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: what was the percentage change in the average spot rate between 2015 to 2016?
Answer: -67%