Ye ol' question: gas or electric?
November 24, 2014 11:04 AM   Subscribe

Help me figure out whether I should be heating my small house with the gas furnace or an electric space heater...

I have a small (1000 square foot) house that is poorly insulated in the Bay Area (so not too cold, but chilly at night). The house has a furnace with gas heat that heats all of the rooms. But I only need heat at night in the bedroom so I thought I was being sneaky by getting an electric space heater. Both the space heater and the gas furnace have thermostats that I can set. I got my first bill though and my electric seemed to be *really* high! There could be a couple other explanations (it's getting darker earlier, I've been running a lot of laundry etc.)

I don't need you to tell me which will be cheaper, but I'd love it if someone could tell me how to evaluate which would be cheaper. Should I use only one on one day and the other on another and compare costs (I get a daily output from the energy company of both gas and electric usage)? Should I look into the actual energy usage of the space heater and try to evaluate from there? Do we just know that gas is more efficient overall, so I shouldn't use the space heater?
posted by Toddles to Home & Garden (6 answers total) 2 users marked this as a favorite
Gas is almost always much cheaper per BTU of heat, but you have to offset that against the fact that the space heater is only heating part of your house. So trying to make a comparison might be a good idea. The problem with trying to compare two heating sources, though, is that how much each of them costs depends on how cold it gets. Most heating cost calculations are based on "degree days" - the number of days x the average number of degrees the outside temperature falls below some threshold. If I were trying to compare, I'd try using the central heat for a few days, then the space heater instead (or whatever combination of settings you think works best), then go to a site that offers degree day data for my area, and compute "heating cost per degree day" for each approach. The gas and electric companies may provide degree-day data along with their usage reports, which may be tuned for your exact location, but here's the national weather service data for SF - "HDD" = heating degree days, "CDD" = cooling degree days.
posted by mr vino at 11:18 AM on November 24, 2014 [2 favorites]

Buy a p3 kill-a-watt, for about 20 bucks, and plug your space heater into it. Read the kilowatt-hour usage every day, and make a note of it.

Then you will be able to evaluate your electric bill, starting in the top tier of usage, and see how much your space heater is contributing to your electric bill. This is "Should I look into the actual energy usage of the space heater ..."

You can also check the kill-a-watt month-to-month, as it will accurately total energy for that long, but if you have a power failure or unplug the unit it will forget, and you'll lose a month's worth of usage data. Reading it day-to-day will allow you to formulate an answer in about a week.
posted by the Real Dan at 11:19 AM on November 24, 2014 [1 favorite]

Google tells me that a kilowatt hour converts to 3,412.14163 BTU. The space heater probably draws 1800 watts. That's an inefficient way to generate heat, a heat pump is 4x as efficient, but a bit of Googling says that the 1800 watt space heater, run for an hour, or 1.8 kilowatt hours, will generate about 6.14 BTU. A good heat pump I think has a conversion factor of around 4, so will generate closer to 24½ BTU, but that requires somewhere to pump the warm from (ie: outside), so you're using a plain ol' inefficient resistance heater.

Our PG&E tiers for electricity start at 15 cents per kilowatt hour, and run up to tier 4, 65 cents per kilowatt hour.

One Therm of gas generates 100,000 Btu. I'm not sure what we actually pay, but a quick search suggests that PG&E is running about a buck and a half per therm.

So using the space heater, at tier 1 you're paying about $4.39 for the same amount of heat you'd get for $1.50 in gas, and at tier 4 you're paying $19.

Heating your bedroom alone may use 3x fewer BTUs, so at tier 1 that tradeoff makes sense, but it's unlikely to use 12x fewer BTUs. So unless you're nowhere near the higher tiers (and it's likely you're near them), heating the whole house with gas makes more sense.
posted by straw at 11:23 AM on November 24, 2014 [3 favorites]

But I only need heat at night in the bedroom so I thought I was being sneaky by getting an electric space heater.

Follow-up question... do you need heat in the ROOM or are you okay with just heat in the BED? I get that the space heater has a thermostat in it so it's not heating you up too much when you are sleeping, but it may also be that you just need a non-chilly bed and a lot of blankets and a mattress pad warmer (which can also be on a thermostat). You can look at some of the numbers here for using one of them on a low setting overnight. I use mine to heat my bed up every night (and keep my house otherwise cold at night) and my electric bill remains low. Your library may also check out kill-a-watts which might be a better investment if you only need to check this out once.
posted by jessamyn at 1:32 PM on November 24, 2014 [2 favorites]

I think you're leaving out whether you're keeping the central gas furnace on at a minimum temperature all of the time, which I think would be wise. For example, you might keep the whole house gas-furnace-heated to 60 degrees, and your bedroom electric-space-heated to 66-68 degrees, or whatever's comfortable. This way you're also protecting your house & everything in it from damage due to high humidity & condensation (likely a big problem, especially where you live). How much cash this would save you as compared to just keeping the furnace on at 66-68 hard to say, and I don't really have an answer to your question. I think the Real Dan's suggestion is a good place to start.

We recently converted from 40-year-old electric baseboard heaters to an air-source, electric ductless heat pump in our 900-square-foot home in the PNW. It works great, but one of the back bedrooms was slightly chilly. We needed a dehumidifier, and after getting one, I realized that a dehumidifier also has a small space-heating effect, and now that back bedroom is the same temperature as the rest of the house (68 degrees), and bone-dry to boot.
posted by bennett being thrown at 3:45 PM on November 24, 2014

Response by poster: To answer the questions that came up...
1. Could you just heat the bed?
Alas, new baby, so up and down all night long. A warm room is preferred, as a warm bed will only make the up and down much harder.

2. Do you leave the gas furnace on 60 degrees all night and the electric space heater higher?
I like this idea - I might try that tonight and see how it goes.

This all been very helpful - thanks!
posted by Toddles at 10:12 PM on November 24, 2014

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