Hybrid Water Heating System Integrating Split AC Waste Heat and Solar PV for Energy Efficiency

Water heating is an important energy demand in residential, commercial, and industrial sectors. Conventional electric and gas water heaters are highly energy-intensive and contribute substantially to greenhouse gas (GHG) emissions. In tropical countries such as Indonesia, air-conditioner (AC) systems account for 30-40% of total electricity consumption, and their condensers continuously release waste heat into the environment. This waste not only signifies a substantial loss of recoverable energy but also intensifies the urban heat island effect.

Indonesia simultaneously possesses abundant solar energy resources, averaging 4.8 kWh/m²/day, and the government has set a target to expand photovoltaic (PV) capacity to 0.87 gigawatt (GW) by 2025. Harnessing both waste heat from split AC systems and solar PV power, thereby providing a promising pathway to reduce reliance on the national grid (Perusahaan Listrik Negara-PLN, Indonesia’s state-owned electricity utility), lower operating costs, and contribute to the country’s Net Zero Emission 2060 target.

In tropical climates, where cooling demand is high and solar irradiance is abundant throughout the year, coupling AC waste heat recovery with PV electricity creates a unique synergy. The higher the cooling load, the greater the recoverable heat and the available solar power. This complementary behavior makes hybrid AC–PV heat pump systems particularly efficient, sustainable, and well-suited to tropical regions—an aspect rarely emphasized in previous studies.

Extensive studies have been conducted on solar-assisted heat pump (SAHP) systems. Zohri et al. [1] showed that integrating solar collectors, PV, and photovoltaic/thermal (PV/T) with heat pumps significantly enhanced water heating and drying. Kedar et al. [2], Najaf and Aslan [3] extended this concept through hybrid desalination combining evacuated tube collectors (ETC) and compound parabolic concentrators (CPC), achieving competitive water yields. PV/T heat-pipe systems have also reported combined thermal–electrical efficiencies of 22.9-24.6% [4]. Floating PV has been recognized as a feasible pathway for capacity expansion in water-rich regions [5].

As baselines, thermosyphon–riser collectors achieved 70% efficiency [6], vacuum tube collectors with reflectors reached 86% [7], while induction heating provided an alternative non-solar route [8]. Pressurized collectors with reflectors produced water at 318 K [9], and air collectors showed cost-effective heating [10]. The introduction of baffles improved heat transfer in solar air collectors by 35% [11], while a parabolic trough collector (PTC) paraboid-integrated with double chillers achieved synergistic operation.

Sustainability-oriented designs have also gained attention. Collectors fabricated from recycled materials have achieved efficiencies of up to 62% [12], while broader reviews emphasized renewable energy as a long-term solution [13, 14]. Similarly, solar dryers showed significant performance improvements: non-integrated vs. novel-modified designs increased efficiency by 9-55%. Experimental methods such as halogen-lamp simulators enabled controlled PV testing.

Thermal storage integration has become another major study avenue. Solid–liquid phase change material (PCM) systems improved thermal stability, while solid–solid polyethylene PCMs provide an alternative option for heat storage applications. Low-cost media such as sand hill reduced thermal fluctuations by 5.2℃. PV/T coupled with PCM maintained water above 34℃ [15] and stabilized PV performance under forced convection.

Numerical and Computational Fluid Dynamics (CFD) simulations have further advanced the existing literature. Studies showed that incorporating central heat exchangers in solar ponds enhances thermal efficiency, while cone receivers improve dish concentrator performance. Hybrid PTC + cascade still distillation achieved 6 kg/m²/day. The Capderou model validated solar resource estimates with a root mean square deviation (RMSD) of 7%, and nanofluid-enhanced collectors improved thermal conductivity.

Studies on PV system optimization include orientation and tilt studies (e.g., 30° tilt in equatorial conditions) [16] and azimuth tracking, which increased output by 28-40% [17]. Advances in PV/T optimization included sustainability indices, finned collectors [18], concentrated solar power (CSP)-integrated thermal design [19], and PV–thermal co-sizing for nearly zero-energy buildings [20]. Broader integration studies highlighted the importance of reliability in renewable mechanical systems [21], while exergy-based analyses of solar stills [22] and nanofluid applications expanded evaluation frameworks. Air collectors with fins and baffles also showed significant numerical performance improvements.

