31 , 2015 , the price was r$ 218/mwh .
after the expiration of contract with eletropaulo , tiet ea's strategy is to contract most of its physical guarantee , as described in regulatory framework section below , and sell the remaining portion in the spot market .
tiet ea's strategy is reassessed from time to time according to changes in market conditions , hydrology and other factors .
tiet ea has been continuously selling its available energy from 2016 forward through medium-term bilateral contracts of three to five years .
as of december 31 , 2016 , tiet ea's contracted portfolio position is 95% ( 95 % ) and 88% ( 88 % ) with average prices of r$ 157/ mwh and r$ 159/mwh ( inflation adjusted until december 2016 ) for 2016 and 2017 , respectively .
as brazil is mostly a hydro-based country with energy prices highly tied to the hydrological situation , the deterioration of the hydrology since the beginning of 2014 caused an increase in energy prices going forward .
tiet ea is closely monitoring and analyzing system supply conditions to support energy commercialization decisions .
under the concession agreement , tiet ea has an obligation to increase its capacity by 15% ( 15 % ) .
tiet ea , as well as other concession generators , have not yet met this requirement due to regulatory , environmental , hydrological and fuel constraints .
the state of s e3o paulo does not have a sufficient potential for wind power and only has a small remaining potential for hydro projects .
as such , the capacity increases in the state will mostly be derived from thermal gas capacity projects .
due to the highly complex process to obtain an environmental license for coal projects , tiet ea decided to fulfill its obligation with gas-fired projects in line with the federal government plans .
petrobras refuses to supply natural gas and to offer capacity in its pipelines and regasification terminals .
therefore , there are no regulations for natural gas swaps in place , and it is unfeasible to bring natural gas to aes tiet ea .
a legal case has been initiated by the state of s e3o paulo requiring the investment to be performed .
tiet ea is in the process of analyzing options to meet the obligation .
uruguaiana is a 640 mw gas-fired combined cycle power plant located in the town of uruguaiana in the state of rio grande do sul , commissioned in december 2000 .
aes manages and has a 46% ( 46 % ) economic interest in the plant with the remaining interest held by bndes .
the plant's operations were suspended in april 2009 due to the unavailability of gas .
aes has evaluated several alternatives to bring gas supply on a competitive basis to uruguaiana .
one of the challenges is the capacity restrictions on the argentinean pipeline , especially during the winter season when gas demand in argentina is very high .
the plant operated on a short-term basis during february and march 2013 , march through may 2014 , and february through may 2015 due to the short-term supply of lng for the facility .
the plant did not operate in 2016 .
uruguaiana continues to work toward securing gas on a long-term basis .
market structure 2014 brazil has installed capacity of 150136 mw , which is 65% ( 65 % ) hydroelectric , 19% ( 19 % ) thermal and 16% ( 16 % ) renewable ( biomass and wind ) .
brazil's national grid is divided into four subsystems .
tiet ea is in the southeast and uruguaiana is in the south subsystems of the national grid .
regulatory framework 2014 in brazil , the ministry of mines and energy determines the maximum amount of energy that a plant can sell , called physical guarantee , which represents the long-term average expected energy production of the plant .
under current rules , physical guarantee can be sold to distribution companies through long- term regulated auctions or under unregulated bilateral contracts with large consumers or energy trading companies .
the national system operator ( "ons" ) is responsible for coordinating and controlling the operation of the national grid .
the ons dispatches generators based on hydrological conditions , reservoir levels , electricity demand and the prices of fuel and thermal generation .
given the importance of hydro generation in the country , the ons sometimes reduces dispatch of hydro facilities and increases dispatch of thermal facilities to protect reservoir levels in the system .
in brazil , the system operator controls all hydroelectric generation dispatch and reservoir levels .
a mechanism known as the energy reallocation mechanism ( "mre" ) was created to share hydrological risk across mre hydro generators .
if the hydro plants generate less than the total mre physical guarantee , the hydro generators may need to purchase energy in the short-term market to fulfill their contract obligations .
when total hydro generation is higher than the total mre physical guarantee , the surplus is proportionally shared among its participants and they are able to make extra revenue selling the excess energy on the spot market .
the consequences of unfavorable hydrology are ( i ) thermal plants more expensive to the system being dispatched , ( ii ) lower hydropower generation with deficits in the mre and ( iii ) high spot prices .
aneel defines the spot price cap for electricity in the brazilian market .
the spot price caps as defined by aneel and average spot prices by calendar year are as follows ( r$ / .

Year
2017
2016
2015
2014
Spot price cap as defined by ANEEL
534
423
388
822
Average spot rate
94
287
689

.

Question: Analyse this data from a financial earnings document. what was the percentage change in the average spot rate between 2015 to 2016?
Choices:
-0.67247
-0.45626
-193
-0.28012
-100.87108
Correct choice index: 0