How to Estimate Your Daily Power Use
A simple method for figuring out how much electricity you actually use — the first step before sizing any solar setup.
I guessed my power needs when I bought my first inverter. It was too small by half. The fridge alone drew more at startup than I planned for the entire cabin.
Sizing solar starts with one question: how much power do you actually use in a day? Not how much you wish you used — how much you use now, or will use, with real appliances and real habits.
List everything that plugs in
Walk through your cabin — or the setup you’re planning — and write down every device that draws power. Don’t skip the small stuff yet. Include:
- Fridge and freezer
- Lights (count bulbs and note wattage if you can)
- Phone and laptop chargers
- Water pump
- Router or modem
- Fan, heater, or stove fan if electric
- Anything you run on a generator now
For each item, note whether it runs continuously (fridge cycles all day), daily for a set time (laptop for three hours), or occasionally (power tool for twenty minutes).
I missed my router the first time through. It runs 24/7 at low wattage, which adds up quietly over a month.
Find the wattage
Wattage (W) is the number on the label, in the manual, or in an online spec sheet. It’s how fast a device draws power at any given moment.
If you only see amps (A) and volts (V), multiply them: watts = amps × volts. A 120V device drawing 2A uses about 240W.
Some labels show kilowatts (kW) — that’s 1,000 watts. A 1.2 kW kettle is 1,200W.
When you can’t find a number, use a rough estimate and mark it with a question mark so you know to verify later. A laptop charger is often 60–90W. LED bulbs are usually 8–12W each.
Calculate watt-hours per day
Watt-hours (Wh) measure total energy used over time. This is the number that matters for battery sizing.
Formula: watts × hours used per day = watt-hours per day
Examples from my cabin:
- LED lights: 10W × 4 hours = 40 Wh
- Laptop: 65W × 3 hours = 195 Wh
- Router: 10W × 24 hours = 240 Wh
- Fridge: trickier — it cycles on and off. I used a power monitor for a week and averaged 800 Wh per day. Without a monitor, budget 600–1,200 Wh for a small efficient fridge.
Add every line item. My daily total came to about 2,400 Wh (2.4 kWh) in summer and closer to 3,200 Wh in winter when lights and the stove fan run more.
Account for surge loads
Some devices draw a brief spike of power when they start — well pumps, fridge compressors, circular saws. Your inverter must handle the peak even if the daily average looks fine.
Check startup (surge) ratings separately. My fridge pulls about 120W running but spikes near 800W when the compressor kicks in. If the inverter can’t handle that spike, it shuts down — which is how I learned my first one was undersized.
Rule of thumb: note the highest surge device and make sure your inverter exceeds that number with some headroom. Running two surge-heavy devices at once means adding their peaks together.
Build in margin — but not too much
Add 20–30% to your daily watt-hour total for cloudy days, inefficiency, and the things you’ll forget to count. I aimed for 25% margin and still wished I’d gone slightly higher my first winter.
More margin costs more in panels and batteries. Less margin means cutting usage on dark days. There’s no perfect number — it depends on your budget and how much inconvenience you’ll tolerate.
If you’re on a tight budget, size for your actual usage and plan to reduce load on bad days rather than oversizing the whole system upfront.
What this guide won’t cover
Panel count, battery bank sizing, and wiring — those come after you have a reliable daily watt-hour number. I also won’t recommend specific brands or monitors here.
Where to go next
If the vocabulary still feels fuzzy, read Solar Basics: Panels, Batteries, and Inverters Explained first.
For the bigger picture of self-sufficient living beyond power, What Self-Sufficient Living Actually Looks Like puts daily energy use in context with water, heat, and food.