Power is a rate and energy is a total, and your utility only bills you for the total. Kilowatts are the speedometer, kilowatt-hours are the odometer, and the single number that sizes a solar system is the odometer reading on your electric bill: kWh used. Everything else on that page is arithmetic built on it, or a fee solar cannot touch.
Speedometer and odometer
Picture your dashboard. The speedometer says 60 miles an hour. That is a rate, how fast you are moving right now. The odometer says 120 miles covered. That is a total. Drive at 60 for two hours and the odometer picks up 120. Rate times time equals total.
Electricity works exactly the same way. A kilowatt (1,000 watts) is the speed: how fast power is flowing this instant, whether a panel is making it or an air conditioner is burning it. A kilowatt-hour is the distance. Run something at 1kW for one hour and you have used 1kWh.
The thing doing the using can be big or small, and the hours matter as much as the watts. A fridge sips somewhere in the 100W to 250W range while its compressor runs, cycles all day, and lands around 1kWh to 2kWh per day. A typical central AC pulls 3kW to 5kW while running. A 4kW unit that runs a combined six hours on a hot July day is 24kWh from one appliance. Small speed for long hours can out-eat big speed for short hours, and the bill only sees the totals.
Why every box uses a different unit
Once the rate and the total are separate things in your head, the spec sheets sort themselves.
The bill is in kWh because the utility charges for the total that crossed the meter. They do not care whether you used it fast or slow.
An inverter is rated in kW because its job is a rate. The EG4 12000XP is rated 12kW continuous. That is the width of the pipe, the most house it can carry at one moment, and it says nothing about how long.
A battery is rated in kWh because it is a tank. One EG4 wallmount holds 14.3kWh. How long that lasts is pure division by your draw: a house sipping 500W gets around 28 hours out of it, a house pulling 5kW gets under three. Same tank. The speed of the draw does the rest.
So a system quote always carries both numbers. The kW covers your biggest moment, the kWh covers your longest stretch, and one never substitutes for the other.
Where the kWh hides on the bill
Every utility lays the page out differently, but the number is always there. Look for a line called kWh used, total usage, energy usage, or billed kWh, usually near the meter readings. Some co-op bills around the panhandle tuck it into the meter reading table as three numbers: previous reading, current reading, and the difference. The difference is your month. If the paper defeats you, the online account almost always shows usage more clearly, with a year or more of history.
Most bills also print a small bar chart, twelve or thirteen months of usage side by side. It is the most useful thing on the page. Tall bars are July and August with the AC grinding. Short ones are the mild months. Anyone sizing a system should look at that shape first. Our guide to reading the bill walks the whole page line by line.
Your real rate is a division problem
Bills split the price of power into pieces: an energy charge, a fuel charge, taxes, franchise fees, storm surcharges. No single line tells you what a kWh actually costs you. So skip the line items and divide. Total dollars over kWh used. A $180 bill for 1,200 kWh (round practice numbers, every bill differs) works out to 15 cents per kWh. That blended rate is what one kWh of solar production is worth at your house, fees and taxes included. Fuel charges drift, so run it on two or three bills and take the middle.
What solar removes, and what it never removes
Solar attacks the usage side. Shrink the kWh you buy and the energy charge, the fuel charge, and the taxes riding on both shrink with it. But somewhere on the page is a flat monthly amount labeled customer charge, base charge, or service availability charge. That is the cost of being connected to the grid at all, and it survives any system size. When a yard-sign pitch promises a zero bill, that fixed charge is the tell that the pitch is a sales line.
One more trap: seasons. An August bill and a January bill from the same Florida house can differ by a factor of two or more. Size from August alone and the system overproduces most of the year. Size from January and the first summer eats you alive. The number that behaves is the yearly total, twelve months of kWh added into one figure.
Try this at home
Pull a real bill, yours, and extract four numbers: the shape of the usage chart, the twelve-month kWh total, the blended rate (total dollars over kWh), and the fixed monthly charge. Write them down. That is the ground truth every honest sizing conversation starts from, it takes ten minutes, and it is exactly what we ask for first. Bring the yearly total to the system builder and the sizing math starts on solid ground.
Sources
- EG4 12000XP datasheet: 12kW continuous output rating. Check the current sheet before designing.
- EG4 WallMount Indoor battery datasheet: 14.3kWh nameplate per unit.
- Appliance draw ranges (a fridge at 100W to 250W and 1kWh to 2kWh per day, central AC at 3kW to 5kW) are typical ranges for illustration, never a promise about a specific appliance.
- The $180 for 1,200 kWh and $210 for 1,400 kWh bills are invented round numbers for arithmetic practice, labeled as such.
- Rate times time equals total is physics, no document needed.