How this is worked out

Every figure on this page comes from either a stated source or a labelled assumption. Here is each one, in plain words, with its value.

Temperature

Each city's hourly outdoor temperature follows its measured summer, winter and shoulder-season high and low, peaking at 3pm and bottoming out at 3am — a smooth curve shaped like a day actually feels, not a straight line between two numbers.

Cooling (HVAC)

The calculator steps your home's indoor temperature through the day one hour at a time. Each hour the home warms toward the outdoor temperature, gains 0.26°F from people and appliances, and cools by however long the air conditioner runs. Heating isn't modeled.

How fast the home warms depends on how well it holds its temperature. Each hour it closes a share of the gap between indoor and outdoor:

These come from our pilot homes. Measured over the stretches when their air conditioners were off, the three central-AC homes closed 1.5% to 3.5% of the gap an hour and the two window-AC rooms 4% to 6%. Low and high bracket what we measured; they are not a survey of all housing. With the outdoor temperature equal to the indoor, the central-AC homes still warmed 0.1 to 0.3°F an hour, which is where the 0.26°F comes from.

Your preference sets the comfort range, in degrees Fahrenheit from your preferred temperature:

Without a manager, the thermostat holds the middle of that range. With one, the calculator finds the cheapest way to run the air conditioner that keeps the home inside the range every hour. On a flat price that means letting the home sit warmer, because a warmer home gains heat more slowly. When some hours cost more, it also cools the home ahead of them and lets it warm slowly through them. If you have solar, cooling that your spare solar covers is priced at what exporting that power would have earned, so with a low export credit the manager cools the home while the sun is up. The indoor temperature chart above shows all of this.

The air conditioner is sized to run flat out when it is22.5°F hotter outside than the setpoint, so a leakier home gets a bigger unit. A 2,000 sq ft medium home's unit draws 2.86 kW (20 BTU per square foot at SEER2 14) and cools at 1.05°F an hour at full run. Those two are assumptions. They are set so a medium home holding a steady setpoint uses what the earlier version of this calculator gave. While running, the pilot central-AC homes cooled 0.7 to 1.1°F an hour, so if anything the model under-states how fast a home can be cooled ahead of a peak.

For a 2,000 sq ft Chicago home with medium insulation at the widest range, the manager saves 26% on cooling at a flat price. On a 10¢/35¢ rate that peaks from 4pm to 9pm it saves54%: it cools the home to 71.8°F by 4pm and lets it rise to 73.7°F by 9pm. In Sacramento on the same rate it uses5% more electricity than the thermostat and still costs 46% less.

A replay of the product's actual overnight schedules on three real homes near Chicago and New York, over summer 2026, predicted 23% to 53% saved against the same middle-of-the-range baseline: predicted, not metered, and cooling only. On those homes' rates most of it came from using less energy. How much comes from timing depends on how far apart your cheap and expensive hours are, and how well your home holds its temperature.

Other home use

Lights, appliances and everything else default to 8 kWh a day plus 0.004 kWh per square foot, spread across the day in a shape built from typical household use — more in the evening, less overnight. None of it is shiftable or managed; you can edit the daily total.

Solar

Solar figures are three years of measured sunlight (2023 to 2025) on a 20-degree roof or a vertical balcony panel, in your own city's climate. Every hour is derated by 80% for real-world panel and inverter losses, then by a shading factor you choose — a rule of thumb, not a measurement:

• Mostly unshaded: 100%
• Some trees or obstructions: 80%
• Mostly shaded: 55%

EV charging

Daily charging need is whichever is smaller: your daily driving distance at 0.3 kWh per mile, or your target charge level times the battery's size — divided by a 90% charging efficiency. Without a manager, the car charges at full power from the moment it's plugged in. With one, it charges in whichever hours of the plug-in window are cheapest, counting any leftover solar as free or discounted power first. If the window is too short to finish, both ways say so.

Battery

A home battery and a plug-in battery, if you have both, are treated as one pool: their capacity and power add together. Charging loses 10% to round-trip efficiency. A reserve is always kept back, in case the day runs long:

• low: 30% reserved
• medium: 20% reserved
• high: 10% reserved

Without a manager, the battery only charges from solar you'd otherwise export, and only discharges to cover your own load — it never buys from the grid and never sells back to it. With one, it also buys at the day's cheapest hours and sells at its most expensive ones, whichever pair of prices works out best over the day. A battery on a flat rate has no cheap-versus-expensive hours to work with, so it earns nothing from timing there — it's only useful for storing solar.

Prices and the grid

Each hour, what the home draws from the grid is priced at that hour's rate; anything it sends back is credited at a share of that same rate, by export credit:

• Full credit: 100% of the price
• Half credit: 50% of the price
• No credit: 0% of the price

The default is no credit when your only solar is a balcony system, and half otherwise — you can change it. Prices from wholesale markets carry a delivery adder (default 8¢/kWh, editable) added before anything else, since a wholesale price alone never reaches your meter.

How close this is to your bill

Every figure here is an estimate. The weather and prices are measured, and the heat-up rates come from real air conditioners in our pilot homes, but your home is described by a few settings and stated assumptions about how buildings gain and lose heat. A real home has its own walls, windows, sun, air leaks, occupants and equipment, and those change how its energy flows. Read the results as a guide to what a manager could do for a home like yours, not as a quote or a promise of savings. An energy management tool that uses real data from your home or building should give more accurate results.

What this doesn't cover

• Heating. The model only cools: on days that need heat it counts no energy for it, and the pilot data behind it is from summer cooling.
• Water heaters, robot vacuums, demand charges, and tiered or baseline-allowance billing.
• Utility-specific net-metering rules — export credit is one of the three simple choices above, not your actual utility's tariff.
• An account or a saved report on a server. Nothing you enter here is sent anywhere or stored anywhere but your own browser's address bar.
• Live prices. Every curve on this page was baked in when the site was built.
• A hardware price. Payback only appears once you enter a cost yourself.
• Product or installer recommendations.

In your AI assistant

The same math is also available as read-only tools an assistant such as ChatGPT or Claude can call directly — see using it in your assistant.

Where the numbers come from

• Open-Meteo (CC BY 4.0)
• Data provided by Grid Status