Recent studies provide updated information. Wiriyasart and Kaewluan [23] experimentally showed AC waste heat recovery for water heating, validating its potential in hybrid systems. Znaczko et al. [24] reported a 12.7% improvement in thermal storage efficiency using adaptive control strategies in solar hot water systems. Satpute et al. [25] used CFD to optimize PVT absorber configurations, achieving thermal–electric efficiency improvements up to 75%. Finally, Icaza [26] conducted techno-economic assessments of PV-assisted heat pumps, showing competitive PP and reduced energy costs. In the Indonesian context, previous studies confirmed the technical feasibility of AC waste heat recovery using helical coil heat exchangers, achieving water heating above 50℃. However, these studies still relied entirely on grid electricity, which limited their sustainability.

Despite significant progress in the field, critical gaps persist. Most existing studies focus exclusively on either solar-based systems or air-conditioning (AC) waste heat recovery, rather than integrating both approaches. Few integrate real-time PV electricity with AC waste heat recovery under tropical field conditions, and even fewer evaluate thermal performance, economic feasibility, and environmental impact comprehensively. This present study addresses these gaps by experimentally developing and analyzing a hybrid system that combines split-type AC waste heat recovery with solar PV input under tropical conditions.

These results highlight the scalability of the system. The 0.5 PK unit is suitable for single-household applications with modest demand, while 1 PK and 2 PK units can serve multi-user households or small-scale commercial facilities. Importantly, the hybrid mode shows a balance between performance and grid independence, making it particularly relevant for Indonesian households where energy reliability is often a concern.

The hybrid HPWH system showed strong thermal performance across all modes. PLN mode produced the highest heating rates, PV-only mode showed reliable standalone operation with slightly reduced efficiency, and the hybrid mode provided stable and resilient performance. The results not only validate PV–grid integration but also align with energy efficiency and sustainability goals for tropical regions.

3.2 Contribution of solar PV

The contribution of solar PV supply was clearly observed across all capacities and operating modes tested in this study. The experimental results showed that PV-only operation produced slightly lower final water temperatures compared to PLN (grid) operation, while Hybrid PV–grid operation consistently narrowed this performance gap. For the 0.5 PK unit, water temperature under PV-only mode reached 42.7℃, approximately 2.0℃ lower than the 44.7℃ achieved under PLN operation, while the hybrid mode improved the outcome to 43.4℃. For the 1 PK unit, the differences were similarly small: PLN produced 49.5℃, PV-only reached 48.4℃, and the hybrid mode delivered 48.9℃, a reduction of only 0.6℃ from the grid baseline. Finally, for the 2 PK unit, the grid mode achieved 57.7℃, PV-only reached 56.1℃, and hybrid operation delivered 56.9℃, a difference of less than 1℃.

The results confirm that the 1.1 kWp PV array was capable of reliably supporting compressor operation across capacities, with hybrid mode ensuring nearly identical outcomes to grid supply.

The robustness of PV-only operation can be attributed to high solar resource availability in tropical regions, where daily irradiance averages 4.8–5.2 kWh/m² [16]. Although modest reductions in final water temperature occurred under PV-only mode, these deviations were small enough to validate PV as a reliable energy source for HPWH operation. The hybrid configuration further mitigated these variations by supplementing PV input with grid electricity, ensuring stable compressor loads and reliable hot water delivery while simultaneously reducing dependency on PLN supply. Comparable outcomes have been reported in PV/T–PCM studies, where water temperatures above 34℃ were consistently maintained under solar-only configurations [15].

Similarly, optimization studies in equatorial regions emphasize that PV system performance can be maximized through tilt adjustment, with a 30° angle identified as optimal for annual solar capture [16]. These results further reinforce the suitability of the PV setup used in this study.

Three key factors explain the small differences observed between PV and grid operation: (i) fluctuations in solar irradiance during intermittent cloud cover, which affected compressor load stability, (ii) the limited PV array size, optimized primarily for a 1 PK compressor, leading to tighter margins for 2 PK operation, and (iii) conversion losses in the inverter–battery system, typically ranging from 10-15% [4].

