By Dan Strawn

Summer has arrived in Denver, and we just had our first scorcher. You felt it. I felt it. Even the dog looked offended. And now, with Utility Companies’ time-of-use rates kicking into full swing, you’re probably wondering: How do I keep my home cool without setting my wallet on fire?

Let’s get real. Expect too many numbers and some good ol’ fashioned engineering.

tldr; save a lot of money by tolerating temperature in your house in a slightly wider range!

Let’s Talk About Thermal Batteries

A thermal battery is any material or system that stores heat (or coolness) and releases it over time, helping to smooth out temperature swings. It’s not a battery in the traditional electrical sense—no lithium-ion here—but it serves a similar purpose. It stores thermal energy when it’s cheap or easy, and releases it when it’s expensive or hard to control.

The idea: Turn your house into a thermal battery.

Step One: Understanding How Your Home Stores Heat

The materials in your home all handle heat differently.

• Metal (like in radiators) = very conductive, heats up and cools down fast.
• Stone, brick, plaster, drywall = slow to warm up, slow to cool off (high thermal mass).
• Air = fast-moving, doesn’t store heat well.

To figure out how well your home stores cool air, try this simple experiment:

✏️ Home Thermal Decay Test

1. Night before: Cool your home to a baseline temp—72°F works well.
2. At Noon: Note the outdoor temperature and indoor temp (72°F).
3. Turn off your AC for 2 hours.
4. At 2 p.m.: Record the new indoor temp.

📊 What You’ll Learn:

You’ll now have a temperature rise (ΔT) over 2 hours.

For example:

• Outside temp = 94°F
• Start indoor temp = 72°F
• End indoor temp = 76°F
• ΔT = 4°F over 2 hours

This gives you a sense of your home’s thermal inertia—basically how quickly it heats up. The lower the ΔT, the better your home holds cool air (your “thermal battery” is working well). A little trial and error will help you dial in your own system! You might also see that insulation would help, and it arms you to know your real savings!

Step Two: Weaponizing Time-of-Use Pricing

Our utility energy charges are based on Time of Use (TOU). That means electricity is cheap when demand is low, and stupidly expensive when demand is high.

Here’s how I game the system using my house as a thermal battery:

Time	Action	Reason
6 a.m.–1 p.m.	Cool house to 72°F	Electricity is cheap.
1 p.m.–3 p.m.	Let house warm to 75°F	Prices rising, but not peak yet.
3 p.m.–9 p.m.	Raise thermostat to 80°F (AC off)	Electricity cost triples. Ride it out.
9 p.m.–midnight	Cool house back to 72°F	Recharge your thermal battery.

🔍 The Math Behind It

Let’s say your AC uses 3.5 kW when running.
Electricity rates in Denver (approximate):

• Off-peak (before 1 p.m. / after 9 p.m.): $0.10/kWh
• Mid-peak (1 p.m.–3 p.m.): $0.20/kWh
• Peak (3 p.m.–9 p.m.): $0.30–$0.35/kWh

Scenario A: No Thermal Battery Use

You run your AC normally all day (on/off cycling):

• 10 hours of run time = 35 kWh
• Let’s average cost at $0.22/kWh
• Total: $7.70/day

Scenario B: Dan’s Thermal Battery Strategy

Time	Energy Used	Cost/kWh	Total
6a–1p (AC on full)	10 kWh	$0.10	$1.00
1p–3p (partial AC)	4 kWh	$0.20	$0.80
3p–9p (AC off)	0 kWh	$0.30	$0.00
9p–12a (recharge)	7 kWh	$0.10	$0.70
Total:	21 kWh	—	$2.50/day

Daily Savings: ~$5.20/day, or $150/month during the summer.

And I’m not sitting in a sauna. My house drifts from 72°F to ~78°F by 9 p.m.—which is tolerable, even comfortable, with good air circulation and ceiling fans.

Final Thoughts

You’re not just fighting the heat—you’re fighting a pricing model that works against you for staying cool at the wrong time. By pre-cooling and using the thermal mass of your home, you’re storing coolness like energy in a battery.

Yes, it takes a little testing and a few spreadsheets (don’t tempt me…), but the payoff is real. Your house has more brains than you think—it just needs a good operator. Check out our little calculator for fun!

Stay cool out there.
– Dan