Unlike chemical batteries, gasoline generators consume a minimum amount of fuel even at zero current draw. [The battery equivalent is the inverter idle current, but this is typically only a few watts.] To a first approximation, internal combustion generator fuel consumption (liters per hr, c) as a function of energy output (kW, w) is linear [c(w) = pw + i], where the power-proportional consumption (p) and idle consumption (i) tend to be inversely proportional. Manufacturers usually advertise only a single value of this function, the “runtime at half of max load”. When published, c(0) is generally 30-60% of c(maximum sustained instantaneous power).
Typically, small 2-stroke generators are initially cheaper and more efficient at low output powers (lower i) than their larger 4-stroke brethren, but less efficient at high output loads (higher p). The ETQ TG1200 2-stroke gasoline generator ($140-$200), produced by Eastern Tools, advertises unusually efficient consumption: i=.0682gal/hr, p=0.114gal/kWhr. It produces 120VAC utilizing a “computer friendly sinewave alternator (with THD level under 6%)”. [I have not verified the output waveform's sinusoidal purity.] These fuel consumption coefficients are calculated from the manual, which states a tank capacity of 1.0 gallons and runtimes of 8 hours at 500W and 5.5 hours at 1000W. Preliminary analysis of the experimental data indicates i=0.0739gal/hr, p=0.127gal/hr/kW for a TG1200 unit. [UPDATE: See post on further measurements and analysis for final numbers.] These coefficients are 11.4% and 8.4% higher than advertised, respectively, but still place the TG1200 in the upper echelon of small generator efficiency. At $2.50/gallon gasoline, the latter numbers equate to $0.19/hr at idle, adding $0.32 per kW output. In chemical battery terminology, the TG1200 has an “inverter idle draw” of 48.5A (582W).
ETQ TG1200 - Experimental setup
The experimental setup is pictured above. A (new) ETQ TG1200, fueled with 87 octane gasoline mixed at a 50:1 ratio with Pennzoil 2-stroke oil, powers an analog clock “uptimer”. The clock is normally powered by a single AA battery (1.25 to 1.5V); it is converted to 120VAC operation by a 3VDC adapter and a resistive divider (two 36 ohm resistors). The generator is run on a rubber mat atop a cement floor, ungrounded, 12.8 Celsius ambient, cooled by an external box fan. The first hour of (idle) operational data is discarded, as some fuel is expected to be absorbed at first run. Various halogen bulbs are combined to achieve the non-idle power draws.
The source data will be published after further measurements at higher output powers. [The initial measurements were clustered between 0 and 600W to determine i precisely.] Below is a preliminary trendline.
ETQ TG1200: 2009-12-22 preliminary trendline
