How to Calculate Daily Energy Usage Before You Buy Solar

Most people buy solar panels like they buy a shirt guessing the size. In solar a medium guess costs you $2,000 extra or leaves you with a dead system at 2am. Learning how to calculate daily energy usage is the single most important step before buying anything. It takes 30 minutes and saves thousands.

How to Calculate Daily Energy Usage

The GridFree Golden Rule

The only formula you need:

Watts × Hours Used Per Day = Watt-Hours (Wh) Wh ÷ 1000 = Kilowatt-Hours (kWh)

That’s it. Everything else in solar sizing flows from this calculation.

Think of an energy audit like a multi-point vehicle inspection. You can’t fix the performance problem if you don’t know where the leaks are. You can’t size a solar system correctly if you don’t know what you’re actually consuming.

A Simple Worked Example

Fridge: 150W × 8 hours = 1,200Wh
LED lights (4 bulbs): 40W × 6 hours = 240Wh
Phone charging: 10W × 2 hours = 20Wh
Laptop: 60W × 4 hours = 240Wh
CPAP (no humidifier): 50W × 8 hours = 400Wh
Total: 2,100Wh = 2.1kWh per day

Daily Load Worksheet

Copy this into a notebook and fill it in for your home:

Appliance	Watts	Hours/Day	Watt-Hours/Day
Refrigerator
Lighting
Phone/Tablet
Laptop
CPAP
TV
Other
TOTAL

The Hidden Watts: Identifying Your Largest Loads

The Vampire Load Warning

Appliances pull watts even when switched off. Phone chargers, TVs on standby, cable boxes, gaming consoles, microwave clocks. A typical home has 50–100W of phantom load running 24 hours a day that’s 1,200–2,400Wh per day from devices doing nothing useful. Unplug what you don’t use. It matters more off-grid than anywhere else.

The Inductive Load Reality

Motors don’t start gently. A fridge compressor rated at 150W running watts needs 400–600W at startup. A well pump rated at 800W needs 2,000W+ to start. This is called startup surge and it’s why your inverter must be sized for peak demand not average demand. Size your inverter for running watts only and your system will shut down every time the fridge cycles.

The Anker SOLIX C1000 Gen 2 handles compressor surge with its 2000W surge capacity one reason it’s the recommended starter station for off-grid beginners who want to run a fridge without tripping their system.

For real startup surge numbers see our 1000W Portable Power Station Reality Check.

The Heating and Cooling Problem

Electric space heaters are watt monsters. A 1500W space heater running 4 hours = 6,000Wh per day. That single appliance uses more power than everything else in the example above combined. For off-grid use resistive heating loads are almost always replaced with propane or wood heat. If you plan to run electric heating off-grid your system cost doubles or triples.

Reading Your Ontario Hydro Bill

For Ontario readers the fastest shortcut to your daily usage number is already on your hydro bill. Look for:

Average Daily Consumption – shown in kWh
Monthly Usage – divide by 30 for your daily average
Time-of-Use breakdown – shows on-peak vs off-peak consumption

Ontario Time-of-Use rates mean calculating WHEN you use power matters as much as how much. On-peak rates ($0.18–$0.22/kWh) apply weekday mornings and evenings. Off-peak rates ($0.08–$0.10/kWh) apply nights and weekends. A solar system that covers your off-peak usage has very different economics than one sized for peak demand.

Pull your last 3 hydro bills. Find the daily average on each one. The highest number is your worst-case day that’s the number you size for.

The Kill-A-Watt Method

For older appliances or anything you’re not sure about a plug-in energy meter gives you real numbers in real time. Plug it between the wall and the appliance. Run it for 24 hours. Read the actual watt-hours consumed.

Most useful for:

Older fridges real consumption is often 2–3x the nameplate rating
Chest freezers
Well pumps
Any appliance that cycles on and off

Search Amazon for Kill-A-Watt meter budget around $30–$50 CAD. It pays for itself the first time you discover your old fridge is consuming 800Wh/day instead of the 300Wh you assumed.

Once you know your real daily load pair a Renogy 100W Solar Panel with the Anker SOLIX C1000 Gen 2 as your starting point the 1056Wh capacity covers the 2.1kWh example load for roughly half a day, giving you a realistic baseline to size up from.

Winter vs Summer: Size for Your Worst Month

Winter: More lighting hours dark by 4:30pm in Ontario. Heating loads increase. Solar production drops with shorter days, lower sun angle, and potential snow coverage. Your system must survive January. If it does summer takes care of itself.

Summer: Fans and AC loads increase but longer days mean more solar production. Generally easier on off-grid systems than winter.

The rule: Calculate your worst-case month usually January in Ontario. Size your system for that number. Everything else is a bonus.

Pro Tip: Don’t guess appliance wattages. Most appliances have a nameplate label on the back or bottom showing voltage and amperage. Multiply volts × amps = watts. A fridge showing 120V / 1.5A = 180W running wattage. Always confirm with a Kill-A-Watt meter for anything that cycles on and off the nameplate number and the real number are often very different.

The Verdict

Skipping the energy audit is the single most reliable way to fail at solar. You will either oversize and overspend or undersize and run out of power when you need it most.

How to calculate daily energy usage takes 30 minutes and a piece of paper. Do it before you buy a single panel, battery, or inverter.

Your homework: Pull your last 3 Ontario hydro bills. Find the daily average consumption on each one. Write down the highest number. That is your sizing baseline. Everything else builds from there.

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How to Calculate Your Daily Energy Usage (Before You Buy a Single Panel)