Clean Energy Facts

Understanding Renewable Energy and Wind Energy Integration

Maui Electric is deeply committed to integrating renewable energy on our grids. This includes wind power from large facilities; photovoltaic ("PV"), mainly customer-sited private rooftop systems that create electricity for customer use and send the excess to the grid and small amounts of hydroelectric; and biofuel power.

By the end of 2016, over 37 percent of electricity used by Maui Electric customers came from these renewable resources. We've come a long way, already surpassing the 2020 Renewable Portfolio Standard goal of 30 percent from renewable resources. We realize much remains to be done as we continue to do what's right for our community by adding even more clean energy in a safe, reliable and affordable way.

renewable graph maui
Annual Maui Renewable Portfolio Standard

Wind power is an important resource on Maui and a critical part of our efforts to reduce our use of imported fossil fuels.

The output from Maui's three wind facilities varies, depending on changes in wind speed and direction, as the chart below illustrates. While our goal is to integrate as much wind energy on our grid as possible, we are also committed to keeping the lights on for our customers with safe, reliable service. To provide a steady supply of energy to meet customer needs, two wind facilities use large energy storage systems and some Maui Electric firm power generators are operated to compensate for the wind's variability.

Effect of Wind Variability on Frequency

Electricity generation must match demand (consumption by end users) instantly at each moment. When generation supplied to the system exceeds electricity used by customers (also called an "excess energy" situation), Maui Electric first reduces output from some of its oil-fired generators and may take some units off-line. However, there are limits to how low a firm generator can be turned down and whether a unit can be taken off-line. After taking these steps, if energy is still in excess, an as-available generator, like a wind facility, may need to be restricted or "curtailed." The amount of curtailed energy is the difference between the available output versus amount of wind energy that can be accepted by the grid.

In accord with Hawaii Public Utilities Commission (PUC) Decision & Order No. 31288, Maui Electric is providing the following information:

  • quantity of wind energy accepted per month
  • quantity of wind energy curtailed per month
  • estimated cost to customers of curtailment, calculating "the difference between the average cost of curtailed wind energy and the average monthly energy cost of MECO's fossil generation."

Maui Electric notes that calculating the actual cost of curtailment to customers is more complex than the calculation defined above. The electric grid could not provide continuous power to customers if firm generation was simply turned off to eliminate curtailment of wind energy.

As-available (also called "intermittent" or “variable”) resources such as wind do not provide the ancillary services1 that help maintain the stability of the electric grid. To reliably integrate more wind energy and reduce wind curtailment would incur additional costs above the cost of purchasing more wind energy. Such costs could include modifying generators or the transmission and distribution system. These additional costs could exceed potential savings from simply accepting more wind power (identified in the table as Estimated Cost of Curtailment).

The cost of curtailment is expected to increase in the last months of each year because contracts between Maui Electric and two of Maui's wind farms include tiered pricing. That means Maui Electric buys energy at decreasing prices as the year-to-date quantity exceeds each tier. Because the cost of curtailment calculation shown in the table below uses these reduced prices, once the total energy (accepted plus curtailed) reaches the contractual tiers, the difference widens between the hypothetical costs of wind energy versus fossil generation.

Savings realized by electric customers due to purchase of wind energy have also been added to the table below. This methodology is consistent with the calculation of the cost of curtailment and represents the difference between the actual cost to purchase the wind energy and the estimated cost to produce that energy using traditional fossil fuel generation.2

2017
 WIND ENERGY ACCEPTED (GWh)EST SAVINGS FROM WIND ENERGY PURCHASED ($000)WIND ENERGY CURTAILED (GWh)ESTIMATED COST OF CURTAILMENT ($000)
Jan 14.6 -1,072 0.9 -103
Feb 15.3 -978 1.4 -162
Mar 10.7 -852 0.4 -49
Apr 16.6 -1,128 1.2 -147
May 22.5 -1,710 2.3 -303
Jun 26.9 -1,674 1.2 -143
Jul 31.6 -2,159 0.8 -58
YTD 138.1 -9,572 8.2 -965

Baseline (2013)
 WIND ENERGY ACCEPTED (GWh)EST SAVINGS FROM WIND ENERGY PURCHASED ($000)WIND ENERGY CURTAILED (GWh)ESTIMATED COST OF CURTAILMENT ($000)
Jan 11.3 694 5.8 89
Feb 22.3 1,536 14.8 193
Mar 10.7 510 3.5 18
Apr 10.8 559 1.2 29
May 15.0 743 3.5 23
Jun 30.9 1,788 7.3 -20
Jul 24.2 901 2.7 99
Aug 30.9 1,382 2.4 99
Sep 27.4 1,368 2.7 449
Oct 15.0 617 0.7 112
Nov 16.1 761 1.0 178
Dec 15.8 924 1.2 206
YTD 230.3 11,783 46.7 1,475


Note: Totals may differ from the sum of individual amounts due to rounding. To view complete monthly data beginning in 2013, please click here.

In addition, the PUC required Maui Electric to submit a System Improvement and Curtailment Reduction Plan by September 3, 2013. That plan includes specific system improvement measures, estimated cost of the measures, impact on curtailed wind energy, and net benefit of implementing the measures. The plan can be viewed here. In December 2016, the Hawaiian Electric Companies submitted an updated Power Supply Improvement Plan, as required by the PUC. A copy can be found here.


1Ancillary Services are provided by generating resources to help maintain the stability and power quality of the electric grid, including frequency and voltage regulation. Ancillary Services include:

  • ability to commit;
  • ability to dispatch and control from moment to moment the output of a generating unit;
  • ability of a generator to provide reactive power to control voltages;
  • ability of the generator to follow load to help balance supply and demand to regulate frequency;
  • ability of a generator to provide operating reserves - upward reserve in the event of sudden and unpredictable loss of output from another generator or downward reserve in the event of a loss of demand from the system (such as if distribution circuits unexpectedly trip out of service due to a motor vehicle accident);
  • ability of a generator to provide supplemental service (to start up and shut down as needed to serve demand in event other generation is not available).

Although not defined as an ancillary service in most utility jurisdictions, in island electric systems such as Hawaii's, a generating unit's ability to "ride through" an unstable condition on the electric grid without tripping off line is also an important characteristic necessary to help maintain the stability and power quality of the electric grid.

2The estimated savings from wind energy purchased by Maui Electric is calculated as the difference between monthly payments to the wind farms for energy delivered and the estimated average monthly cost of fossil generation had that energy been produced by Maui Electric units. The same average fossil fuel cost used in the cost of curtailment calculation is applied to the wind energy accepted to arrive at the cost of fossil generation.