Solar farm on terraced farmland versus rooftop PV
A rural council wants to meet a renewable-energy target. A developer offers a 1.2 MWp solar farm on 2.4 ha of abandoned terraced farmland, currently used for occasional grazing. The alternative is rooftop PV on 400 households, averaging 3 kWp each, but grid connection at the substation is limited to 800 kW export and the utility will curtail output above that. The solar farm would need a new 1.5 km line through a valley. Local farmers support the solar farm because it pays rent; hikers oppose losing the terraced landscape. The council has 14 months to deliver and a budget of €900,000. Assume a local yield of about 1,550 kWh per kWp per year.
1. Compare the annual generation and land-use implications of the two options.
Read the worked answer
Generation
- Solar farm: 1.2 MWp × 1,550 kWh/kWp = 1,860 MWh/year.
- Rooftop: 400 × 3 kWp = 1.2 MWp, so also about 1,860 MWh/year at the same yield.
Land use
- Solar farm: 2.4 ha of terraced farmland, plus a 1.5 km line and access tracks. It changes the landscape character and may affect grazing and soil.
- Rooftop: about 15–20 m² per 3 kWp system, so roughly 0.6–0.8 ha of roof, with no greenfield land take and no new valley line.
The generation is essentially equal, but the land-use burden is very different. That does not automatically decide the case, because rooftop schemes have higher transaction costs and the grid constraint applies to both.
2. Analyse the effect of the 800 kW export limit on each option.
Read the worked answer
The substation can accept only 800 kW export, while both options have 1.2 MWp of panels. Output above 800 kW at midday will be curtailed unless it is stored or used locally.
- Solar farm: output is exported in bulk, so curtailment is direct. If midday output exceeds 800 kW for a significant number of hours, annual losses could be several percent to over 10%, depending on the inverter rating and generation profile. The developer may size the inverter at 800 kW AC, which reduces clipping but also reduces peak output.
- Rooftop: household demand can absorb some generation behind the meter, especially in the evening and morning, but PV peaks at midday when household demand is low. Export will still be constrained. The advantage is that self-consumed electricity avoids curtailment and reduces bills.
The grid limit therefore favours a solution that maximises self-consumption and includes storage or demand shifting, rather than simply adding more panels.
3. Recommend a delivery mix for the council and justify it against the budget, timeline and landscape concerns.
Read the worked answer
A defensible mix is:
- Prioritise rooftop PV on as many of the 400 households as the budget allows, with a coordinated bulk purchase and a short connection queue. Rooftop avoids the 2.4 ha land take and the valley line, which addresses the hikers' landscape objection directly.
- Add a modest battery or managed hot-water diversion at community or household level to shift midday export and make better use of the 800 kW limit.
- Do not approve the full solar farm on the terraced land unless the grid study shows the curtailment is acceptable and the land is genuinely degraded, not merely abandoned. If a solar farm is still needed to meet the target, require it on lower-value land with no terrace loss and no new valley line.
The budget of €900,000 is tight for 400 rooftops, so the council may need to phase the scheme or reduce the number of households. The judgement is to spend first on the option with the lowest land-use conflict and the highest public acceptance, then revisit the solar farm only for the residual target.