Designing the Electrical System

Designing a tiny house involves tackling tons and tons of semi-separate, interlocking design problems (in the sense of "math problem," rather than problem problems). One problem that I've been wrestling with lately is electricity. Oversimplified, the big question is just a binary:

Do I need 30 amp or 50 amp electrical service?

But it's tricky. I'm learning a lot about electricity and wiring these days, but I'm still working on this one. Today I sat down and calculated the total wattage and amperage of all the electrical appliances I plan to have to determine whether I need a 50 amp RV electrical hookup, or if I can make do with 30 amps.

In case that's as opaque a concept to you as it was to me a few weeks ago, here are some electricity basics:

The three main units that we use to measure electricity are volts, amps, and watts. (Ohms, too, but only if you're getting technical.) The explanation of these three units that I have most commonly heard--so I assume it's correct (I'm not an expert yet!)--is an analogy with plumbing: voltage (V) is like water pressure, amperage (A) is like the flow rate through the pipe, and wattage (W) is related to the other two via an equation: 

W =  A * V

Wattage is a measurement of how much work the electricity can do per second: the higher the wattage of a heater, the more heat it puts out per second; the higher the wattage of a lightbulb (type of lightbulb--LED, CFL, etc.--being equal), the brighter it shines; the higher the wattage of a speaker, the louder the sound it can produce. Watts are also a standard measure of energy production and consumption.

In the United States, the voltage of our standard household electricity is 120/240, meaning that electrical cords have one or two wires, each carrying 120V. (You also see 110V and 115V listed as the standard. I think it has to do with the fact that voltage drops as it travels through long wires; 120 is the ideal, but...) Most devices have a cord with just one hot wire and take 120V. Major electric appliances--like oven/ranges, clothes dryers, and water heaters--need incredible amounts of electricity, so they get two hot wires, for 240V.

Most appliance spec sheets list the appliance's required voltage and either its wattage or amperage. The watts or amps are what you want to know to calculate how much electricity you're consuming. If you're planning an electrical system, you'll want to know exactly how much electricity you need to consume, and build the system big enough (actually at least 25% bigger than big enough, for safety) to supply it.

So how big can those systems be? Typically, tiny houses mimic recreational vehicles in terms of utilities hookups so that they can access the most commonly available resources. Here's a good article explaining RV electrical systems. Basically, RVs come with either 30A or 50A systems. The 50A systems are somewhat less common, and are actually much bigger than the 30A ones. Why? Because 50A RV extension cords have two hot wires, each at 120V, versus the 30A cord with just one hot wire. That means that rather than being 167% the size of a 30A system, a 50A system is 333% the size. Theoretically, you can use 12,000 watts in a 50A system, versus 3,600 in a 30A one. With a 50A system, I suppose you could have 240V appliances, too.

My house will have only small, 120V appliances, so that's not a reason to go big with a 50A system. But nearly all my appliances will be electric; I have a strong aversion to combusting flammable gases and liquids in my itty bitty living space. Right now, I'm planning to have everything except the hot water heater run on electricity. 

So what does that "everything" consist of? Mainly kitchen appliances, to be honest. I love to cook! I will also have two exhaust fans to control odors and--crucially--moisture, which can both become serious problems in a tiny house. I will heat my house with two panel heaters, which are low-profile, wall-mounted, and energy-efficient. Aside from that, some overhead lights and some outlets for other small devices, and we're good.

I want to be able to go with a smaller, 30A system. But that may not be possible. Here are the results from my calculations today:

Bathroom: LED light (0.1A), exhaust fan (0.9A), heater (2.1A)
Total Bathroom: 3.1A

Kitchen: overhead LED lights (0.3A), two induction burners (each max 10.3A), microwave/convection oven (7.5A), fridge (1A), multicooker (8.3A), exhaust fan (0.2A)
Total Kitchen: 37.9A

Living/Bedroom: computer--charging (0.2A), cell phone--charging (0.02A), light (0.1A), heater (3.3A)
Total Living/Bedroom: 3.62A

TOTAL: 44.62A  /  5,354.4W

(Plus there may be things like Christmas lights, an outdoor outlet for God-knows-what, maybe I'll acquire an electric toothbrush or a set of speakers, maybe I'll decide on a hardwired smoke/CO detector instead of battery-operated, etc. etc. etc.)

Just the kitchen alone is a major electricity draw. But it gets worse: since you're not supposed to exceed 80% of your system's capacity, with a 30A/120V system, I could only actually sustain 24A, or 2,880W, of consumption. So while, say, cooking dinner on a cold winter's night, I could have both heaters running (5.4A), the kitchen exhaust fan, fridge, and lights on (1.5A), and have enough left over (17.1A) to operate the oven and one induction burner on a medium setting, or the oven and the multicooker both on maximum, or one induction burner on maximum and the other on low.

That means I could not prepare a meal with three hot elements (e.g. roast chicken with mashed potatoes and peas, or stir-fried tofu with steamed bok choi and rice). Nor could I cook multiple meals at once, like I often do on the weekends to supply leftovers for the coming week's lunches and rushed dinners. I could still manage something with simpler dishes, casseroles, one-pot meals, that sort of thing. But for me, cooking is a hobby--my favorite hobby--not just a way to feed myself. I would be sorely restricted on a 30A all-electric system.

But isn't simplification and reduction the point of tiny houses? Shouldn't I embrace it, even in my cooking and eating habits?

Well, yes. But let's not lose perspective, here (," she tells herself). Having a tiny kitchen inherently forces a certain--huge--amount of simplification and reduction. I'm more than willing to forgo the specialty appliances and ten different knives for different purposes and that sort of thing. I'm even willing to forgo some normal appliances, like a full-sized range, oven, and fridge. So, I'm already super limited, compared to what I would be capable of in a normal house's kitchen. But I'm not willing to compromise my capacity more than that. So for now, yes, simplification is the point, but there's a line to be drawn--and there it is.

So it looks like I'm leaning toward a 50A system. That way, when I have all 50A available to me, I can use literally everything in my house at once without beginning to approach an overload--good, safe. I can have more separate circuits, so the whole house doesn't go dark if I trip one. I may have to plug into a 30A supply sometimes, and then I'll be restricted in my cooking and/or heating capacity. And, of course, I may have to drastically rethink my aversion to flames if I decide to go off-grid and use solar panels to supply electricity.

But even if I don't, whichever way I cut it, I'll be using a bare fraction of the energy that a typical full-sized house with 200A/240V service uses.

So in conclusion: Woohoo, tiny houses!