If you have a 12 VDC power system, like the battery of a PV solar system or car, would it be more efficient to boil water for that cup of tea with that 12V straight from the battery, or use a 240 VAC mains kettle via a ~90% efficient inverter instead? That’s the question that [Cahn] decided to answer experimentally, using a bulky 3 kW inverter and a collection of electric kettles.Although the used amount of 500 mL of water is boiled much faster in the 2,200 Watt mains kettle than in the 150 and 350 Watt low-voltage kettles, this obvious difference is somewhat irrelevant if you’re only concerned with efficiency. To measure the power used a Victron smart shunt was used with each run, keeping in mind that a perfect efficiency for heating 500 mL from room temperature to boiling is around 43-44 Wh.With two runs per kettle, the 240 VAC kettle used 65-70 Wh. The first ‘150 Watt’ kettle pulled nearly 200 Watt to boil the water after about 20 minutes, using 62-64 Wh. The second ‘150 Watt’ kettle pulled around 180 Watt, took 23-25 minutes and used 68-74 Wh. Finally, the ‘350 Watt’ kettle drew over 420 Watt and used 50-56 Wh in just over 8 minutes.When you look at the final results, it’s interesting to note that the low-voltage kettles got both first and last place in this contest, even when factoring in the inverter losses for the 2.2 kW kettle. This makes it quite obvious that the issue at hand is less about DC vs AC or mains vs low-voltage.The 350 Watt kettle is clearly better designed, featuring a level of insulation that the cheap 12V kettles lack, while pumping more energy into the water at a much faster pace due to the higher current.Of course, this also shows the whole headache of using 12 VDC appliances like this, as you can only pull so much current from a cigarette lighter socket, while connecting directly to the battery and its juicy 100 A or more poses its own logistical problems. Taking the inverter losses as the price to pay for convenience is thus another totally valid option whenever you’re out camping or at that off-grid cabin.