Rosie has always followed a rule of thumb when advising Rosie on the House listeners about how big a replacement air conditioning unit should be: 400 square feet per ton. But that rule is out of date. Due to the increased energy efficiency in homes today, it is no longer possible to base the size of your air conditioner solely on the size of your home, and therefore there is no good rule of thumb; but there are some guidelines. When it comes to air conditioning, it’s actually better to err on the side of smaller. A unit that’s too big for the home won’t run long enough to wring out the humidity. Instead, it will cool your home too quickly, and then cut itself off before it has time to deal with the humidity. High humidity indoors can lead to problems, including but not limited to, your discomfort and mold growth. Also, an air conditioning unit that cuts on and off every few minutes because it cools the house so quickly will use more energy than one that stays on long enough to do its job with the humidity.

That said, you don’t want to buy a unit that’s too small, either. An undersized unit will run all the time without ever making your home as cool as you’d like it to be. An air conditioning unit that’s just the right size for your home will run long enough to bring your home to the temperature you like, and it will cycle the moisture out of the air and give you the most comfort, and energy efficiency. To figure out how big your new air conditioning unit should be, consult with an air conditioning professional who will use a computer to do a “load calculation” for your home. That calculation will take into consideration not only the size of your home, but which products the builder used to build it. The calculation will consider how thick your wall and ceiling insulation is, how many windows and doors your home has, and whether the glass in them is single-, double- or triple-pane. Which direction the windows face, and whether your windows, doors and other openings in the home (like entry points for electric and cable wires, for instance) are sealed up tight will make a difference as well.

It seems like Tesla might be making a battery for your house. Would that be cool? installing a through the wall ac unitBut why would you need a house battery? ac unit on vs. autoI can think of a couple of uses:how much does an inside ac unit cost For an off-grid house you might like to power it with solar or wind power. Unfortunately, neither of these two sources provide constant energy. If you could store the energy in a battery, you could use this during the night or calm weather. Many people keep a gasoline powered generator for their house. I have one that I don’t use too often, but it’s awesome when you need it. What if you had a battery that you could use for your house in times of power outages? That would be cool.

It seems like the power company would like everyone to have a battery. With a house battery, you could reduce power spikes on the grid. When you turn your air conditioner on, it draws a large current for a short period of time(here’s an explanation of why the current spikes). With a battery this current demand could be leveled out (I guess). But that’s not why you are here, is it? You want to know how big of a battery you would need. We need some starting values. First, how long do you want to run your house on a battery? I think Elon Musk (from Tesla) said one week. The next big thing is the power usage. I think a fair assumption is a constant 2000 Watt power usage. Clearly a house would need more than 2000 watts at some point in the day. However, at night you wouldn’t need much power such that the average for the day could be 2000 watts. If you don’t like that value, you can put your own numbers into the calculations. If I know the power and the time, I can use the definition of power to calculate the energy stored in the battery.

Having the power in Watts is fine (since a watt is a Joule per second) but I need the time in seconds. Now I can calculate the stored energy in the battery.But what the heck is a Joule? Sure, it’s a unit of energy but is that a large amount? Here’s a simple experiment you can do yourself. Take a textbook and put it on the floor. Now pick it up and put it on a table. In order to lift the book, you need energy (to change its gravitational potential energy). A book is about 1 kg and you lifted it about 1 meter. That makes a change in energy of about 10 Joules (don’t forget the gravitational field is 9.8 N/kg). So now you know about Joules. Now for the battery size. The dimensions of the battery will depend on the type of battery. The current Tesla vehicles use a lithium ion battery. According to Wikipedia, the lithium ion battery has an energy density from 0.9 – 2.23 MJ/L (mega Joule per liter). I would imagine that Tesla would only use THE BEST BATTERIES! That would put its energy density at 2.3 x 109 Joules per cubic meter.

If I call the energy density σ, then I can find the battery volume: Putting in values for the energy density and the battery energy: So just half a cubic meter. That’s not too bad. If you want to put this inside your house, you could make it as tall as a wall (let’s say 2.5 meters). Now from a design view maybe a battery should be just 5 cm thick. This means that it would have to be about 4 meters wide. Ok – that wouldn’t work. If increase the thickness to 10 cm, it would just be 2 meters wide. Of course, this only really works because of the high energy density for a lithium ion battery. If you used an akaline battery (like AAs) it has a lower energy density at around 1.8 MJ/L such that it would be slightly larger house battery (but not rechargable). You can look at the Wikipedia page on energy density to get an estimate for house batteries of different materials. Just for fun, what if you made a penny battery (from copper-zinc pennies and acid)? How big of a battery would you need in that case?