Quick answer
The highest electrical demand for a specified site or grid, time period and measurement method; interval demand, equipment surge and tariff billing demand can differ.
What Is Peak Load?
Answer: Peak load is the highest electrical demand observed for a defined site or grid, time period and measurement method. Power is measured in kW or MW; energy consumption is measured in kWh. A utility’s interval-average demand peak can differ from a brief equipment-starting surge, and the tariff determines which measured peak becomes billing demand.
The Oregon Department of Energy’s 2024 report, in its Peak Electricity Demand section, explains peak load at the service-area scale. The same maximum-over-a-period concept is useful for a facility, but the boundary and interval must be specified.
Four Different Peaks
| Measure | What it describes |
|---|---|
| Equipment surge | Brief starting or transient demand relevant to equipment response |
| Facility load peak | Highest demand of the defined building or process |
| Grid-import peak | Highest measured import after on-site generation/storage effects |
| Billing demand | Value selected by the applicable tariff, potentially using windows, history or another basis |
The utility system’s peak can occur at a different time from a customer’s own maximum. A battery supplying a building can reduce grid import while the building’s underlying load remains unchanged. Keep the load and import series separate.
Instantaneous Power and Interval Demand
An interval meter can calculate average power from energy used during the interval. For a hypothetical 15-minute interval containing 50 kWh of grid-import energy:
Average demand = 50 ÷ 0.25 = 200 kW.
That is an interval average, not a statement that every instant was 200 kW. For example, assumed constant demand of 300 kW for 5 minutes and 150 kW for 10 minutes yields 25 + 25 = 50 kWh, and therefore the same 200 kW average. The brief 300 kW level still matters to physical equipment.
Use the utility’s actual block or rolling interval, units, timestamps and aggregation rules. An hourly series can conceal a 15-minute peak. If the billing interval is finer than the available data, record that limitation rather than treating the hourly maximum as the actual billable peak.
Peak Load and Demand Charges
The U.S. Department of Energy utility-rate guide separates energy, demand and fixed charges. It also distinguishes time-of-use demand, non-coincident demand, look-back ratchets and system-peak charges.
For a hypothetical simple tariff, assume billing demand equals this month’s maximum interval import, with 200 kW billed at $12/kW for the month. Its demand component is $2,400. This is not a current utility rate or a complete bill.
A hypothetical 80% look-back floor based on a previous 250 kW peak would instead create a 200 kW floor. Reducing this month’s measured peak to 180 kW would not lower billing demand below 200 kW under that assumed rule. Actual tariffs can use different histories, percentages, minimums and charge components; read the current schedule.
How Solar Changes the Measured Peak
Solar can reduce grid import when its output coincides with the relevant load interval. Annual solar energy or PV nameplate power does not prove a given demand reduction. Use aligned load and production series at the same boundary and time resolution.
Suppose one interval has 200 kW facility load and 60 kW coincident PV output, producing 140 kW import without storage. But another interval with 180 kW load and no PV still imports 180 kW. The new peak across those two intervals is 180 kW, not 140 kW. Evaluate every relevant interval.
Peak shaving evaluates ways to lower these peaks through load management, generation or storage. A battery is not automatically required: feasible scheduling changes can also help. Its permitted power, available energy and recharge behavior constrain any proposed reduction.
Practical Handoff
Designers should retain the source load data, meter boundary, interval, time zone/daylight-saving treatment, tariff version and seasonal peaks. Request a representative history and reconcile interval totals and peaks to bills. Monthly bills identify the charge structure but do not reveal the full shape or duration of peaks.
Installers and facility teams should verify the documented monitoring boundary, sensor orientation, phase assignment and control configuration. Measurement may be reconstructed from supported sensors; there is no universal instruction to place every CT physically at the utility meter. Use qualified installation and the equipment design.
Sales teams should show energy and demand estimates separately, with the relevant tariff assumptions and limitations. Avoid fixed demand-charge percentages, universal building peak hours or guaranteed savings. When evaluating SurgePV, ask which interval-data, load and tariff inputs the workflow supports.
Common Questions
What is peak load in solar design?
It is the maximum of a defined demand series. Distinguish building load from grid import after PV and storage, and specify the time interval.
Is peak load the same as energy consumption?
No. Demand is power in kW or MW. Energy accumulates over time in kWh or MWh; identical energy totals can have different demand peaks.
Can solar reduce peak load?
It can reduce import at coincident intervals. The maximum remaining import and the applicable tariff determine whether that produces a demand-charge reduction.
Related Glossary Terms
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