Demand charge electricity

For the vast majority of the last century, the relationship between a homeowner and their electric utility was defined by a simple, transactional metric: volume. The consumer utilized a certain volume of electricity, measured in kilowatt-hours (kWh), and the utility billed for that total amount at the end of the month. This model, often likened to buying gasoline for a vehicle or water for a garden, prioritized the amount of the commodity consumed over the manner in which it was used. However, a profound transformation is currently reshaping the residential energy landscape across the United States. Driven by the modernization of the electrical grid, the integration of distributed energy resources like rooftop solar, and the increasing strain of peak load events, utility companies are migrating toward a more complex billing structure known as "demand charge" pricing.
Formerly the exclusive domain of large industrial facilities and commercial enterprises, demand charges are rapidly entering the residential sector. This shift represents a fundamental change in how energy costs are calculated, moving from a purely volumetric model to one that accounts for the intensity of usage. For the American homeowner, particularly those investing in solar photovoltaic (PV) systems or electric vehicles (EVs), this transition introduces new financial risks and opportunities. A lack of understanding regarding demand charges can lead to "bill shock," where monthly costs remain high despite low overall energy consumption. Conversely, an informed approach to load management—leveraging behavioral changes and technologies like battery storage—can unlock significant savings. This report provides an exhaustive analysis of residential demand charges. It explores the physics distinguishing power from energy, the economic rationale driving utilities to adopt these rates, the specific mechanics of demand calculation, and the intricate relationship between demand charges and solar energy. Furthermore, it offers a detailed guide to appliance‑level power requirements and outlines strategic methodologies for load management, ensuring that homeowners are equipped to navigate this new era of the electric grid. 1

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Part I: The Physics of Billing – Energy vs. Power

To navigate the complexities of modern electric bills, one must first establish a rigorous understanding of the physical concepts that underpin the grid: energy and power. While often used interchangeably in colloquial language, these terms represent distinct physical quantities that impose different costs on the electrical infrastructure.

1.1 Defining the Core Metrics

The distinction between energy and power is the foundation of demand‑based billing.

Energy (kWh): Energy represents the capacity to do work over a period of time. It is a cumulative measurement. In the context of a residential bill, it is quantified in kilowatt‑hours (kWh). This measures the total volume of electricity that has flowed into the home during the billing cycle. It is analogous to the volume of water in a tank or the distance traveled by a vehicle.
Power (kW): Power represents the rate at which energy is consumed or generated at a specific instant. It is an instantaneous measurement. In the context of billing, this is referred to as "demand" and is quantified in kilowatts (kW). It measures the intensity of the electrical flow at any given moment. It is analogous to the flow rate of water through a pipe or the speed of a vehicle. 1

1.2 The Speedometer Analogy

The most effective heuristic for distinguishing these concepts is the automotive analogy, which appears frequently in utility literature to educate consumers.

The Odometer (Consumption/kWh): The odometer in a car records the total distance traveled. If a vehicle travels 100 miles, the odometer registers 100 miles. It makes no distinction between a journey that took two hours at a leisurely 50 miles per hour and a journey that took one hour at a racing speed of 100 miles per hour. In both scenarios, the distance (consumption) is identical. Traditionally, electric bills have functioned like an odometer; the utility charged for the distance traveled (kWh), regardless of the speed. 1
The Speedometer (Demand/kW): The speedometer indicates the vehicle's speed at a specific moment. Driving at 100 miles per hour requires a significantly more powerful engine, better tires, and a more robust suspension system than driving at 10 miles per hour, even if the distance traveled is the same. Demand charges function like a tax on the speedometer's highest reading. The utility observes the "top speed" (peak demand) the household reached during the month and applies a fee based on that maximum intensity, regardless of how long that speed was maintained. 4

1.3 The Hydraulic Analogy

A second, equally powerful analogy involves fluid dynamics. Consider the task of filling a 5‑gallon bucket with water.

Consumption: The water itself represents the energy. Whether the bucket is filled drop by drop over an hour or blasted full in ten seconds, the final result is 5 gallons of water.
Demand: The demand is represented by the diameter of the hose required to fill the bucket. To fill the bucket in ten seconds requires a massive fire hose with high pressure (high demand). To fill it in an hour requires only a thin garden hose (low demand). The utility company charges for the "size of the pipe" they must dedicate to the property to ensure the customer can fill their "bucket" as instantly as they desire. 4

1.4 Mathematical Implications for the Homeowner

The implications of this distinction are mathematically profound. Consider two households, House A and House B, both using 10 kWh of electricity.

