How do I work out what to redraw in onUpdate()?

You can limit the amount of times onUpdate actually does things. I have a basic sample of using a boolean and timer to only redraw every 2 seconds. I don't wear my watch while sleeping and it drops less than 2% battery overnight. hard to say since they won't give us decimal values ;)

You won't see much savings with this example because it's only 1 thing being drawn, but on my WF with a couple of things (date, hr, body battery, time, seconds, steps, temperature) it makes a noticeable difference in the average in the simulator's profiler. About half, for a short sample period. You could add other bools and checks and draw a black (if that's your bg color) rectangle over the seconds to clear that part of the screen and only redraw the text of that part. It could probably get complicated with a lot of data.

Sorry for the unformatted code, I tried the other option which told me I'm blocked.

import Toybox.Application;
import Toybox.Graphics;
import Toybox.Lang;
import Toybox.System;
import Toybox.WatchUi;
import Toybox.Time.Gregorian;

class SimpleWatchFaceView extends WatchUi.WatchFace {
var _canUpdate = false;
var _lowPwrMode = false;
var _blanked = false;
var _updateInterval = 2000;
var _updateTimer = new Timer.Timer();

function initialize() {
System.println("view initialize");
WatchFace.initialize();
}

function startUpdateTimer() {
System.println("starting update timer");
_updateTimer.start(method(:onTimerUpdate), _updateInterval, false);
}

function stopUpdateTimer() {
if (_updateTimer != null) {
System.println("stopping update timer");
_updateTimer.stop();
}
}

function onTimerUpdate() as Void {
System.println("onTimerUpdate");
_canUpdate = true;
WatchUi.requestUpdate();
}

// Load your resources here
function onLayout(dc as Dc) as Void {
System.println("onLayout");
}

// Called when this View is brought to the foreground.
// Restore the state of this View and prepare it to be shown.
// This includes loading resources into memory.
function onShow() as Void {
System.println("onShow");
_canUpdate = true;
startUpdateTimer();
}

// Updates the view.
// Called every second in high power mode.
// Called once a minute in low power mode.
function onUpdate(dc as Dc) as Void {
System.print("onUpdate: ");

if (_lowPwrMode && !_blanked) {
System.println("low power mode");
dc.setColor(0, 0);
dc.clear();
// for debugging
/*dc.setColor(Graphics.COLOR_PURPLE, -1);
dc.drawText(208, 208, Graphics.FONT_SMALL, "Low Power Mode", Graphics.TEXT_JUSTIFY_CENTER | Graphics.TEXT_JUSTIFY_VCENTER);*/
// end debugging
_blanked = true;
return;
}

if (_canUpdate == false) {
System.println("skipping");
return;
}

System.println("drawing");

// clear screen
dc.setColor(0, 0);
dc.clear();

// time
dc.setColor(Graphics.COLOR_DK_RED, -1);
dc.drawText(208, 208, Graphics.FONT_TINY, System.getClockTime().sec.format("%02d"), Graphics.TEXT_JUSTIFY_CENTER | Graphics.TEXT_JUSTIFY_VCENTER);

// Only allow timer tick to update screen.
//_canUpdate = false;
//startUpdateTimer();
}

// Called when this View is removed from the screen. Save the state of this View here.
// This includes freeing resources from memory.
function onHide() as Void {
System.println("onHide");
stopUpdateTimer();
_canUpdate = false;
}

// Terminate any active timers and prepare for slow updates (once a minute).
function onEnterSleep() as Void {
System.println("onEnterSleep");
stopUpdateTimer();
_lowPwrMode = true;
_blanked = false;
//WatchUi.requestUpdate();
}

// The user has just looked at their watch. Timers and animations may be started here.
function onExitSleep() as Void {
System.println("onExitSleep");
_canUpdate = true;
_lowPwrMode = false;
WatchUi.requestUpdate();
}
}