There are two wireless networks to think about here:
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The one the emonPI uses communicate with the emonTX etc, I think it’s 433 MHz but have no experience, I am sure someone else will jump in
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The WiFi connection between your emonPI and you access point. and other devices on the WiFi such as Kassa Plugs and PIZero’s
My entire setup works on WiFi, reporting back to the emonPI, which is also on WiFi as it’s close to the consumer unit and running an Ethernet cable to my network cupboard was a pain. With WiFi it’s a case of you get what you pay for. I ditched the “free” WiFi hub provided by my ISP some years ago and put in my own system using a Ubiquity Edge Router and a UniFi Access Point Lite installed in the roof space
When I purchased my 1990’s house in 2007 I ran CAT5 cable to everywhere (except the consumer unit cupboard!). However since installing the AP-Lite I have stopped using wired connections for most things, the cheap (GBP 9) PIZeroW’s I have round the house just connect to it fine. The Ethernet cables are sometimes repurposed to provide DC power to the PIZero.
Using Ubiquity kit (other similar “prosumer” brands are available) you can control more than one AP on same SSID to increase the range and also create VLAN’s to separate your energy monitoring/control network from your “Windows” network. and “Guest” (internet only) network.
However if you are building from scratch as others say the incremental cost of conduit in a build is neglible, splash out and put some nylon rope in it as well, so you can easily pull cables later.