Compare two fictional proposals delivering the same 1500 kWh of useful electricity per year for 12 years. A (local panels and storage) has initial embodied emissions 4000 kg CO₂e and operating emissions 0.05 kg CO₂e per delivered kWh. B (a grid-only plan) has initial emissions 400 kg CO₂e and operating emissions 0.35 kg CO₂e/kWh. The supplied rates include operating losses; ignore replacement and disposal only for this first calculation. Find total emissions over the stated horizon and the equal-emissions time. Then recommend how the decision should account for this constraint: the storage room reduces accessible indoor space and equipment noise concerns neighbours. Do not assume these fictional factors describe actual products.

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Over 12 years: A 4900 kg CO₂e, B 6700 kg CO₂e. Equal-emissions time 8 years. A has the lower modelled total; adoption also depends on the stated service and stakeholder constraints. Both proposals deliver the same useful service, so their operating emissions can be compared on a common boundary. Over 12 years the delivered energy is 1500 × 12 = 18000 kWh. Add initial and operating contributions. A gives 4000 + 18000 × 0.05 = 4900 kg CO₂e; B gives 400 + 18000 × 0.35 = 6700 kg CO₂e. A starts with 3600 kg CO₂e more embodied emissions but saves 450 kg CO₂e each year of operation. Dividing gives an equal-emissions time of 8 years under the constant-rate assumptions. A carbon ranking is only one decision input: the storage room reduces accessible indoor space and equipment noise concerns neighbours. Discuss who receives the benefits, who pays upfront, and whether either proposal reliably delivers the promised service before choosing. Make the recommendation conditional on verified service life, repair and replacement needs, disposal and representative operating factors. The stated calculation intentionally omits those effects, so it cannot establish the full life-cycle impact or guarantee that the ranking survives changed assumptions.

A pavilion energy proposal

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