Power Consumption

Power Consumption

technology – energy management


Power Consumption


Things to consider before going solar.


By DENNIS ERSKINE


Truly Transparent Solar
 

AS MORE FAMILIES LOOK TO SOLAR POWER GENERATION for their home, this is a good time to outline the items which should be considered. This applies not only to sizing the system, batteries or not, but to the economics and expectations you’d have before taking this road. Before we get started, getting a handle on a few of the metrics will help make sense of this discussion.

When discussing power consumption, you’ll see numbers such as kW (Kilowatts) and kWh (Kilowatts per hour). 5kW equals 5,000 watts, or fifty 100 watt light bulbs. 5kWh is 5,000 watts consumed over a period of one hour. Your 1,200 watt microwave is 1.2kW, your TV is typically in the range of 0.4kW (400 watts) and your electric dryer is around 5.6kW (5,600 watts). If you ran your dryer for an hour, that would be 5.6kWh. 2kW would power a deep freeze and one element of an electric stove. Central air conditioners average 3.0kW to 5kW while the fan on your air handler consumes 1.4kW to 2.4kW. A 5 ton unit consumes up to 17kW.

In some jurisdictions, as with the example we’re going to be using, a residence is limited to a 10kW maximum solar system. This is known as a 10kW peak system. “Peak” becomes important — that’s peak at any given point in time. Unless your solar panels move to follow the sun, it’s only during a brief period during the day you’ll actually see peak production. Don’t expect to achieve 10kW all day, every day. While your local codes may not place a peak limit, it is very important to be aware of your local codes. Some areas and some Home Owner Associations will have restrictions on panel location. Many fire codes will limit how close panels can be installed to the peak and edges of your roof. At the same time, if you have an older home, it would be a good idea to have a structural engineer verify your roof loading. This will allow you to close in on what you could expect to produce from your solar system which then feeds into the economics and expectations for your installation.

 

BESIDES ROOFTOPS, SOLAR PANELS CAN BE MOUNTED ON THE GROUND PROVIDING EASY ACCESS AND THEREFORE ARE EASIER TO SERVICE.

If you’re fortunate enough to live in a rural area, you may have the choice to have a ground mounted installation. Other than potentially having such an installation in your daily view, there are advantages with ground mount systems. Not only are they easier to service; but, if you live in an area where a snow blower is pre-requisite, ground mount panel arrays are very easy to clear of snow cover. Snow cover on your panels means little to no solar power production, which, by extension, means you’re not saving money and the economics of solar suffer.

In summer, you’ll have longer days and therefore more solar production; however, if you have central air conditioning, that extra production will need to be considered. In the winter, days are shorter and you’ll be using power from the grid generally only for heating, cooking, lights and TV.

 

An important economic decision is how your electric provider treats solar installations. In our example case, it’s located in a “Net Metering” area. In this case, the power company charges $0.12052 per kWh for power consumed. On the flip side, they credit your account at exactly the same rate for the excess power you produce and send to them. In some areas, it’s not the same. The power you buy from the power company is at their published retail rate but the excess power you send to them is purchased at their wholesale rate. If the power supplier is paying you wholesale, then you might want to store your excess production in a battery. But just because that’s the case that is not a reason to jump into battery storage! If the difference between wholesale and retail is $0.04 per kWh, it would take a couple decades for that difference to pay for a $10,000 battery. This is a case where you might restrict using the dryer, dishwasher or other higher power users during daylight hours when you have peak production and forgo the battery.

 
 

test case

Let’s look at some real results from a 5.44kW peak system. The first chart is of actual solar production for the dates shown. On March 11th, total production was 30.22kWh, March 12th only 7.14kWh, March 13th 2.3kWh and zero production on the 14th. This loss of production was the result of a winter storm.

Power chart 1

Below is the power from the electrical grid over the same period of time. You can see that on March 11th very little power was provided by the grid — generally only early morning and after dark consumption. But as solar capture was down during the snow storm, the house had to rely on the electric company for power. This then brings us to the battery discussion and whether or not a battery back-up system is warranted.

Power chart 2

Batteries are expensive. Installed they can run from $9,000 to $12,000 each. Much of the marketing literature will suggest you can use your stored power at night and during power outages (BTW, your power supplier will not allow you to have solar power during an outage since almost all solar installations do not have a transfer switch.) During the day, solar production is used both for your home needs and to recharge the batteries. Your excess production doesn’t go back to the grid but is stored for later use. From the examples above, on the night of March 11th, the batteries would be used for electrical needs. On the following day, the solar production would not have been enough to both power your current needs and recharge your batteries. Looking then at the next several days, you’d not have any stored energy and your batteries would be excellent boat anchors.

 
Kohler generator
 

Another buy/don’t buy battery decision would be based on whether or not you have a well! The typical battery back-up systems designed for residential use would supply on the order of 13kWh to 15kWh. They typically will supply a peak of around 7kW and support a continuous draw of 5kW. So, let’s look at a well pump. Well pumps will draw 5kW to 7.5kW but require three to five times that at start up. This will vary based upon the horsepower of the pump, depth of well and gallons per minute pumped. So in a typical well that draws in excess of 15KWh, you would be exceeding the capacity of a single battery. Remember that electric dryer? It exceeds the 5.0kw continuous draw of a single battery. So for most installations, you’ll need more than one battery and your cost is now getting prohibitive.

In the system we’ve been using as an example, the homeowner opted to install a whole-house generator for about the same price as a single battery. This was the better option to cover power outages. Also being in a net metering environment, the economics of batteries made very little sense and with a very long investment return window. At $0.12052 per kWh, it would take 82,973 kWh to offset the cost of a battery.

Getting to large scale acceptance of battery back-up and/or battery powered homes will take a reduction in costs to the homeowner, as a first step. Capacitance, or the capacity of battery systems, will need to increase as well. While there are advantages to a battery powered home, the economics make it prohibitive for a large majority of the country. Those areas prone to the worst ravages of climate disasters will continue to justify battery back-up systems particularly if outages are generally short term. If outages are greater than a day, a back-up generator is a better option. When battery systems become more affordable I think you’ll see them as a viable solution versus typical back-up generators.


 
Dennis Erskine

DENNIS ERSKINE
CEO ERSKINE GROUP

Dennis has been in the residential technology industry for over 20 years. He currently serves as Chairman of CEDIA’s Certification Commission working toward development of ANSI/ISO accredited technician certifications within the industry.