Despite these constraints, the system exhibited notable operational stability. The PV-only mode achieved water heating performance comparable to that of grid-powered operation, while hybrid operation effectively eliminated the remaining performance differences.

In practical terms, these results highlight the potential of integrating PV with HPWH systems to improve household PV utilization. Rooftop PV systems in Indonesia are often underutilized because daytime demand does not fully match PV generation. By channeling surplus PV output to operate the compressor, excess electrical energy can be efficiently converted into stored thermal energy, thereby supplying hot water at an almost negligible marginal cost. This not only improves the economic return of PV installations but also increases the capacity factor of the system, aligning with similar outcomes reported in PV/T hybrid studies [1].

On the scientific side, the integration of PV with waste heat recovery enhances the energy utilization factor (EUF) by combining electrical and thermal outputs in a unified system. Compared to standalone PV or conventional AC waste heat recovery, the hybrid configuration ensures that both energy streams are exploited simultaneously, thereby reducing system losses and improving overall efficiency. This dual-use approach aligns with broader trends in PV/T study, where hybrid strategies have been shown to deliver superior long-term performance compared to single-function systems [6, 7].

From an energy policy standpoint, the feasibility of integrating PV systems with heat pump water heaters (HPWH) reinforces Indonesia’s renewable energy transition by enhancing the economic and functional value of rooftop PV installations. The ability of PV to substitute for grid electricity in water heating reduces household peak demand, specifically in urban areas where AC ownership is widespread. By reducing reliance on PLN during peak hours, such systems can contribute directly to national targets for renewable energy expansion and household-level sustainability [1, 16].

Collectively, the experimental results validate solar PV as a robust and reliable contributor to HPWH operation across different AC capacities. PV-only mode consistently sustained compressor operation with minimal performance reduction, while hybrid mode nearly matched grid-supplied outcomes. Beyond technical feasibility, PV integration enhances household energy utilization, supports national renewable energy policies, and aligns with sustainability objectives in tropical regions.

3.3 Energy consumption and operating cost

The analysis of energy consumption highlights the substantial efficiency gains of the hybrid HPWH system compared with conventional electric resistance water heating.

A baseline resistive heater of equivalent capacity required approximately 16 kWh/day, translating to monthly expenditures of around IDR 720,000, based on average residential electricity tariffs. By contrast, the HPWH–PLN configuration consumed only 5.5 kWh/day, reducing costs to approximately IDR 247,500 per month. The Hybrid PV–grid mode achieved an even lower grid demand of 0.95 kWh/day, corresponding to a monthly cost of about IDR 42,750. Finally, under PV-only operation, the 0.5 PK unit achieved full independence from the grid, while the 2 PK unit reduced grid electricity use by over 80%. These results underscore the transformative cost advantages of integrating PV with AC-based HPWH systems.

When benchmarked against regional studies, the observed savings are remarkable. Prior investigations of solar-assisted heat pumps (SAHP) in Southeast Asia generally reported cost reductions of 60-70% relative to resistive heating [1, 4]. In this study, however, reductions reached 82-94%, substantially higher than typical reports. The discrepancy can be attributed to Indonesia’s heavy reliance on resistive water heating, which operates at near 100% conversion efficiency but lacks the performance multiplier provided by heat pumps. By leveraging the high COP of AC-based HPWH systems, combined with renewable PV electricity input, the hybrid system shifts the baseline dramatically. This dual optimization explains the superior savings achieved in the present work.

The observed energy savings are primarily attributed to the thermodynamic efficiency of the heat pump cycle, which recovers waste heat from the condenser rather than relying exclusively on electrical resistance for water heating. Unlike conventional heaters that convert all electrical energy directly into heat, the hybrid HPWH amplifies useful heat output through the vapor-compression process, achieving a COP between 2.5 and 3.5. Consequently, each unit of electrical input transfers 2.5-3.5 units of heat energy to the water, significantly reducing overall electricity consumption. The integration of PV power further offsets compressor loads during peak irradiance, enabling renewable substitution for grid electricity. Similar mechanisms were reported by   Zohri et al. [1] and Wiriyasart and Kaewluan [23], who attributed 60-80% energy savings to combined solar utilization and waste heat recovery. The present study extends these results under real tropical conditions, achieving up to 94% reduction in grid electricity use—surpassing typical SAHP benchmarks due to continuous solar availability and high cooling demand in tropical climates. These thermodynamic and operational synergies also translate into measurable economic benefits, as supported by the techno-economic evaluation of PV-assisted heat pump systems by Icaza et al. [26], which reported comparable cost reductions under similar hybrid configurations.