House A (The Low Demand User): Runs a single 1,000‑watt (1 kW) space heater for 10 hours.
- Calculation: 1 kW × 10 hours = 10 kWh.
- Peak Demand: 1 kW.
House B (The High Demand User): Turns on ten 1,000‑watt space heaters simultaneously for 1 hour.
- Calculation: 10 kW × 1 hour = 10 kWh.
- Peak Demand: 10 kW.

Under a traditional billing plan, both houses pay the exact same amount for 10 kWh. However, House B placed ten times the strain on the grid's transformers and wires during that single hour. Under a demand rate plan, House B would pay a significantly higher bill—potentially ten times the demand fee—despite having the exact same energy footprint as House A. 4

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Part II: The Economic Rationale – Why Utilities Implement Demand Charges

To the average consumer, the introduction of demand charges often feels like an arbitrary price hike or a deliberate obfuscation of billing. However, from the perspective of utility economics and grid engineering, demand charges are a mechanism to align revenue with the physical realities of infrastructure maintenance.

2.1 The Fixed Cost Dilemma

The costs incurred by a utility company can be broadly categorized into two buckets: fixed costs and variable costs.

Variable Costs: These are costs that fluctuate with the amount of electricity generated. They include the fuel (coal, natural gas, uranium) burned in power plants or the electricity purchased on the wholesale market. These costs align perfectly with volumetric (kWh) billing. When a customer uses less energy, the utility burns less fuel, and costs go down.
Fixed Costs: These are the costs of building and maintaining the grid itself—the poles, wires, substations, transformers, and the power plants themselves. These costs are largely static. A transformer on a telephone pole costs the same to maintain whether electricity is flowing through it or not.

Utilities argue that traditional volumetric billing fails to recover these fixed costs equitably. If a customer installs solar panels and reduces their net consumption to near zero, they still require a robust connection to the grid for the moments when the sun is not shining. If their bill is based solely on kWh, they effectively pay nothing for the maintenance of the wires and poles they use as a backup. Demand charges allow the utility to decouple the cost of delivery (the wires) from the cost of the product (the electricity). 8

2.2 The "Stadium" Capacity Model

Infrastructure planning is driven by peak events, not averages. This is often explained using the "Stadium Analogy."
A sports stadium is built to accommodate the crowd of the largest possible event—the Super Bowl or a championship game. If the stadium owners built a facility based on the average daily attendance (which might be near zero on non‑game days), the venue would be woefully inadequate for the main event. The cost of the stadium is determined by its maximum seating capacity (peak demand), not by how many people sit in it on a Tuesday morning.
Similarly, the electrical grid must be engineered to handle the single hottest hour of the year, when every air conditioner, pool pump, and industrial machine is running simultaneously. If the grid is not sized for this peak, equipment melts, and blackouts occur.

The Cost Driver: The size of the wires and transformers is dictated by the customer's peak usage. A customer who draws 20 kW requires thicker wires and a larger transformer than a customer who never exceeds 5 kW.
The Billing Logic: Utilities view the demand charge as a "capacity reservation fee." The customer is paying for the right to draw that amount of power at any moment, and the utility must keep that capacity available, just as a stadium must maintain a seat for a ticket holder even if they only show up for one hour. 4

2.3 Fairness and Cross‑Subsidization

A central argument for residential demand charges is the prevention of cross‑subsidization between solar and non‑solar customers. In a standard net‑metering arrangement, a solar customer might use the grid heavily in the evening (creating costs) but export heavily during the day (earning credits). If the credits cancel out the costs 1:1, the solar customer pays zero.
However, the utility still incurred costs to deliver power in the evening. In this scenario, non‑solar customers (who pay standard rates) are theoretically subsidizing the grid maintenance for solar owners. By implementing a demand charge, the utility ensures that every customer pays for their strain on the infrastructure (the kW peak) regardless of how much energy (kWh) they offset. This structure is aggressively pursued by utilities in states with high solar penetration, such as Arizona and California, to protect their revenue base against the "death spiral" of declining sales. 3

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Part III: The Anatomy of a Demand Bill

Understanding the theory is only the first step. For a homeowner to manage their costs, they must understand the specific mechanics of how the bill is calculated. The devil is in the details of the "interval window" and the "ratchet clause."

