We explore the oper­a­tions, bal­anc­ing require­ments, and costs of the West­ern Elec­tric­i­ty Coor­di­nat­ing Coun­cil pow­er sys­tem under a strin­gent green­house gas emis­sion reduc­tion tar­get. We include sen­si­tiv­i­ties for tech­nol­o­gy costs and avail­abil­i­ty, fuel prices and emis­sions, and demand pro­file. Meet­ing an emis­sions tar­get of 85% below 1990 lev­els is fea­si­ble across a range of assump­tions, but the cost of achiev­ing the goal and the tech­nol­o­gy mix are uncer­tain. Deploy­ment of solar pho­to­voltaics is the main dri­ver of stor­age deploy­ment: the diur­nal peri­od­ic­i­ty of solar ener­gy avail­abil­i­ty results in oppor­tu­ni­ties for dai­ly arbi­trage that stor­age tech­nolo­gies with sev­er­al hours of dura­tion are well suit­ed to pro­vide. Wind out­put exhibits sea­son­al vari­a­tions and requires stor­age with a large ener­gy sub­com­po­nent to avoid cur­tail­ment. The com­bi­na­tion of low-cost solar tech­nol­o­gy and advanced bat­tery tech­nol­o­gy can pro­vide sub­stan­tial sav­ings through 2050, great­ly mit­i­gat­ing the cost of cli­mate change mit­i­ga­tion. Pol­i­cy goals for stor­age deploy­ment should be based on the func­tion stor­age will play on the grid and there­fore incor­po­rate both the pow­er rat­ing and dura­tion of the stor­age sys­tem. These goals should be set as part of over­all port­fo­lio devel­op­ment, as sys­tem flex­i­bil­i­ty needs will vary with the grid mix.