Building Integrated Photovoltaic Thermal (BIPV/T) systems are promising solutions for serving local electricity and heat demands in Net Zero Energy Buildings (NZEB). Despite BIPV/T offering clear energetic and space saving advantages compared to separate BIPV and solar thermal, overheating occurs when no thermal demand exists, resulting in reduced yields, stagnation damage, and excessive fluid pressures. Whilst continuous fluid flows mitigate overheating, corresponding parasitic demands and space requirements are significant (pumps, large storage tanks or heat rejection equipment). This two-part study examines an alternative approach to BIPV/T, addressing overheating by combining BIPV and Integrated Collector-Storage Solar Water Heater (ICSSWH) concepts. Solar heating capabilities of ICSSWH collectors are well established and their overnight heat loss characteristics provide passive overheating control. BIPV-ICSSWH approaches have yet to be investigated extensively. This paper (Part 1 of 2) reviews state-of-the-art and performance benchmarks in BIPV/T and ICSSWH; proposes new performance metrics enabling fairer comparisons; and develops a heat transfer model for BIPVICSSWH facade elements employing Planar Liquid-Vapour Thermal Diodes (PLVTD) to regulate absorber temperatures and heat losses. Multi-day solar thermal collection, photovoltaic generation, and overnight heat retention behaviours are simulated in different climates. The modelling results (experimentally validated in Part 2 of 2) suggests BIPV-PLVTD-ICSSWHs with single transparent covers and zeta approximate to 90% PLVTD diodicity achieve eta(T,col) approximate to 60% solar thermal efficiency at N approximate to 0.035m(2)K.W-1, PV/T performance ratio PRT3 approximate to 75%, and heat loss coefficient U(r,sys)A(sys)/u approximate to 20 W.m(-3) K-1. The novel BIPV-PLVTD-ICSSWH approach can reduce maximum stagnation by 20 degrees C compared to conventional BIPV/T and therefore support NZEB realisation during global efforts to tackle the climate crisis.