Accounting is a black art. The "cost" could be the monthly kerosene bill or the payments for a solar system. The payments for a solar system are dependent on the interest rate on the loan, which is finally a matter of public policy. The kerosene bill is also an artifact. Those numbers hardly reflect the real labor costs or the real resource costs.
What is really the cheapest source of energy? If the final costs could be known and accounted for wouldn't it be that the monthly cost is hardly important. The final results of pollution and depletion on the planet's population can't be know or measured, but the folly of fossil fuel consumption makes the final cost of solar equipment seem tiny.
The solar equipment could last for decades, and its money cost could be amortized over that long period. The money cost of kerosene is likely to go up due to scarcity, while the money cost of the solar alternative could be accounted for as zero after the equipment has been paid off.
Multi-dimensional good judgment, not money cost, is the only sane guide to investment decisions. The short-term money cost which rules our thinking and our economy is a form of blindness. When we confuse a rate of payment with a final cost, our accounting is a farce.
Barry