Although absolute monetary savings are moderated by relatively low national electricity tariffs, relative efficiency improvements remain highly significant. Annual household savings of up to IDR 8 million were recorded, depending on capacity and mode. For smaller-capacity units such as 0.5 PK, the system approaches complete energy independence, virtually eliminating household reliance on PLN. Larger-capacity units (1–2 PK), while consuming more energy in absolute terms, deliver greater monetary savings over time due to the scale of hot water delivered. This dual adoption pathway reflects the versatility of the system: smaller households benefit from energy autonomy, while larger households or commercial facilities realize substantial financial returns through long-term operating cost reductions.

The results are in line with international literature that highlights the economic viability of hybrid SAHP and PV/T systems. Several studies have shown that the combination of renewable power and heat pump technology reduces operating expenditures and improves life-cycle cost-effectiveness for households [15, 16]. The particularly high cost savings in this study are further reinforced by Indonesia’s tropical climate, where consistently strong irradiance ensures high PV yields, while high cooling demand guarantees continuous availability of condenser waste heat for recovery [6, 7].

 In applied practice, the results confirm hybrid AC–PV water heating as a highly cost-effective solution. For low-capacity households, near-total grid independence minimizes vulnerability to tariff fluctuations and power outages. For higher-capacity applications, the absolute financial savings strengthen the economic rationale, particularly when supported by government incentives or net-metering schemes for rooftop PV adoption [16].

From a scientific perspective, the results validate hybrid system integration as an effective optimization pathway. By simultaneously exploiting the thermal dimension (waste heat recovery) and the electrical dimension (PV substitution), the hybrid HPWH achieves EUFs that are unattainable by conventional single-source systems. This conclusion is consistent with current trends in PV/T hybrid studies, which emphasize the superiority of dual-use strategies for sustainable residential energy solutions [1, 15].

Regarding societal and regulatory aspects, the implications are equally significant. At the household level, large-scale adoption could directly reduce peak electricity demand, thereby alleviating stress on national infrastructure during high-demand periods. In line with Indonesia’s rooftop PV subsidy programs, hybrid HPWH systems can accelerate residential adoption of renewable energy technologies, contributing to the government’s targets for expanding solar penetration while simultaneously lowering household energy costs [16].

In general, the energy consumption and cost analysis shows that hybrid HPWH systems provide compelling financial advantages in addition to technical and thermal benefits. With grid dependency reduced by about 94%, annual savings exceeding IDR 8 million, and scalability validated across capacities, hybrid AC–PV water heating emerges as a cost-effective and sustainable solution for both residential and small-scale commercial applications in tropical regions.

3.4 Economic viability: ROI and payback

The economic viability of the hybrid HPWH system was assessed using PP and ROI metrics, which together provide a comprehensive picture of financial feasibility. Across all operating modes, the results confirm that the system is not only technically sound but also financially attractive under Indonesian conditions.

For the HPWH–PLN configuration, the PP was estimated at 5.3 years, with an ROI of 383%. These values are consistent with regional reports on solar-assisted heat pumps (SAHP) in Southeast Asia, which typically document PP in the 4-6 year range, depending on system size and tariff structure [1, 4]. The relatively short PP reflects the efficiency advantage of heat pumps compared to resistive water heating, which directly consumes electricity without a performance multiplier.

Under PV-only operation (HPWH–PV), the PP extended slightly to 5.8 years, with an ROI of 350%. The longer PP arises from the additional upfront cost of PV modules, mounting structures, and storage components. However, the system still shows strong profitability, as PV electricity replaces nearly all grid demand, yielding substantial long-term energy independence.

The Hybrid PV–grid mode provided the most compelling ROI, reaching 824%, far above the 500-700% ROI typically reported for European SAHP installations [2, 5]. This notable outcome reflects the dual advantages of reducing dependence on the electrical grid while harnessing the abundant solar resources characteristic of tropical regions. However, the hybrid system required a longer PP of 8.2 years due to higher capital expenditure. This trade-off shows the financial dynamics of hybrid adoption: higher upfront investment can delay cost recovery but significantly enhances profitability over the system’s lifetime.

