Wiring a Generator to Your House

We're going on day 5 without utility power at my house in Pennington, New Jersey.  Thanks to Wattvision, we know our home's exact load profile, so we were able to match our generator to our needs, saving tens-of-thousands of dollars in the process.  I'd like to share how we did it, so you can do it, too.  Let's first take a look at the typical load presented by our house on a given day, as recorded by our Wattvision system last Thursday, October 25, 2012, in the pre-Sandy world:

So looking at the graph, with time on the X-axis and watts on the Y-axis, you'll notice that we never use more than 5000 watts, and only come close to that for a very short period in the day.  Zooming in, we were close to 5000 watts for just a minute or so that night:

Just for fun, try to guess the average rate of consumption for the whole day, in watts.  Go on, give it a guess.  

It turns out that, on an average day, we use an average of about ~660 watts throughout the day.  What that means is, if we had a power source that generated just 660 watts, coupled with a 100% efficient battery that could accept and return power perfectly, we could power our whole house on just a 660 watt constant power supply.  But that's not possible in this universe, so let's get a generator...

Choosing a Generator

Looking at our data on the typical day above and other days, we know that we'll never need more than 5 kilowatts for typical use in our home at this time of year.

Now, the wiring in our house can support 400 amps of current at 120 volts, so that's a cool 48,000 watts.  Most typical American homes support up to 24,000 watts.  So you can understand why a generator salesperson might want to sell you a system that can produce 20 kilowatts or more. "Because you never know!"  But in our case, we have the data and the confidence that we won't need that.

During Hurricane Irene last year, we spent several days without power.  On one of the last days, we purchased our 5.5kw gasonline-powered generator.  This is a standard generator that you might find at your typical hardware store, but it also has a 120v/240v NEMA L15-30 outlet.  We fired it up, and wired it directly to our home's well-water pump.  I still remember the excitement we felt when we restored running water to our house.  And then, while we were celebrating, the power came back.  Thankfully we saved the generator, and it's come in handy during Hurricane Sandy.

Wiring it up...

So you have a generator -- how do you wire it to your home's breaker box, ensuring your whole house is back online?

The first thing you'll need to verify is that your generator has a NEMA 240v/120v L15-30 or similar outlet.  This is a special round, typically 4-prong plug that has neutral, ground, and two "hot" 120v lines.

The next thing you'll need to do is contact an electrician, to set up either a generator interlock, or a separate generator-powered breaker panel.  The emergency circuits breaker box is a common route, but isn't ideal -- you have to select the subset of circuits in your house you want powered by your generator.

In our case, we want to use everything in our house, so we opted for a generator interlock kit.  The interlock is essentially a switch that allows you to either power your house from the utility or from your generator.  When the power goes out and you know it'll be out for some time, you go to your breaker panel, flip a switch, plug in your generator, and fire it up.

If you try to wire your generator to your breaker box as a DIY project, you probably will not be in compliance with local electrical codes.  Also know that it's extremely dangerous.  For example, if your ground wiring is not done properly, you may inadvertently put 200+ volts on 120 volt lines, immediately frying bulbs and electronics connected to those lines, and potentially starting fires.  Don't do it.

So how much does it cost to run?

When powered by the utility, we use about 15 kWh / day at the current rate.  At 17 cents per kWh, we have some of the most expensive electricity in America, but it's still only about $3/day at the moment, thanks to our Fall climate here in NJ, and our optimization with Wattvision.

The generator uses about 8 gallons of fuel if we were to run it constantly for 24 hours.  Assuming you can get a gallon of gas for about $3.50, that's about $28/day.

But, we can regulate our consumption -- we turn the generator off at night, for example.  By optimizing our use, we've been running the generator for less than half the day.  So about an extra $10/day over what we were paying normally.  A decent rate if we're talking about only a few days without power every year.

Of course, that's putting aside the environmental cost to run our generator. But that's a blog post on its own. :)

What about Batteries?

The energy required to run our home for a typical day last week was about 15 kilowatt-hours.  So how many AA batteries is that?

A single high-end rechargable AA battery is rated at 1.2v at 2 Ah. So that's 2.4 Wh.  So to run my house for a day, we'll require at least 6,250 AA batteries. Putting aside the cost of the management electronics and all the wiring and little boxes we'll need to buy for these batteries, that will cost > $15,000.  Ouch. (You can buy a pack of 8 batteries for about $20 on Amazon).

How about deep-cycle 12 volt batteries?  You can buy a single battery for ~$130. This battery produces 55Ah and contains 660 watthours of energy. you'll need about 22 of these batteries, and it will cost about $3000 -- a little more reasonable.

Final word...

Since we have electrically-pumped well-water here in NJ and the natural gas line to our house was not disrupted, the 5.5kw generator wired to our breaker box restored our house to life as normal.  Granted, we can't think about running one of our 3 A/C units, which easily pull 5kw each, but in a pinch, the generator is just what we need.

Hopefully our power will be restored soon -- we're busy at work producing Wattvision 2, so we can ship it out to kickstarter backers in January. :)

7 responses
Had you considered a natural gas/propane/multi fuel generator?
Yes, had considered that. They are quite expensive to install, and given that we're only without power for a few days a year we felt it wasn't worth the cost-benefit. You'll need a plumber to set it up on your gas line, get a concrete pad outside your house, etc, from what I understand. Let me know if there are more inexpensive alternatives out there -- because propane or natural gas is a bit more convenient, for sure. ;)
If you use batteries, you should use bigger ones than AA. Car batteries are cheap an reliable, just get 20 batteries with 74 Ah and it should get you through one day (with 15 KWh, longer if you reduce your power consumption). You can also use a generator to charge the batteries and use them as a buffer, since the generator will output more power than is needed at any moment and this would be lost energy without a buffer.

20 car batteries should cost around $1500-$1600. You could also use solar panels to charge them during the day. 2000 Watt panels should be ok and you'll only need the generator (or the grid) during the winter ;-)

Have you taken into consideration Time of Use (TOU) cost differentials and/or Peak Demand charges? If your electric supplier uses either of these factors for rate determination it may be cost effective to offset high peaks with supplemental generation. In other words, in some cases running your generator during periods of high demand or when TOU charges are at there highest, the additional power produced by self generation, AS A SUPPLEMENT, may save money.
Yes, time of use billing is important to us, it's in the works. You're exactly right, it can make sense to offset high peaks with on-site generation.
2 visitors upvoted this post.