Choosing between rooftop PV and a solar water heater
The Camilleri family in Naxxar uses 3,800 kWh of electricity a year, of which 1,800 kWh heats water in an electric cylinder. Their south-facing roof has 18 m² of usable area. They have €6,000 and consider two options. Option A: a 3 kWp solar photovoltaic system costing €4,500. In Malta it should generate about 1,400 kWh per kWp per year. Without a battery they expect to use 40% of the output on site and export the rest. Imported electricity costs €0.20/kWh; exported electricity earns €0.10/kWh. Option B: a 2.5 m² solar water heater costing €2,800. It should supply about 70% of the annual hot-water energy, with the electric cylinder providing the rest. The family expects to stay in the house at least 15 years. A neighbour says photovoltaic panels are useless in winter, when the family uses most electricity, and that the solar heater is always better because it saves hot water directly. The family asks which option to choose.
1. Calculate the simple annual saving and payback time for each option, stating your assumptions.
Read the worked answer
PV: 3 kWp × 1,400 kWh/kWp = 4,200 kWh/year. Self-used 40% = 1,680 kWh, saved at €0.20 = €336. Exported 60% = 2,520 kWh at €0.10 = €252. Total saving = €588/year. Simple payback = €4,500 ÷ €588 = 7.7 years. Solar water heater: 70% of 1,800 kWh = 1,260 kWh saved at €0.20 = €252/year. Payback = €2,800 ÷ €252 = 11.1 years. Both are within the 15-year horizon, but the PV has the shorter simple payback and saves more each year. The result depends on the 40% self-use assumption: if only 30% is self-used, PV saving falls to about €546 and payback rises to 8.2 years. The calculation also ignores degradation, maintenance, export tariff changes and the time value of money, so it is a screening tool, not a full life-cycle comparison.
2. Evaluate the neighbour's two claims using the data and your knowledge of seasonal demand and solar output.
Read the worked answer
Winter output is lower, not zero. In Malta a well-oriented panel might produce roughly 2 to 3 kWh per kWp per day in winter and 5 to 6 in summer, so the annual figure of 1,400 kWh/kWp already averages the seasons. The family's winter use may be higher for lighting and hot water, but the PV still offsets imported units whenever the sun is up, and the exported units earn money. The solar heater does save hot water directly and is efficient, but it only addresses 1,800 kWh of the 3,800 kWh load, and it can overproduce in summer. It also has maintenance and roof-space costs. 'Always better' is therefore too strong: the heater has a longer payback here, while PV saves across the whole load. The best choice depends on the load profile and whether the family can shift use, such as running the washing machine in daylight.
3. The family could instead spend €6,000 on both a smaller PV system and a solar heater. What additional information would you need, and what trade-offs would guide the decision?
Read the worked answer
I would need the hourly load profile, the roof layout and shading, the hot-water tank size, any subsidy or export limit, and the maintenance costs of each system. A combined system could use 10 to 12 m² for PV and 2.5 m² for the heater, but the budget of €6,000 may force a smaller PV array. The trade-offs are: lower import bills and shorter payback (PV) versus direct hot-water savings and less use of roof area per kWh saved (heater); summer overheating and stagnation (heater) versus winter output and export tariff risk (PV); and embodied carbon and end-of-life disposal for both. A combined system is more resilient because it saves both electricity and hot water, but it is harder to control cost. I would model two or three sizes rather than assume that spending the full budget on one option is best.