3.1 The Interval Window

Demand is not measured instantaneously in nanoseconds; it is averaged over a specific time window. The standard interval for residential demand is either 15 minutes, 30 minutes, or 60 minutes, depending on the utility.

The Measurement: The digital smart meter records the total energy consumed in each interval (e.g., 1:00 PM to 1:15 PM, 1:15 PM to 1:30 PM). It then multiplies this to find the average power during that time.
The Selection: The utility's billing software scans roughly 2,880 intervals in a month (assuming 15‑minute intervals). It ignores 2,879 of them and selects the single highest interval.
The Charge: The customer is billed based on that one "bad" 15‑minute period. This means a homeowner can be perfectly efficient for 29 days and 23 hours, but if they host a party and run the oven, AC, and dryer simultaneously for 20 minutes, their entire month's demand charge is set by that one event. 1

Impact of Interval Length:

Shorter Intervals (15 min): Much harder to manage. Short spikes (like a microwave running for 10 minutes) can set the peak.
Longer Intervals (60 min): Easier to manage. Short spikes are averaged out with periods of lower usage. For example, running a 10 kW strip heater for 6 minutes in a 60‑minute interval only registers as 1 kW average demand (10 kW × 0.1 hours), whereas in a 15‑minute interval, it would register much higher. 5

3.2 On‑Peak vs. Off‑Peak Demand

Not all demand is created equal. Many modern rate plans distinguish between "On‑Peak" demand and "Off‑Peak" or "Maximum" demand.

On‑Peak Demand: This charge applies only to the highest peak recorded during specific "On‑Peak" hours (e.g., 4:00 PM to 9:00 PM on weekdays). This is typically the most expensive charge, often ranging from $10 to $20 per kW. The goal is to discourage usage when the grid is most stressed.
Off‑Peak / Maximum Demand: Some utilities also charge a secondary, lower rate (e.g., $3 to $5 per kW) for the highest peak recorded at any time during the billing cycle. This covers the local infrastructure costs (the transformer on the street) which must handle the load regardless of the time of day. 9

3.3 The Ratchet Clause: The Trap Door

One of the most punitive features of commercial and some residential demand rates is the "Ratchet Clause." This mechanism links the current month's bill to the highest peak recorded in the previous year.

Mechanism: The billing demand for any given month is defined as the current month's peak OR a percentage (e.g., 60‑80%) of the highest peak recorded in the previous 11 months, whichever is greater.
Scenario: A homeowner runs their AC hard in July, hitting a peak of 10 kW. The utility has a 60% ratchet.
Consequence: In December, the homeowner goes on vacation and uses almost no power, peaking at only 1 kW. However, the ratchet clause kicks in. The minimum billable demand is 60% of July's 10 kW = 6 kW. The homeowner pays for 6 kW of demand in December, despite only using 1 kW.
Implication: A single mistake in July can financially penalize the homeowner for an entire calendar year. This makes consistent load management critical. 14

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Part IV: The Solar Intersection and the "Duck Curve"

For homeowners interested in solar power, demand charges present a specific and often frustrating challenge. The interaction between solar generation and household demand patterns is often misaligned, leading to lower‑than‑expected savings.

4.1 The Misalignment Problem

Solar panels produce energy according to the position of the sun. Production ramps up in the morning, peaks at solar noon (roughly 12:00 PM to 1:00 PM), and declines to zero by sunset.
In contrast, the typical American household operates on a schedule that conflicts with this curve.

Morning: A small spike in usage as the family wakes up and prepares for work/school.
Mid‑Day: Low usage while the house is empty. This is exactly when solar production is highest.
Evening: A massive spike in usage (The "Evening Peak") as the family returns home, turns on air conditioning, cooks dinner, does laundry, and watches TV. This occurs just as solar production is vanishing. 15

4.2 The "Duck Curve" Phenomenon

This mismatch creates a phenomenon known to grid operators as the "Duck Curve."

The Belly: During the day, solar floods the grid, and net demand drops (the belly of the duck).
The Neck: As the sun sets, solar generation disappears rapidly. Simultaneously, human demand skyrockets.
The Ramp: The grid must ramp up conventional power plants aggressively to meet this sudden shortfall.

