Laptops are where there's no free lunch. Every CPU cycle used is something that consumes your battery life.
Look for ones that basically use a little CPU during your normal work as possible. Unfortunately in practice, it means these things consume a lot of battery, and you should look to see if your AV supports disabling them:
- Realtime scans beyond on-execution/on-open (on-creation, on-access)
- Scanning heavily changed but low risk areas like the browser cache (downloads that get executed are moved out of the browser cache to elsewhere)
- Scanning all extensions
- Periodic/background/startup scans
- Network Filtering (SSL inspection and network stream filtering are all high CPU usage)
Of course, balance this with your desired level of protection.
In practice, I've found Kaspersky to be pretty high impact on battery life, Norton as well when network traffic is heavy (though Norton is better than Kaspersky at IMMEDIATELY stopping all background tasks when running on battery power). ESET is good if you disable network scanning (the SSL filtering aspect is fairly CPU heavy), but note that with ESET in particular, disabling network scanning severely reduces your protection against trojan dropper/downloader sort of threats where a highly obfuscated first stage downloads a much easier to identify second stage.