Design and Integration of a Retrofit PV–Battery System for Residential Energy Savings and Thermal Comfort ^†

Abstract

This study presents the design and implementation of a prototype dual-function photovoltaic window system that integrates flexible solar panels for dynamic shading and a compact lithium battery for local energy storage. The methodology involves developing an experimental setup where translucent, flexible photovoltaic panels are retrofitted onto a standard residential window. The system is connected to a charge controller and a small-capacity lithium-ion battery pack. Key performance metrics, including solar irradiance, power generation efficiency, reduction in thermal transmittance, and battery state of charge, are continuously monitored under varying real-world environmental conditions. The integrated panels can significantly reduce solar heat gain, thereby lowering indoor ambient temperature and reducing the building’s cooling load. Simultaneously, the system will generate sufficient electricity to be stored in the lithium battery, providing a self-contained power source for low-draw applications such as lighting or charging personal devices. This research highlights the viability of developing cost-effective, multi-functional building components that transform passive architectural elements into active energy-saving and power-generating systems in terms of green environment goals.

1. Introduction

Energy consumption in buildings represents one of the largest contributors to global energy use and greenhouse gas emissions. The building sector accounts for approximately 30–40% of total primary energy demand worldwide and around 36% of CO₂ emissions, driven mainly by heating, cooling, and lighting requirements [1,2]. In Mediterranean countries such as Greece, the high solar irradiance and prolonged cooling season further increase electricity consumption for air conditioning, making thermal management and renewable integration key sustainability priorities [3].

Windows are critical components of the building envelope, providing daylight and ventilation but also representing major sources of heat gain and loss. Studies have shown that up to 30% of a building’s heating and cooling load is associated with fenestration surfaces [4]. However, this same surface area presents a valuable opportunity for systems that convert solar radiation into electricity while providing solar shading and improving thermal comfort [5,6]. In this dual role, photovoltaic (PV) windows and façade-integrated systems can reduce cooling loads by limiting solar heat gain and simultaneously generating clean, renewable power for on-site use.

The integration of PV modules into windows or balcony structures has evolved rapidly in recent years, driven by the availability of flexible thin-film technologies and advanced encapsulation materials. Modern semi-transparent and flexible PV panels can be laminated on curved or lightweight substrates to provide partial transparency, uniform appearance, and structural adaptability. According to Dallaev et al. [7], flexible solar panels exhibit conversion efficiencies of 12–18% and can maintain mechanical stability under repeated bending cycles, allowing installation on doors and window frames. Their low weight and reduced balance-of-system requirements make them ideal for decentralized residential systems. Furthermore, life cycle assessments of semi-transparent PV windows show energy payback times as low as 13.8 years and greenhouse gas payback times of about 10.4 years—well within the 25-year service life of typical building components offering long-term environmental and economic benefits [8].

In addition to energy generation, PV windows significantly influence daylight and heat transfer. Li et al. [5] reported that semi-transparent PV windows improve indoor visual comfort while reducing solar radiation penetration. In warm climates, such as southern Europe, this translates directly into lower cooling energy demand. For instance, optimized PV shading devices can reduce a building’s total cooling energy by 15–25% depending on the orientation and cell coverage ratio [9]. Moreover, PV shading elements prevent excessive heat ingress, lowering indoor temperatures by 2–4 °C in warm climates [10,11].

In Mediterranean regions such as Athens, where summer irradiance often exceeds 800 W/m², such reductions can translate into significant air conditioning energy savings. While photovoltaic systems effectively offset grid electricity during sunny hours, energy generation and demand rarely coincide. Battery storage is therefore essential to maximize solar utilization and increase self-consumption. Hybrid PV–battery systems store surplus electricity during the day and discharge it during evening peaks, improving energy autonomy and resilience [12].

In this project, generated electricity is stored in a portable power station, providing about 1 kWh of usable capacity. Lithium-based batteries offer several advantages over traditional lead-acid types, including higher round-trip efficiency (>90%) and deeper discharge capability, with lithium-iron-phosphate (LiFePO₄) batteries operating for over 20 years under moderate temperature conditions with minimum maintenance, making them ideal for compact, decentralized residential applications [13].

While large PV systems have been studied extensively, small balcony-integrated systems remain underexplored, especially in the context of urban apartments [14]. This study proposes a layout that combines flexible PV panels, energy storage, and smart monitoring in a unified module. Research focuses on the feasibility of flexible PV panels on door and window casings and on evaluating the dual effect of solar shading and renewable energy generation on residential energy savings.