For the solar homeowner on a demand plan, this is disastrous. The demand charge is based on the peak usage. If the household's peak usage occurs at 6:00 PM (when solar is producing very little), the solar panels do almost nothing to reduce the demand charge. The homeowner might generate 100% of their energy (kWh) needs during the day, but because they still pull 10 kW from the grid at 6:00 PM to cook dinner, they pay the full demand fee. 3

4.3 Why Solar Alone Fails

Historical data and customer "horror stories" from regions like the Salt River Project (SRP) territory in Arizona illustrate this failure mode. Customers who installed solar panels without battery storage found their bills barely budged.

Scenario: A customer generates 50 kWh of solar energy during the day but exports it all because they aren't home. They receive a small credit for this export (e.g., 3 cents/kWh).
The Peak Event: They come home at 6:00 PM and turn on the AC and oven. Their demand spikes to 8 kW.
The Bill: They are charged an $80 demand fee (assuming $10/kW). The meager credits from the daytime export are insufficient to offset this fixed fee. The solar system has offset the energy charge but left the demand charge—often 50% or more of the bill—untouched. 17

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Part V: The Offenders – An Appliance‑Level Analysis

To manage demand charges effectively, a homeowner must stop viewing appliances as "machines that do work" and start viewing them as "electrical loads with specific wattages." Managing demand requires a tactical understanding of which devices are the heavy hitters.

5.1 The High‑Wattage Hierarchy

The following table categorizes common household appliances by their typical power demand. Note the distinction between running watts and startup watts (inductive loads like motors often surge at startup).

Appliance Category	Specific Device	Typical Running Demand (kW)	Startup/Surge Demand (kW)	Demand Impact Rating
HVAC (Heating)	Electric Furnace / Strip Heat	10.0 – 20.0 kW	10.0 – 20.0 kW	Critical
HVAC (Cooling)	Central Air Conditioner (3‑5 ton)	3.5 – 5.0 kW	10.0 – 15.0 kW	Critical
Water Heating	Tankless Electric Water Heater	15.0 – 27.0 kW	15.0 – 27.0 kW	Critical
Water Heating	Standard Tank Water Heater	4.5 kW	4.5 kW	High
EV Charging	Level 2 Charger (40‑80 Amp)	7.6 – 19.2 kW	7.6 – 19.2 kW	Critical
Laundry	Electric Clothes Dryer	3.0 – 5.0 kW	5.0 – 6.0 kW	High
Cooking	Electric Oven / Range (All burners)	3.0 – 8.0 kW	N/A	High
Cleaning	Dishwasher (Heater Cycle)	1.2 – 1.8 kW	N/A	Moderate
Pool	Pool Pump (Variable Speed)	0.5 – 2.5 kW	3.0 kW	Moderate
Misc	Microwave	1.0 – 1.5 kW	N/A	Moderate
Misc	Hair Dryer	1.2 – 1.5 kW	N/A	Moderate
Misc	Lighting / TV / Electronics	< 0.5 kW (Total)	N/A	Negligible

5.2 The "Silent Killers" of Demand

While most homeowners suspect the air conditioner, several other appliances can surprisingly ruin a demand strategy.

The Tankless Electric Water Heater: This device is the nemesis of demand management. Unlike a tank heater that heats water slowly over hours (low demand) and stores it, a tankless heater must heat water instantly. To raise groundwater from 50°F to 120°F in seconds requires massive power—often 15 kW to 27 kW. A single shower during peak hours can cost a homeowner over $300 in demand charges in a single month. 20
The Electric Strip Heat (Auxiliary Heat): Heat pumps are efficient, but when the temperature drops too low, they engage "auxiliary" or "emergency" heat. This is essentially a giant toaster coil inside the ductwork. It consumes massive amounts of power (10‑20 kW). A homeowner might think they are running an efficient heat pump, but if the strip heat kicks on for 15 minutes, the demand bill explodes. 23
The Level 2 EV Charger: Plugging in a Tesla or Chevy Bolt is electrically equivalent to turning on a second entire house. A standard Level 2 charger draws ~7‑10 kW. If this is scheduled to start charging at 6:00 PM when the oven and AC are running, the resulting peak (7 kW + 4 kW + 4 kW = 15 kW) is financially devastating under a demand plan. 7

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Part VI: Strategic Mitigation – Behavioral Load Shifting

The first line of defense against demand charges is behavioral modification. This is often referred to as "Load Shifting"—moving energy usage from expensive peak times to cheaper off‑peak times.

