomes possible to plan to use your a/c only when you have surplus spare power, because those times are also the times you most want your a/c running.

So, if the climate warrants it, go ahead and treat yourself, and fit some a/c to your retreat.