Applying to full-time remote work positions when I 'only' work 30 hours a week. How to explain?

There's no reason to be nervous or defensive.

If working only 30 hours per week is important to you, then just state up front "I can only work 30 hours per week."

If forced to explain why, just indicate that this is as many hours as you choose to work.

You'll lose some jobs that want full-time/40 hours. But you don't want to waste any of your time with an employer that doesn't meet your needs.

As someone who is in your shoes (hourly rate, bill less than 40 hours a week, but more than 30, Engineering field (controls, not PE)) I can relate to the anxiety of dealing with that set of interview questions.

Personally, I respond with a question of my own: "Why would you pay for 40 hours of labor when the work can be completed in 30?". Change the tone of the interview to examining output, rather than input. Most engineering firms will allow schedule flexibility so long as the business needs are met and resources are not wasted. If you can be present for mandatory things (meetings, scheduled tasks, etc.) and not create delays for the rest of the team, you will typically be successful selling the reduced hours as a cost savings, rather than a loss of available labor.

There are 2 budgets when it comes to labor as a resource. There is the cost budget (payroll, benefits, department overhead, etc.) and the labor budget (typical manhour expenditure per task/project/etc.). The goal of any hiring process is to maximize the latter, while minimizing the former. Pointing to past performance as an indication that you will provide adequate labor for reduced budget (40 hours worth of work from a typical employee for only 30 hours worth of budget) and you should have no issue.

The key things about working a reduced schedule that you need to be able to answer all relate to business needs. Core hour availability is a big one (not slowing the rest of the team down by not being available). Less common but still prevalent in my field is Time and Material billing. I work less hours, company gets to charge less hours. The upside is, initial quotes can be made with a smaller hour budget in mind, thereby winning more potential work, which balances out the reduced billing with more projects. Benefits (in the USA) which typically are more of a concern for you as the employee, unless they're negotiated in (3/4 typical work hours leads to 4/3 typical per hour benefit cost, changing the hiring cost calculation).

Consider switching from hourly billing to day rates. This way it doesn't matter if you work 6 or 8 hours per day, the price is the same. This is fairly common if you're dedicating your time to one client, and I did this myself in my consulting days.

The additional advantage, and the reason many people do it, is that it shifts the client's perception away from "hourly worker on assembly line", where your pay is tied directly to how many widgets per hour you grind through and the client has a direct incentive to nickel and dime you, to "skilled professional", who is paid for the value they provide and not how many hours they put in.

Billable hours and FTE working hours are not the same. A contractor should only bill for the hours working on the client's tasks, and billing for 30 is absolutely correct. As a FTE (full-time employee), you're entitled to the full agreed-upon salary, as long as you've worked your hours or done all your work and reported your availability.

As a contractor, saying "30 hours" is enough. If a company's taking you on as a salaried FTE, they expect some of the 9-to-5 to be spent non-productively. As long as 40-hour week is doable sometimes, a no-overtime FTE is very close to your preferred schedule. But some employers expect more than 40, and these clearly won't be a fit.

To politely explain this, allowing for FTE employment, but not FTE+overtime, I'd mention this during the interview, e.g.:

• "I usually do my week's tasks in 30 hours, and spend the rest learning"
• "Due to family obligations, overtime is not an option for me".

As an employer and a hiring manager (I've hired about 80 employees with unusual schedules, work practices, disabilities, etc), I'd consider that sufficient and fair disclosure. A recruiter will note your "no overtime" request, and the HM will understand that you're available for

From someone who is both used at managing multiple consultants/the client's perspective as well as working as paid consultant for clients:

It is the norm that consultants only bill effective time spent working and clients usually don't have a problem with that. Usually consultants make up for it with higher rates compared to an employee with salary - who might put in more work hours but not necessarily be 100% efficient during that time. All sound employers know that nobody is 100% efficient for 8 hours straight every single day (particularly not if there are lots of meetings interrupting work and generally clogging things up).

Also, if you aren't a hired employee, you never agreed to spend 40h per week working unless you signed an explicit contract saying as much. Consultants don't apply for full time jobs, which implies employment and work 8 hours per day (out of which x hours are actual effective work). Lunch breaks do not count in those 8 hours, but shorter coffee breaks might and toilet breaks definitely do.

Furthermore, clients may not assume that you bill the equivalent of full time if only because they assume that consultants will have more than one client, which is very common. And if you only billed 30 hours, the client will not expect it to be a "full-time work product" (ie 40 hours).

Now none of this micro-management of time might matter a lot unless you work in project form, where there's deadlines. Or perhaps if you made an offer when accepting the job "I expect this to be 160 hours of work". Even then, you can break the time table down into key dates where certain progress is expected. If you can deliver the results, then the client usually don't care about the exact amount of hours it took to get there.

And since you set your work hours yourself, nobody expects it to be an exact amount each week - could be a few more this week and a few less the next. They might expect you to put in extra hours if close to a deadline and the results aren't ready.

Finally, what most clients keep a watchful eye on is rather the opposite of what you worry about: consultants who bill too many hours compared to what they actually deliver.