There are many variables to consider when deciding between diesel or natural gas-powered generators.
Typically, the price of natural gas is lower than that of diesel fuel. However, natural gas generators can cost up to twice as much as comparable diesel models. Generally, the smaller the generator, the less differential you will see in price.

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While both diesel and natural gas prices fluctuate throughout the year, the cost of diesel is tethered to the global oil supply, so it fluctuates considerably. In a worst case, the cost to refuel your generator could go up hundreds of dollars overnight and could continue to rise unchecked.
Natural gas is a cleaner fuel and picking a gas generator can win you green points.
Natural gas is more abundant, on a global scale, than diesel. However, diesel may be more widely available near your site, as gas lines aren’t ubiquitous. If you need a generator and don’t have pipeline access nearby, diesel makes more sense.
If you plan to use a diesel generator, check the fuel regularly, because diesel can spoil.