This simple, practical guide offers a transparent way to compare the climate-effectiveness of different ways to provide electrical services—specifically, different ways to displace coal-fired electricity. Its worked examples show manyfold to over 50-fold differences in “Climate Effectiveness” (carbon saved per dollar spent) between common options, depending on their relative emissions and costs. To save the most carbon at the least cost requires a metric and an analytic method combining both emissions and costs; neither criterion by itself can yield sound choices to protect the climate. The proposed method needs only basic arithmetic and a hand calculator, and can be applied to any resource comparison for any sort of emission or other consequence. The four worked examples apply the latest U.S. market cost data for 3Q2019, and can be easily updated or modified to fit local conditions or other data sources. Additional issues bearing on resource choices, including relative speed of deployment, are also briefly reviewed.