This has been asked a few times before and it is a possibility. But we need to know a lot more, like what is your type of heat? Are you running computers 24/7? One thing you could do is take a meter reading and also see if any power is being used (old style is with the wheel rotating and new style is digital with "dash marks" showing consumption. Then, after preparing, turn off your main breaker and see if you still have any current consumption. Also, if one of your neighbors homes "go dark", that'd be a good clue as well.

Yes -- you should be able to get instantaneous usage out of your electric meter, either by "timing the disc" on an old-school spinning-dial meter or directly out of most smart-meters (although some emulate the spinning-dial disc display)

Turn off the main breaker and see if your neighbor complains.

@GeorgeAnderson The only equipment I can find in the house is called an "Air Handler". I can't find any other kind of heating equipment that it is attached to. No computers, but some laptops run 8 hours a day.

Electric or gas or other hot water? Electric or gas or other heat? Any outdoor lights on all the time?

@manassehkatz-Moving2Codidact Everything runs on electric, no gas. There is a single outdoor lamp we can't turn off, we hired an electrician but they couldn't figure out how to connect it to the light switch, so it runs 24/7, but that is just one light bulb.

Electric heat and hot water can be real energy hogs. See if you can find a model #/rating/nameplate on the electric heat, which may be built-in to the air handler. Upload a picture of your main electric panel - that may provide some clues. But electric heat can be a lot - you could easily have several kW in heat, if running a few hours a day that will add up very quickly.

What type of electric heating do you have - heat pump, or resistance? How well insulated is the house - ceiling and walls? Is the house drafty, or well-sealed? If things really point toward power theft, you might want to rent or hire an underground cable locator and check all the way around the house. If a neighbor truly is stealing power, the cable is probably not very deep.

Assuming that you have an illegal tap that is running up your bill, the tap would have to be after your electric meter. Unless your meter is remote from your house, a back-alley tap wouldn't affect your bill.

Detached house or town house: do you share walls with neighbours?

Is this 400kwh measured usage? Or 400kwh estimated usage?

You bought a house with electric heat. This is what it's like. Did you not ask the previous owner for utility bills?

We used to own a house with electric heat. The house was always freezing, and the electric bills were astronomical. We finally bit the bullet and switched to gas, and it paid for itself in 2 years, plus we were finally warm. The gas installer told us our old electric furnace was basically "a toaster with a fan."

25F is not “mild”, that’s below freezing, and electric resistance heating is very expensive. Sounds like that’s a major factor.

Look into heat pumps as well. It takes less power to pump heat from outside to inside than it does to just generate it directly (which an electric resistance furnace does). Also maybe the always-on bulb could be switched to LED or CFL. I'm just a bit suspicious that your 'electrician' couldn't figure out how to connect a light bulb to a switch...

"we hired an electrician but they couldn't figure out how to connect it to the light switch" Then, with all due respect, you didn't hire an electrician, but someone playing at being an electrician. Unless you didn't let the electrician open up any walls to actually see the wiring or let them access the panel to check circuit breakers?

@manassehkatz-Moving2Codidact Ok, but it's still not at all relevant, nor is it the reason why electric heating is so expensive. "toaster with a fan" sounds like someone trying to make a point that an electric furnace is cheap junk and that it's therefore expensive to operate (or something?) - it's a wild pejorative that is poking at something which is a total red herring. The only issue here is that electricity is considerably more expensive per joule compared to other forms of energy like gas.

My understanding is that heat pumps aren't efficient below 35F. So even if you've got a heat pump, the backup - electric resistance($$$) - heat is kicking in every night.

@J... I agree with you. I think that may have been the point - the gas installer really looked down on electric heat. The installer may have also been trying to justify why it is so expensive & labor-intensive to install gas (which it is, but in most areas there is payback in lower energy costs).

@G.Ann-SonarSourceTeam A quick search shows the switchover at the 25 - 30 range. If it is close to 25 and OP's house is in area where temperature rarely goes below 25 then it could work quite well.

@manassehkatz-Moving2Codidact The lowest night temperature I observed was 22, that happens occasionally. Rises to 45 F in the day, then back to about 22-25 at night.

Do you have air conditioning? If so, how old is it? Putting in a heat pump to replace heat & air conditioning is not inexpensive. Payback time will depend partly on the difference in efficiency (SEER) on the air conditioning side, which will largely correlate with age of the unit.

@Josh 25F seems mild to me. It's not winter unless you can freeze to death and this just barely qualifies. You can walk around in that without a jacket.

@DKNguyen NOAA disagrees with you: Scientists defined “mild” weather as temperatures between 64 and 86 degrees F. But regardless of the definition of "mild," unless OP is keeping his house at <45F, using electric resistance heating when it's 25F or below is going to be very expensive

@Josh False dichotomy because mild weather is not the same thing a same thing as a mild winter. At least, that's the way my mind works. It's relative to the thing you're talking about. Like a cool star vs a hot star. Or uhhhh, mild steel.

The point is that you have to heat a lot of air approx 50 degrees more than the temp outside. That's not cheap even with 100% efficiency. 60-70% of your energy usage could easily be heating alone.