A house with a parent in it uses things differently. The heat runs more hours, the hot water runs more often, the washer runs more loads, and lights stay on in rooms nobody is sitting in. None of that is a mystery. Most of the increase comes from a few places you can see.
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Read the usage line before the total
Look at the usage on the bill, not only the amount due. Compare the daily average to the same season before the change, if you have those bills. A higher daily average tells you the house is using more. A higher bill with the same daily average tells you the rate or a fee moved. Those are two different problems with two different fixes.
Heat and cooling
The thermostat setting is usually the largest single lever. Decide who owns it, what the comfortable setting is for your parent, and what the setting should be when nobody is in the room. A small change over a full day adds up. Check the rooms used most, and look for drafts around doors and windows where heat escapes.
Hot water and laundry
Count how many loads of laundry the house runs in a week before and after the change. Full loads and cool water are usually the easier choice on the bill than many small hot loads. Count showers and baths too, and check the water heater setting. A running toilet or a dripping faucet adds water use you never see, so listen for them at night.
Lights, plugs and equipment
Walk through the house and count lights left on in empty rooms, extra lamps, second televisions, and any heater or fan that runs all day. For any powered equipment in the home, read the manual for its expected power use or ask the supplier, rather than guessing. Do not change a medical device's settings to save power. Ask the people who supplied it first.
Ask about help before you pay the increase
Many utilities have assistance programs with eligibility rules, and some offer budget billing that spreads the cost across months. Check your utility's own assistance page for the current rules, and look at our local help page for St. Louis programs. Rules change, so read the current criteria yourself before you apply.
Change one thing at a time
Pick one driver, change it, and watch the usage line for two billing cycles before you change the next. If you change five things at once, you will not know which one worked. Keep the notes on the fridge with the other lists so the whole household works from the same numbers.