2. Materials and Methods

For the testing requirements of the experiment, a simple layout was assembled: Four flexible monocrystalline panels by PlusEnergy and a portable power station were applied. Specifically, all 4 panels installed in the door frame are 50 W modules, with operating voltages of 12 Volts [15]. All PV modules consist of Ethylene tetrafluoroethylene encapsulated flexible panels (∼3 mm thick), which makes them extremely lightweight, durable, and easy to install on the door panel. This coating provides excellent heat, UV, and corrosion resistance, so these panels can handle the hot, harsh conditions of the Greek summer. The PV installation is presented in Figure 1 below.

As the panels are robust and easy to install, they were mounted with double-sided tape on the sides of the panels (at the red lines), covering the whole area of the double-glazed door. In this way power is generated while the solar panels simultaneously provide shading to the house living room, thereby reducing direct solar heating and thus the internal room temperature. For validation a HY02B05-wifi Thermostat with a display accuracy of 0.5 °C and 1% probe fault rate, shown in Figure 2, that is already installed, is utilized to manually check the internal room temperature. In addition, the Xiaomi Mi Monitor 2, with ±0.1 °C temperature accuracy and ±1% relative humidity precision, provides constant monitoring through the Xiaomi Home online application via smartphone or tablet [16,17].

The solar array feeds into a Bluetti AC50P portable power station with an integrated Battery Management System. The AC50P has a 504 Wh LiFePO₄ battery, with a maximum of 3000 life cycles and a 700 W (1.05 kW surge) pure-sine output [18]. It accepts up to 200 W of solar, which is the max output of the PV array enabling a full recharge in roughly 3 h. The unit is compact and light, with built-in AC outlets and USB ports providing a combination of battery longevity, high power, and portability. In operation the Bluetti is charged directly by the panels and then provides AC/DC power to charge smartphones and tablets as well as running small smart appliances like a microwave oven. All photovoltaic panels were connected in series for an output of 24 V and 200 W via 8 mm cables, and the final 8 mm Y splitter was used to provide power to the Bluetti power station power adapter.

During testing, the panels continually supply the portable station, which in turn powers the connected loads (phones, IoT devices, etc.). The passive shading effect of the glass mounted panels also keeps the house cooler under sunlight. Testing was conducted at a household located in Athens, Greece, and specifically in the province of Peristeri (Lat, Long: 37.999764, 23.69319) with 1000 total measurements gathered throughout a 10-day period, between 25 September and 4 October 2025. The final layout of this experimental setup is presented in Figure 3.

3. Results and Discussion

Over the 10-day monitoring period, the four flexible PV panels produced a fraction of the home’s electricity with the array generating over 11 kWh (summing hourly PV output). Daily PV production ranged from 0.55 to 1.45 kWh, for example, on Day 9 the panels yielded ~1.28 kWh whereas on a cloudy day (Days 5 and 8) only ~0.69 kWh was generated. Solar irradiance ranged from 10 to 720 W/m² as the sunshades were always elevated but the photovoltaic cells were positioned at 90° to sunlight so solar absorption was limited.

Figure 4 shows a representative daily profile: solar irradiance and PV power peaking at midday, reaching the 200 W array limit.

The battery’s capacity or state of charge (SoC) profile illustrates that surplus PV energy was stored for later use. In several cases (e.g., Days 6–10) the battery ended the day with higher SoC than it began (indicating net energy storage), whereas on the most demanding day (Day 1) the SoC dropped from 99.5% at 08:00 to 24.4% by 18:00, showing heavy discharge. This on-site generation directly translates into energy savings, as the PV output effectively replaces purchased electricity that would be used for charging smart devices or for appliances like a microwave. The measured capacity factor, roughly 95% of installed PV hours, is consistent with techno-economic studies of small PV–battery systems [3,19].

The LiFePO₄ battery’s high efficiency and depth-of-discharge enabled effective buffering of solar production. Over each day, the battery absorbed excess PV power and delivered it during peak load. For instance, on Day 7 the battery SoC rose from 30% at dawn to 52% by evening, storing a net amount of over 120 Wh, whereas on Day 1 the large evening loads drained about 379 Wh or 75% SoC. Figure 5 illustrates SoC and load versus time, showing that during peak sun the battery was often fully charged and during the evening it provided supplemental power for charging or light cooking at 500 W maximum. This behavior, which prevents PV overproduction and supplies stored energy after sunset—exemplifies the core benefit of hybrid PV–battery systems while ensuring minimal losses during cycling. In addition, the battery SoC was kept within the 20% to 90% range, when possible, to extend battery life, measured as the State of Health (SoH) value [20].

