Introduction
We wrote this guide because we made every mistake when installing our solar panel system, and we don't want others to make those since we want everyone to have panels themselves. We didn't know the difference between a kW and a kWh before we started our solar panel journey, so we want to help others understand this exciting technology. Since we didn't know what we didn't know, we ended up with a slightly smaller system than we needed; the cost to add those last few panels is too small for any installer to consider, so let's get your system right the first time!
kW vs kWh
The flow of sunlight is measured in Watts and can be thought of as raindrops:
| Unit | Example of usage | Analogy | Analogy example of usage | |
|---|---|---|---|---|
| 1 Watt (W) | 1 LED night light | one raindrop per second | a light drizzle of rain | |
| 1 Kilowatt (kW) | a microwave oven's rays | 1,000 raindrops per second | a downpour of torrential rain | |
| 1 Kilowatt-Hour (kWh) | electric oven for 24 min | the amount of raindrops collected in one hour | the amount of water in a jacuzzi | |
| 5 Kilowatt-Hour (kWh) | electric oven for 2 hrs | the amount of raindrops collected in five hours | the amount of water in a pool |
Typical electric usage
The amount of electricity you use will vary throughout the day, depending on which items you use and for how long.
For comparison, the following table lists the actual kW and kWh we use.
| Electric item | Size | Remarks | kW usage | Annual kWh |
|---|---|---|---|---|
| Microwave oven | 2.2 cu.ft. | 5-10 min/day | 1.5 - 1.8 | 25 - 50 |
| Dishwasher | 24" x 24" x 35" | 1 kWh per 60 min cycle | 0.6 - 2.5 | 48 - 60 |
| Clothes washer | 27" x 40" x 30" | 1-2 kWh per 30 min load | 1.2 - 3.0 | 96 - 108 |
| Clothes dryer | 27" x 40" x 30" | 2-5 kWh per 60 min load | 2.0 - 6.2 | 312 - 360 |
| Oven | 5.0 cu.ft. | 300 - 425 °F | 5.0 - 9.6 | 850 - 950 |
| Heat pump + AC | 1.5 tons | 68 °F | 0.5 - 1.5 | 1100 - 1500 |
| Water heater | 40 gallon | 30 amp, 120 v, 1 hr/day | 0 - 4.5 | 1600 - 1700 |
| EV wall charger | 32 amp | 40 amp breaker | 1.0 - 7.7 | 400 - 2000 |
| Stove | 5 elements | 2 kWh x 3 hrs/day | 1.2 - 3.0 | 2000 - 2200 |
| Refrigerator | 27 cu.ft. | runs 24/7 | 0.5 - 1.1 | 1450 - 2900 |
| =========== | ||||
| 6400 - 8300 |
Once, for an hour, we were washing and drying a load of laundry while using the oven and stove--all at the same time. That "maximum usage" of 11.5 kW turned out to be good to know since our Powerwall+ is able to supply a maximum output of 5.7 kW, which required the remaining 5.8 kW to come from the Grid.
So, we learned to use heavy appliances at different times--until we added a second Powerwall. (That issue is no longer a problem since a Powerwall 3 can output 11.5 kW)
Luxury electrical usage
| Electric item | Size | Remarks | kW usage | Annual kWh |
|---|---|---|---|---|
| Space heater | 5118 BTU | 170 sqft, 11 hrs/day 5 mo/yr | 0.75 - 1.5 | 1250 - 2500 |
| Heat pump water heater | 50 gallon | 5.0 | 850 - 900 | |
| Jacuzzi | 400 gallon | 6-person (3.5 - 6 kWh/day) | 1.5 - 2.0 | 1250 - 2200 |
| Sauna / steam room | 4-person | 4 - 9 | 150 - 500 |
30-year comparison
