A website should be online for 99% of the time. You will never know when a potential customer visit your website. If a potential client visits your site and it was down, you might end up losing a lead. Or worse, having downtime on a online shopping site could result in direct lost in revenue. Hence, getting your website online "all the time" is crucial for your business. Here is a tool that we use to monitor the downtime of any website.
We recommend using HetrixTools to monitor your website. It is a online tool that checks your website at interval (e.g. every minute) from multiple locations around the world. If your website is down, it sends notification to your email, even to your phone through SMS. The great news is it has free tier which supports up time 15 websites.
Here is a screenshot of their pricing structure, feel free to visit their website for more information.
Note that we are not affiliated with HetrixTools.