6.1 The "One‑at‑a‑Time" Rule

The simplest and most effective strategy for households without automation technology is the "One‑at‑a‑Time" rule. The goal is to prevent the "stacking" of high‑demand appliances.

Scenario A (Stacking):
- AC Running (4 kW) + Oven On (4 kW) + Dryer On (5 kW) = 13 kW Peak.
Scenario B (Staggering):
- Hour 1: AC Running (4 kW). Oven Off. Dryer Off. Peak = 4 kW.
- Hour 2: AC Off. Oven On (4 kW). Dryer Off. Peak = 4 kW.
- Hour 3: AC Off. Oven Off. Dryer On (5 kW). Peak = 5 kW.
- Result: By spreading the tasks over three hours, the monthly billing peak drops from 13 kW to 5 kW, potentially saving over $100. 8

6.2 The "Dinner Gap" Strategy

The most difficult period to manage is the "Dinner Gap"—typically 5:00 PM to 7:00 PM. This is when families need to cook and cool the house, creating an unavoidable conflict. Strategies to mitigate this include:

Pre‑Cooling (Thermal Battery): Run the air conditioner aggressively before the peak window (e.g., from 12:00 PM to 3:00 PM). Cool the house down to 68°F or 70°F. When the peak window starts at 4:00 PM, turn the thermostat up to 78°F. The house acts as a thermal battery, staying cool without the compressor running during the expensive hours. 26
Alternative Cooking: Avoid the main electric oven (3‑5 kW). Use the microwave (1 kW), a slow cooker (0.2 kW), an Instant Pot, or an outdoor gas grill. These alternatives use a fraction of the power. 1
The "Delay" Button: Modern dishwashers and washing machines almost all feature a "Delay Start" button. Load the dishwasher after dinner at 7:00 PM, but set it to run on a 4‑hour delay. It will run at 11:00 PM, well outside the peak window. 1

6.3 The "Forbidden" Appliances

Households on strict demand plans (like SRP's Customer Generation plan) often create a list of "Forbidden Appliances" during on‑peak hours.

Clothes Dryer: Never run between 3 PM and 8 PM.
Pool Pump: Timer set to run only between midnight and 6 AM.
EV Charger: Programmed to start charging at 11 PM.
Dishwasher: Only run overnight. 28

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Part VII: Technological Mitigation – Energy Storage

For many homeowners, strict behavioral monitoring is inconvenient or impossible. The technological solution that solves the demand charge problem without requiring lifestyle changes is Battery Energy Storage Systems (BESS).

7.1 Peak Shaving with Batteries

Batteries like the Tesla Powerwall, Enphase IQ Battery, or LG RESU are designed with "Peak Shaving" algorithms specifically for demand charges.

Operation: The battery software monitors the home's grid usage in real‑time.
The Threshold: The user or installer sets a "demand limit" (e.g., 3 kW).
The Reaction: If the homeowner turns on the oven and usage spikes to 7 kW, the battery instantly detects the breach of the 3 kW limit. It discharges 4 kW of power to fill the gap.
The Result: The utility meter only sees the 3 kW coming from the grid. The remaining 4 kW is supplied by the battery. The demand charge is "shaved" off. 30

7.2 Battery Economics and ROI

The return on investment (ROI) for batteries in demand‑charge territories is significantly different from net‑metering territories.

Energy Arbitrage: In standard TOU markets, batteries save money by "buying low" (charging at night) and "selling high" (discharging in the evening). The spread might be 10‑20 cents/kWh.
Demand Savings: In demand markets, the savings come from avoiding the fixed fee. A battery that shaves 5 kW off the monthly peak in a territory with a $20/kW charge saves $100 per month instantly. This creates a much faster payback period than energy arbitrage alone. Research suggests batteries become economically viable once demand charges exceed $15 per kW. 12

7.3 Sizing the System

To effectively peak shave, the battery must have sufficient power output (kW), not just energy capacity (kWh).

