This is the boring step. It's also the one that decides whether your whole setup works or doesn't.
If you skip this, you'll buy the wrong battery. The wrong inverter. The wrong number of panels. Money down the drain.
Walk Around Your House With A Notebook
Make a list of everything that uses power. Lights, fridge, TV, laptop, phone chargers, washing machine, microwave. Every single thing.
For each one, write down two things:
How many watts it uses. It's printed on the back, on a sticker, or in the manual. A laptop might say 50W. A fridge might say 150W. A microwave might say 1000W.
How many hours a day it runs. A fridge runs 24 hours (but only the motor, on and off). A laptop maybe 6 hours. A microwave maybe 10 minutes (so 0.17 hours).
Multiply, Then Add
For each item: watts × hours = watt-hours per day.
A 50W laptop running 6 hours = 300 Wh per day.
A 150W fridge running effectively 8 hours of motor time = 1200 Wh per day.
Add up everything on your list. That's your total daily power use, in watt-hours.
Divide by 1000 to get kilowatt-hours (kWh), the unit most batteries are sold in.
A typical small off-grid home uses 3-5 kWh per day. A more comfortable home with a fridge, a freezer, and normal appliances uses 8-15 kWh.
Don't Forget The Surge
Some things spike when they first switch on. A fridge motor starting up might draw 3x its normal power for a second or two. Same with pumps, power tools, AC units.
The inverter has to handle those spikes. So when you size your inverter (next step), add at least 25% on top of your normal peak load.
Account For Loss
Nothing is perfectly efficient. The inverter loses 5-15% turning DC into AC. The wiring loses a little. The battery itself loses a bit when it charges and discharges.
A reasonable rule: assume you'll need 20% more battery than your raw daily use number says.
Why It Pays To Be Honest
People always underestimate their power use. They forget the laundry machine. They forget the coffee maker. They guess low because they're hopeful.
Then they buy too small a system, the lights go out in January, and they have to spend more money fixing it.
Take a full week to track your real use, not a guess. The notebook is the cheapest tool in this whole project.
→ Read more about figuring out your wattage
