The simple version
- Watts (W) / kilowatts (kW): how fast power is being used right now. A 100 W bulb uses 100 watts while it's on. 1 kW = 1,000 W.
- Watt-hours (Wh) / kilowatt-hours (kWh): how much was used over time. That 100 W bulb on for 10 hours uses 1,000 Wh = 1 kWh.
Think water: kW is how far the tap is open; kWh is how many gallons ended up in the bucket.
Why it matters for solar
Batteries are sized in kWh (how much they store). Inverters are sized in kW (how much they can run at once). Panels are rated in W or kW, and what they make in a day depends on your sun hours.
| Thing | Measured in | Example |
|---|---|---|
| Your daily use | kWh / day | 7 kWh a day in a tiny house |
| A battery | kWh stored | ~5 kWh per server-rack battery |
| An inverter | kW it can run at once | 6 kW or 12 kW |
| A panel | W of peak output | 400 W |
Find your number
Look at your power bill for the month with the most kWh and divide by 30. That's your daily use, and the one number that sizes everything else. Then try the battery bank calculator, or let a guide size the whole system.
Questions people ask
Is kWh per hour or per day?
kWh is a total amount of energy. It's often quoted per day (kWh/day) to size solar, or per month on your power bill.
How many kWh does a house use per day?
A typical US home uses around 25–30 kWh a day, but it varies a lot. Your power bill shows your exact monthly total; divide your highest month by 30.
What size solar system do I need for 30 kWh a day?
It depends on where you live, because winter sun varies. As a rough guide, 30 kWh a day in a place with about 3 hours of winter sun needs around 14 kW of panels. The guides do this math for your location.
Get the exact setup
A few taps and you'll know exactly what to buy for your situation.