The State of Hawaii has adopted the unprecedented goal of building a 100 percent renewable power system by 2045. This report identifies some of the central challenges in achieving this goal and uses the SWITCH power system planning model to identify solutions to these challenges. A 100% renewable power system must balance electricity supply and demand on two main time scales: diurnal (providing enough power each hour of the day) and seasonal (providing enough total energy on each day of the year). The diurnal balance could be achieved by installing large amounts of primarily solar production capacity, then using batteries, demand response, biofuels or hydrogen production to shift power production and/or consumption between day and night. The seasonal balance may be more challenging. Energy demand during days or weeks with low sunlight could be met by building extra solar and wind capacity, using biofuels, or using hydrogen produced during sunny months. Demand response will likely be less expensive than the other options for day-night energy balancing, and customer-sited solar may be competitive with utility-scale solar; consequently electric utilities may need to become energy integrators and market managers, rather than bulk power providers.It is unclear how much biofuel the State could use without compromising other environmental and energy independence objectives; consequently hydrogen energy storage merits serious consideration. SWITCH or similar models can be used to identify optimal long-term plans; however, a new incentive system is needed to encourage the State’s utilities to develop and implement such plans, regardless of who will own the generating equipment.