HOMER will always load one generator fully (the generator with the lowest marginal cost) before increasing the load on any other generator, yet real systems often share loads equally between generators. The reason why we have not made load sharing an option is because this has only a small impact on economics, and it doesn't usually affect the results. Although one generator may be burning more fuel than the other, the operating hours, maintenance and replacements, and etc are all the same as if the load were shared equally (in the HOMER simulation, that is).

So if there were four equal generators and a 25% operating reserve, then HOMER might have three generators near maximum capacity and the fourth one at minimum load. The extra available power in the fourth generator serves the operating reserve. This is not the ideal case for a real system, but in HOMER, the costs and fuel consumption are exactly the same as if the load was shared equally, so we don't make the extra options and logic to do equal load sharing. If the load increased at all, HOMER would need to turn on a fifth generator, even though the fourth generator is still at minimum load.

HOMER uses the fixed generation cost and marginal generation cost to decide which generators to turn on and which ones to ramp up. The Fixed [hourly] Generation Cost and Marginal Generation Cost [per kWh] are listed in the generator's tab of the simulation results window (see attached). You should be able to replicate these numbers with your own calculation:

Fixed Generation Cost = O&M [$/hr] + Wear cost [$/hr] + Fuel cost [$/hr]
Fixed Generation Cost = O&M [$/hr] + (Replacement cost [$] / Lifetime [hr]) + (Fuel curve intercept [L/hr/kW] * Generator capacity [kW] * Fuel price [$/L])

Remember that to calculate the O&M cost, you have to multiply by the generator capacity. The default O&M cost in the generic generators is 0.03 $/hr/kW — for a 500 kW generator, that's $15/hr. The same is true for the replacement cost, which is $500/kW by default — that's $250,000 for a 500 kW generator.

Marginal Generation Cost = Fuel curve slope [L/hr/kW] * Fuel price [$/L]

HOMER uses the Fixed Generation Cost to decide which generator to turn on: the combination of generators that meets the required operating capacity with the lowest total fixed cost [$/hr] will be used. HOMER then ramps up the generator with the lowest marginal cost first in order to meet the load, followed by the other generators in ascending order of marginal generation cost.