When choosing a generator, your needs determine it’s size. It’s really that simple.
If you just want it for recreational purposes, maybe to run some light power tools off-grid and as emergency backup power to keep your fridge and freezer running through a blackout, so that the food doesn’t spoil as quickly, then your best choice would be a general purpose portable generator, with around 1,500-3,000 watts of electrical power.
But there are even smaller generators available for those who already have a renewable energy system set-up at home, and want to use it as emergency backup power to charge their battery banks. Generators like this may only have 500-1,000 watts electric power capacity, but they are much more fuel efficient than the bigger generators. Tiny generators are also cheap, they can cost as little as $100 ($50 used) and are an ideal purchase if you’re experimenting with do-it-yourself wood gasifiers or biogas digesters as alternative renewable energy sources.
For off-grid construction and light industrial purposes, a larger more powerful generator would be recommended. Something with around 3,500-6,000 watts, depending on what kind of equipment you want to run, and how many people will be using power tools at the same time. These generators are however not as good for recreational purposes because they tend to be very heavy, but are still a decent choice for emergency backup power, if you want a little more energy. However, their fuel efficiency is not as good as the lower capacity generators.
The best and easiest to use emergency backup power generators are however home standby natural gas or propane generators. Propane doesn’t go bad in storage, as long as it is stored in a steel propane tank, unlike gasoline and diesel which often spoil after 6-12 months. Natural gas lines most often continue working even during blackouts. In addition to that, standby generators are hooked up directly to your home electrical system and turn on automatically when a electrical power outage is detected, and don’t require refueling or fuel storage if connected to the natural gas line. Home standby generators are 6,000-7,000 watts at the lower-end, but our recommendation would be a 10000 watt or larger standby generator, which will cover most of your energy needs if you have efficient appliances.
However, if you intend to power a tankless water heater, you may need to aim for something bigger like 20,000 watts. While their total energy consumption may be low, on-demand water heaters tend to require lots of electric capacity (10-20kW).
For farms, workshops and small factories, commercial natural gas or propane generators are the way to go. These range from 20,000 watts to 250,000 watts, but your specific needs determine what size you want to go for. You have to do some calculations to make a reliable estimate.