Choosing a generator
In a solar system, the generator is the backup for grey days and winter. Chosen well, it runs little and burns little. Here are the kinds, and a tool to find the size and what it will cost you in gas.
Tool: What size, and how much gas?Kinds of generators
Quiet (“inverter”)
- For:
- charging batteries, a trailer, a cabin, backup for the fridge and lights.
- The upside:
- It slows down when little is asked: much less gas and noise. Its power is clean, inverter-chargers accept it well.
- Watch out:
- Smaller (about 1,000 to 4,000 W) and pricier per watt. For more power, you pair two.
Contractor (open frame)
- For:
- big start-ups: well pump, tools, 240 V.
- The upside:
- Lots of watts for the price, often with a 120/240 V outlet.
- Watch out:
- It runs at full speed all the time, even to charge a small battery: loud and thirsty. Its power is less stable: some inverter-chargers refuse it.
Dual fuel or propane
- For:
- a cabin where propane is already there (fridge, stove, furnace), or to store fuel for a long time.
- The upside:
- Propane doesn't go stale in the tank and doesn't gum up the carburetor.
- Watch out:
- On propane it gives about 10% less power. Propane vaporizes poorly in deep cold.
With auto start
- For:
- an off-grid system where nobody is there to start it: it starts on its own when the batteries get low.
- The upside:
- The batteries never go too low, even when you're away.
- Watch out:
- It needs an electric starter and a “two-wire” contact that the inverter-charger or controller can drive. Check before buying.
What size, and how much gas?
Check what the generator has to run at the same time. If it charges your batteries, check the charger too.
710 W
running
1,310 W
at start-up
1.3 L
of gas per day
39 L
per month (30 days)
A generator of at least 2,000 W running.
About $2.14 per day, $64 per month.
With batteries and a charger, the generator only runs a few hours, well loaded, then stops: that's where you save the most.
Ballpark figures. Consumption follows the average load (devices are counted as running the whole time the generator runs). We keep a 20% margin under the generator's power, and only one motor starts at a time.
Words to know
- Running watts
- What the generator delivers for hours. The real number to compare.
- Starting watts
- What it gives for a few seconds to start a motor. Often the big number on the box.
- THD
- Power quality: the lower, the cleaner. Under 3%, electronics and inverter-chargers accept it without trouble.
- Eco mode
- On a quiet generator: the engine slows down when little is asked. You sometimes have to turn it off for a big start-up.
- “Two-wire” start
- A simple contact that starts the generator. It's what makes auto start possible.
- Altitude loss
- An engine loses about 3% of its power per 300 m of altitude.
Classic traps
- Buying by the starting watts printed big: a “4,000 W” often gives 3,200 W running.
- Using a big contractor generator to charge batteries: it burns almost as much gas idling as at half load.
- Leaving gas in the carburetor all winter: it evaporates, leaves varnish, and the generator won't start in spring. Drain it or add a stabilizer.
- Running the generator in a shed, a garage or near a window: carbon monoxide kills. Always outside, away from openings.
- Plugging the generator into a household outlet to feed the panel: it's dangerous for you and for people working on the grid. You need a transfer switch installed by an electrician.
To calculate
Next step
Wires and fuses: what size?
Voltage, current and length: we give you the wire gauge, the loss and the fuse that protects it.