Compare two fictional proposals delivering the same 1000 kWh of useful electricity per year for 2 years. A (a roof-mounted system) has initial embodied emissions 2500 kg CO₂e and operating emissions 0.03 kg CO₂e per delivered kWh. B (a supply contract) has initial emissions 500 kg CO₂e and operating emissions 0.3 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 lease ends soon and moving the roof system has not been costed. Do not assume these fictional factors describe actual products.

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Over 2 years: A 2560 kg CO₂e, B 1100 kg CO₂e. Equal-emissions time 7.4074 years. B 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 2 years the delivered energy is 1000 × 2 = 2000 kWh. Add initial and operating contributions. A gives 2500 + 2000 × 0.03 = 2560 kg CO₂e; B gives 500 + 2000 × 0.3 = 1100 kg CO₂e. A starts with 2000 kg CO₂e more embodied emissions but saves 270 kg CO₂e each year of operation. Dividing gives an equal-emissions time of 7.4074 years under the constant-rate assumptions. A carbon ranking is only one decision input: the lease ends soon and moving the roof system has not been costed. 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 community hall may move

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