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A Primer about Power and Energy: Part 2 - Loads and Peaks

Posted by Lester Shen  |  Date October 25, 2012  |  Comments 0

In Part 1 of this Primer we looked at the difference between power and energy. In this blog post we will continue our discussion, focusing a little more on power and the concept of peak loads.

The table below displays some typical home appliances and their wattages, which is the measure of how much power they use, i.e., their load:

If you run all these appliances at the same time, the power requirement for the house will be about 19.7 kW (let's call this your "peak demand"). If your utility company cannot supply this amount of power at peak demand, then you will experience a brown out.

Normally, however, you won't run all of your devices and appliances at once and so your home's actual peak demand will be much less. The actual load pattern for a typical house looks more like this:

The load pattern shows both power and energy. The shape of the curve shows the demand for power over the day. If you are supplying the power to the house, you are concerned with the peaks in the demand over the day (in kW), since they determine how much you need to supply to the house. However, if you are the person who pays the utility bill, you are more concerned with the area under the curve, which represents the energy that your appliances have consumed over the day (in kWh). The summation of all this is how much your electric bill will charge you for that day. The graph shows a peak demand closer to 6 kW than to the 19.7 kW you would need to power all the appliances listed above at once. Peak demand for this day occurs at around 1 pm, 7 pm, and 10:30 pm. Utilites must plan to provide capacity to cover their customers' peak loads, and so they build power plants specifically to be able to respond to the daily peaks in demand.

To better understand the issues of power planning, let’s consider installing a solar photovoltaic (PV) system to supply your home’s electricity. Based on the demand for the typical house above, you might consider installing a 6 kW PV system.

However, you face two constraints when sizing your PV system: PV systems are expensive, and the physical surface area of your roof to accommodate the solar panels is limited. Based on the average roof area of a house, you might have at best 600 - 700 south-facing ft2 for a solar installation. Assuming a solar panel cost of $40 per ft2 (at $5 per Watt and an output of 8 Watts per ft2), the total cost of the array will be about $28,000.

With an output of about 8 W of electricity per square foot of panel, the maximum output for a 700 ft2 array of solar panels will be about 5.6 kW. This will be enough for most times of the day, but won’t be able to cover your peak demand. You need another 50 ft2 of solar panels. Maybe on the garage? Otherwise, if you still hope to go 100% solar, then you will need to figure out how to reduce your peak demand (also known as “shaving your peak”). You can do this through energy efficiency by upgrading your appliances (such as to Energy Star appliances.)

Another approach to shaving peak is to perform energy consuming activities away from peak times (also known as “load-shifting”). This involves behavioral strategies such as setting the dishwasher on delay to wash a load after midnight. Another method to shift load is to store energy in batteries. Batteries can be charged during non-peak times and then you can draw power from them during peak times. Going back to our solar panel example, if we could shave peak through a combination of energy efficiency and storage batteries to flatten our load demand to about 2 kW, then we could cut the size of our solar array to 250 ft2 and the cost of the array to $10,000.

On a larger scale, utilities contend with the same issues. They must produce sufficient power to cope with peak loads during specific times of the day. But building a new power plant is very expensive for utilities, so many are experimenting with ways to use energy efficiency and demand response to reduce peak loads and thus control their costs. Ideally, utilities would like to see as flat a load pattern as possible. In our next blog post we’ll look at the loads to which utilities must respond and some of the options that they are considering to keeping those loads manageable.

Related posts:

Power and Energy Primer: Part 1 - The Light Bulb and the Electric Bill

Power and Energy Primer: Introduction

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