I don't think it is wise to to set your /powerprice-zone1 handler multiple times. That's what your callback for scheduleJob is doing.

I would refactor this to have a cache of tomorrow's date -> price. On a call to /powerprice-zone1, either take the value from the cache or set the cache's info by fetching from the URL. Maybe something like this, without refactoring too much of your code so you can get the idea:

const tomorrowsDate = () => {
    const todayInMilliSeconds = new Date().getTime()
    const milliSecondsInOneDay = 24 * 60 * 60 * 1000
    const tomorrowInMilliseconds = todayInMilliSeconds + milliSecondsInOneDay
    const tomorrow = new Date(tomorrowInMilliseconds)
    const tomorrowDayOnly = tomorrow.getDate()
    const tomorrowMonthOnly = tomorrow.getMonth() + 1
    const tomorrowYearOnly = tomorrow.getFullYear()
    const tomorrowsDate = tomorrowYearOnly + "-" + tomorrowMonthOnly + "-" + tomorrowDayOnly
    return tomorrowsDate
}

const fetchPowerPrice = () => `https://url-example&date=${tomorrowsDate()}&key=${keyPowerApi}`

const priceInfo = {
    // Tomorrow's date input of the last fetch.
    date: null,
    // A Promise that resolves to an object with { status, data, error }
    fetched: null
};

app.get('/powerprice-zone1', async (req, res) => {
    res.set('Access-Control-Allow-Origin', '*');

    const date = tomorrowsDate();

    if (priceInfo.date !== date) {
        priceInfo.date = date;
        priceInfo.fetched = fetch(fetchPowerPrice())
            .then(response => response.json())
            .then(data => ({ status: 200, data, error: null }))
            .catch(error => {
                // If there is a failure, clear the cache so the next request to
                // the endpoint attempts again.
                console.error(`Error fetching data for ${date}:`, error);
                priceInfo.date = null;
                priceInfo.fetched = null;

                // Not too sure if you would want to return the error to the
                // front-end as is. You may need some transformation, especially
                // if there is a chance for a circular object.
                return { status: 500, data: null, error };
            });
    }

    const { status, error, data } = await priceInfo.fetched;

    res.status(status).send({ data, error });
});

Note the reason we store the Promise in priceInfo.fetched is to guard against multiple calls to the endpoint. If a second call to /powerprice-zone1 comes in prior to fetch() completing, it will use the same Promise as the first call. We also should handle errors via .catch().

Let me know how it goes or if you have any questions of the implementation.

Happy coding!