docs: and unit rate info and links

Author: davidsauntson

docs/appliance-calculator/README.md

@@ -18,7 +18,7 @@ The app is iframed into an advice page using the `InteractiveToolRenderer` in th
 
 The iframe is resized as instructed by the `postMessage` sent to the `parent` window via the js in the `app/javascript/modules/resize-iframe.js` module.
 
-There are some more notes [interactive tool docs](https://github.com/citizensadvice/public-website/blob/f019add8aac688659f61548afcff727811d281d4/docs/advice/interactive-tools.md?plain=1#L1).
+There are some more notes in the public website docs: [interactive tool docs](https://github.com/citizensadvice/public-website/blob/f019add8aac688659f61548afcff727811d281d4/docs/advice/interactive-tools.md?plain=1#L1).
 
 ## Contentful Entries
 
@@ -31,3 +31,19 @@ Details of the model fields and their meanings can be found in the energy team's
 #### Cyclical & Time Based Appliances
 
 There are two types of appliance: cyclical and time based.  Cyclical appliances have fixed consumptions each time they are used, like a washing machine.  Time based usages have variable consumptions that depend on how long they are used for, like a TV.  The `CyclicalDailyUsage` and `TimeBasedDailyUsage` objects contain the logic for working out `kWh` consumption based on the data entered by the user.
+
+## Updating the unit rate
+
+The calculator shows a default unit rate for electricity, so that a user doesn't have to find theirs when using the tool.  This is updated
+quarterly to reflect changes in the national average.
+
+It's stored as a config value in [pence per kWh as a config value](../../config/application.rb)
+
+The digital energy team will ask in advance of when the change needs to happen.
+
+## Useful docs
+
+[Appliance calculator analytics summary](https://docs.google.com/document/d/1XHtGyj5pdbtWv-5etS1h5l9g0LHWwzLCbnPM2Y65qCs/edit?tab=t.0)
+[Appliance calculator content model](https://docs.google.com/document/d/1WhrKpLv7HiOKMuSQ_APuD38Pi6w3ouiY7AvZsjTMQQs/edit?tab=t.0#heading=h.502rkuylsq6b)
+[Appliance cost calcs (washing machines)](https://docs.google.com/document/d/1E7pd5Q16FF0sHoev2Dd3_n1DdCLz2w9y_E9I_gJijmg/edit?tab=t.0)
+[Appliance cost calcs (ovens)](https://docs.google.com/document/d/1E7pd5Q16FF0sHoev2Dd3_n1DdCLz2w9y_E9I_gJijmg/edit?tab=t.0)