Financial report context:
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$ / .

Financial table:
year: spot price cap as defined by aneel; 2017: 534; 2016: 423; 2015: 388; 2014: 822
year: average spot rate; 2017: 94; 2016: 287; 2015: 689

Additional report context:
.

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