Four More Amps

Or, how to buy an emergency generator

My wife and I decided to get a generator after Hurricane Matthew proved to us that power outages are just a fact of life where we live. So, this is a quick post about generators and how we ended up picking the one we did.

1.21 gigga-watts!

When buying generators, the first thing you are going to see is the generator’s wattage. This is the number manufacturers splash across the boxes to show that their’s is better than the other guys. It’s a number like “megahertz” or “gigabyes”, it means something, and more is usually “better”, but it doesn’t tell the whole story. So before we get to wattages, let’s do a quick intro to electric power.

Electricity has three parameters we’re interested in today: Voltage, Frequency, and Current. Today, we are not going to worry about voltage or frequency because those things are fixed values by convention. In the U.S. it’s 120 or 240 Volt, 60Hz service. Done.

Current is a different story. Current is the rate at which electricity gets used. If you were to grab a wire in your house, and point at a particular point, current is the number of electrons per second that flow past your finger.

If you have two electric things you want to power, the amount of electricity they use is the sum of their individual uses. That is: $I_{thing_1} + I_{thing_2} = I_{total}$.1 This is in contrast to voltage where, if everything is working right, no matter how many things you plug in, you should always measure 120V.

But generators don’t have amp ratings plastered all over the boxes, they have ‘Watts.’ Knowing either V or I, Watts are easy to calculate $V * I = W$. And, as it happens, we know V well enough. So, because we know V, and we know V isn’t supposed to change, then more Watts $\Rightarrow$ more Amps.

In a big rush

Now, when you first turn things on, those things get themselves in a big rush to get started on whatever it is they do. So things tend to use more electricity when they are first turned on than after they’ve had a little while to settle down a bit. This makes a lot of sense: when you first plug a fridge in, it freaks out because the temp in the fridge is so high, so it does a bunch of work (and used a bunch of electricity) getting everying all cooled off.

Combine these two concepts: 1) your total current draw is the sum of each thing’s draw 2) and that things tend to use more electricity when you first turn them on; you can see that: there are two kinds of loads a power system might see: in-rush power; and normal (“steady-state”) operation.

Inrush Current Illustration

Portable Generators

Generators work by using a gas motor to spin the shaft of an electric motor. Electric motors are funny things, if you put electricity in to them, the shaft spins. On the other hand, if you spin the shaft, you get electricity!

Generators usually have two power ratings: a Max rating; and a Continuous duty rating. The continuous duty rating is the amount of power the generator is designed to provide for an indefinite period of time. For example, if you wanted to run your generator for 12 hours, you can not use more than the continuous duty power. Otherwise the generator works too hard, overheats, and then… BOOM!2

The Max or Surge rating is the amount of power you’re allowed to use for a very short period of time (say: less than five seconds). This extra power allows you to cover small overloads of the generator.

The other things that’s interesting about generators is how they actually generate electricity. The inexpensive caugh cheap caugh generators hook the electric output almost directly to the alternator windings with very little conditioning. This has a number of implications:

  • They have to spin at very specific speeds to make sure you get 60Hz power to run your stuff.
  • They spin at these speeds regardless of electrical load
  • Because they can’t slow down, they are loud… all the time. And your neighbors will want to yell at you.
Alternator-only generator diagram

Higher quality generators include an additional stage before the output: an inverter. This device sits in between the alternator windings and the electrical sockets and synthesizes the voltage at the correct frequency for AC electricity. Because the alternator isn’t directly creating electrical output, this gives some great benefits:

  • Engine speed can vary to meet the load requirement. 60Hz comes out no matter what.
  • Fuel efficiency goes way up (because you aren’t burning extra fuel for no reason)
  • It is sometimes possible to run inverter-based generators in parallel. They can synchronize their AC generation and work together (not possible on pure-alternator generators).
Alternator-only generator diagram

Sizing up your appliances

When planing for a power outage, it’s time to make decisions about how much you can live with – or, more appropriately: live without.

Here is the deal: The more stuff you want to run at the same time the bigger the generator you need… stupid addition.

Here is a list of typical start up and running power for a variety of appliances. I would like to call your attention to the small stove burner (2.1kW). That burner, all by itself, will max out many small generators!

Thing Qty Start up power Running power
Refrigerator or Freezer (Energy Star) 1 1200 192
Microwave Oven 1 1500 1500
Incandescent Lights 4 240 240
Television 1 120 120
Coffee Maker 1 600 600
8-inch electric stove burner 1 2100 2100
Clothes Dryer (Electric) 1 6750 5400
Window Air Conditioner (10,000 BTU) 1 2200 1500
3-ton Central AC (36,000 BTU) 1 5000 3000
Computer 2 500 500
Hot Water Heater 1 4500 4500
Garage Door Opener 1 1420 720

Obviously, we’d need a pretty big piece of equipment if we wanted to run all this stuff. And sometimes, that’s the best idea. If you live in a climate where climate control is necessary all the time (for insurance or health reasons) then having a whole home generator can make absolute sense.

Assuming you don’t want to spend the kind of money to make power interruptions a non-issue, we’ll need to figure out some way to figure out what we need. For us, we decided to break the generator issue down in to multiple parts.

Making a game plan

During a power outage, the most important things to us are:

  • Food preservation (gotta keep the fridge going)
  • Communication equipment – In our neighborhood, cell coverage is spotty. Cable internet service is surprisingly reliable though.
  • Coffee
  • Cooking
  • Heat
  • Hot water for showers

Looking at the list above, some things are no brainers. Fridges are easy to run (200-1200W). So is internet equipment (10W). Coffee isn’t too hard (600W). Stove burner – Ugh (2.1kW). Hot water – Double Ugh (4.5kW). Heat is probably another 5kW – triple ugh. So, even if we didn’t run all this stuff at once. I’d still need something that could pump out 5kW continuous duty. That’s more generator than I want to buy.

So, what I’m going to do instead is get some of this stuff off electric for emergencies. Electric heat pump? Na, let’s get kerosene heaters. Cooking? Propane camping stove. Hot water? I’m going to plump in a propane tankless propane water heater in parallel with the electric water heater.

That leaves fridges, coffee, and internet running on the generator. That is easily covered by a 2kW. The fridge won’t be running all the time, and internet hardly takes up anything, and coffee only during certain times of the day.

Picking one out

In the end, the model I think I’m going to go with is the Honda EU2000i, or maybe it’s bigger brother the EU3000iS. These generators are small, inverter based generators that absolutely meet all our power requirements. Honda sales and service is also readily available in our area, which just isn’t true for most other generator brands.

  1. The letter ‘I’ is used to mean electrical current. I is ‘amps’. ^
  2. Just kidding, the generator turns off your juice. ^
Peter Voorhees
Microelectronics Design Engineer