A breakdown by capacity shows further nuance. The 0.5 PK unit achieved the shortest PP, attributed to its relatively low capital cost. However, its smaller baseline load limited absolute savings, reducing cumulative ROI. By contrast, the 2 PK unit delivered the highest ROI, driven by larger absolute energy savings despite a longer recovery time. This trend underscores the distinction between adoption pathways: smaller-capacity systems are attractive for households prioritizing rapid payback, while larger units suit multi-user households or small commercial facilities aiming for maximum long-term profitability.

These results are consistent with the techno-economic observations of Icaza et al. [26], who emphasized the importance of local conditions—including grid tariffs, PV investment scale, and component efficiency—in shaping ROI and PP outcomes. In Indonesia, relatively low electricity tariffs lengthen PP compared to regions with higher tariffs. However, the system’s ability to substitute grid electricity with PV energy compensates for this limitation, boosting ROI through cumulative lifetime savings.

Operationally, the results validate hybrid AC–PV systems as flexible solutions for diverse user groups. Households with modest energy needs can achieve near-complete grid independence with smaller systems, while larger households or businesses can benefit from significant annual savings—up to IDR 8 million per year—through higher-capacity installations. Both pathways present attractive options depending on user priorities: short-term recovery versus long-term profitability.

In terms of performance analysis, the analysis highlights the importance of integrating ROI alongside PP in evaluating sustainable technologies. Payback alone may underestimate the long-term value of hybrid systems, particularly in tropical contexts where PV availability and AC loads are both consistently high. ROI analysis, which captures cumulative lifetime savings, offers a more complete measure of economic sustainability. This aligns with global trends in renewable energy economics, where lifecycle evaluations increasingly inform system design and policy [2, 5, 15].

From a policy perspective, the high ROI and favorable PP position of HPWH systems as promising candidates for inclusion in green financing mechanisms, carbon credit initiatives, and renewable energy subsidy programs. By combining financial returns with CO₂ emission reductions, hybrid adoption can support Indonesia’s renewable energy targets and reduce household reliance on PLN during peak demand. Policy instruments such as rooftop PV subsidies and net-metering could further accelerate adoption, making hybrid systems a cornerstone of sustainable household energy strategies [16, 26].

A detailed comparison among the three AC capacities (0.5, 1, and 2 PK) reveals distinct trade-offs between cost and environmental performance. The 0.5 PK unit, with its lower capital and operational cost, achieved the shortest payback period (≈ 5.3 years) and the lowest daily grid electricity consumption, reaching near-complete energy independence under PV-only mode. However, its smaller load results in a lower absolute CO₂ reduction of around 2.1 tonnes per year. In contrast, the 2 PK system, while requiring higher upfront investment and longer payback (≈ 8.2 years), delivered the greatest total energy savings and emission reduction—up to 4.7 tonnes CO₂ annually—due to its higher heating load and continuous waste heat recovery. The 1 PK system represents an intermediate configuration, balancing moderate cost, energy savings, and sustainability impact. These results demonstrate that system capacity directly influences both financial and environmental performance: smaller systems offer faster economic returns, whereas larger systems maximize absolute decarbonization potential. Such capacity-based optimization aligns with findings by Icaza et al. [26] and Zohri et al. [1], who also reported that scaling in hybrid heat pump systems enhances total lifetime efficiency and emission reduction in proportion to compressor size.

The economic analysis validates the hybrid HPWH as both feasible and attractive, offering distinct benefits for different users. With ROI values up to 824% and PP as short as 5.3 years, the system shows clear financial advantages that complement its technical and environmental performance. These outcomes reinforce the hybrid HPWH as a scalable, sustainable solution for household and commercial energy use in tropical regions.

