# What does “efficiency” refer to for a refrigerator?

A problem from my textbook mentions a refrigerator working with $$35\,\%$$ efficiency. I thought that in this case the coefficient of performance (COP) was used, specifically $$Q_\mathrm{cold}/W,$$ which is different from the COP of a heat pump.

Let's say it refers to $$W/Q_\mathrm{hot} = 0.35,$$ but the refrigerator works between two reservoirs: one at $$\pu{0 °C}$$ and the other at $$\pu{20 °C}.$$ If we use the temperature ratio for efficiency, it can't be above $$35\,\%.$$

So, what does $$35\,\%$$ refer to? Does it refer to $$35\,\%$$ of the maximum efficiency?

• @ToddMinehardt Hmm, for thermal pumps like refrigerators, is not the thermal efficiency rather $\eta = \frac{Q_\mathrm{in}}{Q_\mathrm{out}}$ ? With $\lim_{T_1 \rightarrow T_2}{(Q_\mathrm{in})}=Q_\mathrm{out}$ for the ideal Carnot-like case? Your efficiency seems to me rather for heat engines which have rather complementary efficiency to the one of heat pumps. Commented Nov 17, 2021 at 9:13
• P.S.: Like $\eta_\mathrm{pump} + \eta_\mathrm{engine} = 1$ for the same $T$ range and Carnot-like cycles. Commented Nov 17, 2021 at 10:36