As mentioned before, the PV installed in window panels served a dual role as passive solar shading. Temperature data confirm a modest cooling effect: during peak insolation the indoor air was consistently a few degrees cooler than outdoors. For example, on Day 1 the outdoor temperature reached 31 °C at 14:00, whereas the living room stayed near 29 °C under the shaded window. On average in the afternoon hours, indoor temperature was about 1–2 °C below the outdoor reading, except when ambient temperature was below 19 °C so shading had a minimized effect. This finding agrees with a prior report that PV-integrated shading can lower indoor heat gain by 2–4 °C [21]. Air conditioning was not required except at peak midday hours where temperature exceeded 30–32 °C. Weather on most days was sunny, except Day 4 and 7 which were partly cloudy and Days 5 and 8 which were partly overcast; however, this did not affect the PV as the irradiation was sufficient to charge cover battery charging. By reducing solar heat ingress, the system lightens the air conditioning load, a major energy consumer in Mediterranean climates, and thus further contributes to energy savings. Figure 6 illustrates the internal and ambient temperatures of each day throughout the 10-day period.

The solar contribution translates directly into lower utility bills. Over 10 days, the system generated roughly 11 kWh. At contemporary Greek household rates, €0.22/kWh represents roughly €2.50 in avoided electricity costs according to Eurostat [22]. In context, a full year at this 10-day yield rate, with approximately 410 kWh generated, the system would save on the order of €90 annually. While the demonstration period was short, the data indicate meaningful cost offsets: every kWh of PV power essentially displaces grid energy. Prior analyses of similar PV–battery systems report levelized generation costs around €0.50/kWh comparable to retail tariffs, suggesting that even small installations can be economically competitive. Thus, the experiment confirms that the proposed system can reduce electricity expenses, with payback improving as component costs decline.

Table 1. Summarizing results of this paper in each sector.

Category	Value	Comments
Energy Generation	11 kWh	Total produced
	0.55 kWh–1.45 kWh	Daily
	200 W	Peak PV power
	410 kWh	Annual projected
	10–720 W/m2	Solar irradiation
Thermal Comfort	1–2 °C	Reduced indoor temperature
Energy Management	75% SoC or 379 Wh	Maximum battery discharge
	95%	PV capacity factor
	500 W	Maximum load
	20 to 90% SoC range	Battery life prolonging
	Minimum solar power losses	PV overproduction limitation
	Battery SoH reduction	0%
Projected Savings	€90	Annually
Total Purchase Cost	€300	Components and cables
Usability and Practicality	Ready to use system	Ease and safe installation
	Gains	Cost-effective, robust, user-friendly

summarizes the results of this paper regarding energy generation and management, thermal comfort along with projected savings:

4. Conclusions

In this paper a hybrid PV–battery system was implemented consisting of 4 × 50 W flexible solar panels and a portable power station by Bluetti. The experimental results demonstrate that the proposed low-cost PV–battery system can effectively harvest solar energy while providing passive shading and thermal comfort benefits. Over the 10-day trial, the four-panel flexible PV array produced over 11 kWh of energy (about 0.55–1.45 kWh per day). The integrated LiFePO₄ battery efficiently buffered this output: on most days the battery ended with a higher state of charge than it began (net charging), and on high-demand days it supplied substantial evening power (e.g., ~379 Wh or 75% SoC supplied at peak load on Day 1). The system maintained a high effective capacity factor (≈95% of peak hours). In parallel, the window-mounted panels provided a cooling effect: indoor air temperatures remained about 1–2 °C below outdoor air temperatures during peak sun.

These combined effects translated into tangible energy savings. Every kWh of PV generation displaced grid consumption, amounting to roughly EUR €2.50 of avoided electricity costs over 10 days (equivalent to ~€90/year). Although the demonstration period was short, the data indicate that even a small-scale installation can yield meaningful cost offsets. Air conditioning operation was limited except midday hours where temperatures were highest, so if this aspect is added, the benefits can be doubled with the same layout, but this project is left for future work. Overall, the prototype proved to be a robust, user-friendly solution that simultaneously generates clean electricity and reduces solar heat gain. These findings confirm the feasibility of an inexpensive, easy-to-install PV–battery monitoring tool for residential use, improving both energy autonomy and thermal comfort. For future work four main points must be reviewed:

• Cost-effectiveness and scaling: A detailed techno-economic analysis should be conducted so the model is scaled regarding incentives and electricity tariffs.
• Retrofitting in urban apartments: Broader implementation requires addressing practical installation issues in high-density buildings.
• User accessibility and smart interfacing: Future work can incorporate real-time monitoring dashboards empowering occupants to manage consumption with ease.
• Grid-assisted functionality and smart home integration.