Capacity (kWh): How long the battery lasts.
Power (kW): How much usage it can cover at once.
The Limitation: A single Tesla Powerwall 2 has a continuous power output of 5 kW. If a homeowner has a spike of 15 kW (AC + Oven + EV), a single Powerwall can only cover 5 kW. The remaining 10 kW will still hit the grid. Therefore, high‑demand households often require 2 or 3 batteries stacked together to cover the whole‑home load during surges. 30

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Part VIII: Case Studies and Regional Landscape

The implementation of demand charges varies significantly across the United States. While some regions use them aggressively, others are just beginning to experiment.

8.1 Arizona: The Epicenter (SRP & APS)

Arizona is the testing ground for residential demand charges in the US.

Salt River Project (SRP): SRP is notable for implementing mandatory demand charges for all new solar customers. Their "Customer Generation Price Plan" (E‑27) includes a demand charge that applies to the single highest 30‑minute interval during on‑peak hours (2 PM – 8 PM in summer).
- Horror Stories: Solar customers who failed to manage their demand have reported bills remaining high despite solar production. One user noted a single day of misconfiguration (charging the EV during peak) cost them over $62 in demand fees for the month. 19
Arizona Public Service (APS): APS offers demand rates as an option (Saver Choice). These plans offer very low energy rates (kWh) in exchange for a demand fee. APS also includes a "Demand Charge Credit" policy: if a customer has an accidental spike, they can call the utility once a year to request forgiveness and have that peak removed from the bill, a consumer‑friendly feature absent in many other territories. 33

8.2 Emerging Markets

Colorado (United Power): United Power utilizes a demand rate to ensure fairness among members in their cooperative. They focus heavily on education, using the speedometer analogy to help rural members understand why running stock tank heaters and welders simultaneously drives up costs. 1
Midwest and Southeast: Utilities in states like Kentucky, Alabama, and Georgia are increasingly proposing demand charges (sometimes called "Grid Access Fees" or "Capacity Charges") as solar penetration grows. In Kentucky, attempts to make these mandatory for all residential customers met with significant public backlash due to the unpredictability of bills. 13

8.3 The Regulatory Debate

Consumer advocates and solar industry groups generally oppose mandatory residential demand charges. They argue that:

Complexity: They are too difficult for the average person to understand and manage.
Lack of Control: Customers cannot always control when appliances run (e.g., a water heater cycling on).
Solar Penalty: They disproportionately harm solar economics, acting as a barrier to renewable adoption.
Utilities counter that demand charges are the only scientifically accurate way to bill for grid capacity and that without them, non‑solar customers are unfairly subsidizing solar owners. 13

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Part IX: Conclusion and Future Outlook

The transition to demand charge electricity represents one of the most significant shifts in the history of residential energy billing. It redefines the relationship between the consumer and the grid, moving from a passive "plug‑and‑play" model to an active "manage‑and‑optimize" relationship.
For the homeowner, this shift carries both risk and reward. The risk is evident: a lack of awareness can lead to punitive bills that erode the savings of solar or conservation efforts. The "ratchet clause" and the "one bad interval" rule mean that the grid is no longer forgiving of inefficiency.
However, the reward lies in the potential for optimization. By understanding the difference between the odometer (kWh) and the speedometer (kW), and by deploying strategies ranging from simple appliance staggering to sophisticated battery automation, homeowners can take control of their energy destiny. As smart home technology evolves, appliances will increasingly communicate with the grid to manage these peaks automatically—the water heater will pause when the oven turns on, and the EV will throttle down when the AC kicks in.
Until that automated future arrives, the burden of management falls on the user. The most successful homeowners in this new era will be those who embrace the "One‑at‑a‑Time" rule, leverage the power of storage, and treat their electricity usage with the same strategic planning they apply to their finances. In the world of demand charges, timing is everything.

Key Takeaways for the Empowered Homeowner:

Identify Your Rate: Check your bill immediately for "Demand Charge," "Peak Capacity," or "kW" charges.
Audit Your Appliances: Know which devices are the 5 kW+ monsters (HVAC, Oven, Dryer, EV).
The Golden Rule: Never run two heavy appliances at the same time during peak hours.
Solar Reality Check: If you have a demand charge, solar panels alone will likely not eliminate your bill. You need a battery to shave the peak.
Use Tech: Smart thermostats and delay timers are your cheapest tools for load shifting.
Pre‑Cool: Use the thermal mass of your home to store "coolness" before the peak window begins.

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