3.5 Environmental impact

The environmental performance of the hybrid HPWH system was highly significant when benchmarked against conventional electric resistance water heating. Based on Indonesia’s official grid emission factor of 0.85 kg CO₂/kWh, a resistive water heater consuming 16 kWh/day emits approximately 13.6 kg CO₂/day. In contrast, the HPWH–PLN configuration reduced emissions to 4.7 kg CO₂/day, while the Hybrid PV–grid mode lowered emissions even further to only 0.8 kg CO₂/day, representing a 94% reduction. Under PV-only operation (HPWH–PV), emissions were eliminated, as the compressor was powered exclusively by renewable solar electricity.

When analyzed by capacity, the scalability of benefits becomes clear. At 0.5 PK, PV-only operation enabled near-total elimination of emissions, reflecting the lower load and higher likelihood of full PV substitution. For the 2 PK system, which still required partial grid support under hybrid operation, the largest absolute reductions were observed due to the higher baseline load. Specifically, shifting from 13.6 kg/day under resistive heating to 0.8 kg/day under hybrid mode avoided approximately 4.7 tonnes of CO₂ annually. To contextualize this impact, the reduction is equivalent to offsetting emissions from a gasoline vehicle traveling nearly 20,000 km per year [6].

These reductions are to be noted when compared with international benchmarks. Global SAHP literature typically reports 70–90% emission reductions [15], while the hybrid HPWH system in this study exceeded 94% in hybrid mode and achieved complete elimination in PV-only mode. The superior performance can be attributed to three main factors. First, the tropical solar resource in Pontianak (4.8 kWh/m²/day) ensured consistently high irradiance, enabling reliable PV operation [16]. Second, the prevalence of AC use in tropical households created substantial recoverable waste heat that would otherwise be rejected to the ambient environment [4]. Third, the inefficiency of resistive heating as a baseline amplified the relative benefits, as substitution with high-COP heat pumps led to disproportionately large emission reductions [7].

Environmentally, the results confirm that even small-scale hybrid systems yield significant environmental benefits. Households with modest demand can adopt 0.5 PK PV-dominated systems for near-zero emissions, while 1–2 PK systems maximize absolute reductions suitable for multi-user households or small commercial facilities. Therefore, the adoption of hybrid AC–PV systems provides both micro-level household benefits and macro-level contributions to national decarbonization strategies.

From an innovation standpoint, this study provides empirical evidence that integrating PV electricity with AC waste heat recovery is an effective emissions mitigation pathway for tropical regions. By simultaneously leveraging renewable substitution and waste heat utilization, the system achieves higher EUF and a lower carbon footprint than either approach in isolation. This aligns with contemporary results emphasizing the importance of integrated renewable–mechanical systems in achieving deep household decarbonization [1, 15].

From a socio-economic perspective, the results strongly advocate for the integration of hybrid AC–PV systems into national climate and energy strategies. Indonesia’s commitment to Net Zero Emissions by 2060 and its nationally determined contributions (NDCs) require household-level interventions to reduce electricity demand and carbon intensity. Hybrid water heating represents a viable entry point, targeting Indonesia’s widespread AC ownership. Furthermore, linking such systems to carbon credit schemes, rooftop PV subsidies, or green financing initiatives could accelerate adoption by lowering upfront costs while rewarding emission reductions [26].

Prospective environmental assessments show that HPWH systems achieve superior performance in both relative and absolute emission reductions when compared with conventional benchmarks. With 94% reductions under hybrid operation and complete elimination under PV-only mode, the system shows a scalable and impactful pathway for household decarbonization. Its adoption would directly contribute to Indonesia’s national climate goals and strengthen global efforts toward sustainable energy transitions.

In addition to the quantitative emission reductions, the proposed hybrid AC–PV HPWH system offers practical applicability for households and small businesses in tropical and hot-climate regions. In residential use, the system can be integrated with rooftop PV installations and existing split-type AC to provide simultaneous space cooling and water heating, effectively using waste heat that would otherwise be discharged to the environment. For small commercial facilities such as hotels, restaurants, or laundries with continuous cooling and hot-water demand, the system can operate year-round with minimal grid dependence. Large-scale adoption of such systems could substantially reduce household and commercial electricity consumption and lower CO₂ emissions, supporting national decarbonization policies in tropical countries.

This work is supported by the Ministry of Higher Education, Science, and Technology of the Republic of Indonesia (Grant numbers: 132/C3/DT.05.00.PL/2025, 23/LL11/KM/2025, 113/II.3.AU.21/SP/2025).