How to Calculate Your Daily Energy Use Before You Buy Solar
Every sensible solar design starts with one number: how much electricity you use in a day. Get that number wrong and everything built on it, from panel count to battery size, will be wrong too. The good news is that it takes an evening and a pen.
Step 1: List what you want to power
Write down every device and appliance that will run on the system. For a grid-connected home you can skip the list and use your bills (see below). For a cabin, camp, RV or backup system, the list is the whole job.
Step 2: Find the power rating in watts
Look on the nameplate or label of each device. If it shows amps and volts instead, multiply them to get watts. For devices that cycle on and off, such as refrigerators, a plug-in energy meter gives a more honest figure. In Canada, appliances with an EnerGuide label list annual energy use in kWh, which you can divide by 365 for a daily figure.
Step 3: Estimate hours of use per day
Be realistic. Lights, chargers, pumps and TVs each have their own pattern. Be honest about winter, when you may use more lighting and run more equipment.
Step 4: Multiply and add
The formula is simple:
Watts × hours per day = watt-hours (Wh) per day. Divide by 1,000 to get kilowatt-hours (kWh).
Here is a worked example. The numbers are illustrative only; use your own nameplate values.
| Device | Watts (example) | Hours per day | Watt-hours per day |
|---|---|---|---|
| LED lighting | 40 | 5 | 200 |
| Laptop and phone charging | 60 | 4 | 240 |
| Refrigerator (from an EnerGuide label of 365 kWh per year) | n/a | n/a | 1,000 |
| Water pump | 400 | 0.5 | 200 |
| Total | 1,640 Wh = 1.64 kWh per day |
Step 5: Account for the seasons
Your winter number is often higher than your summer number, and winter is exactly when solar produces the least. If your property is used year-round, size from the winter figure. If it is a summer-only camp, size from the summer one.
Step 6: Add a margin
Inverters, wiring and batteries all lose some energy, and loads tend to grow. Many designers add a margin of roughly 20% to 30% on top of the calculated figure. Treat that as a starting point and adjust it for your situation.
If you are on the grid: use your bills
Your utility bill shows kWh used in each billing period. Divide by the number of days in the period to get kWh per day. Look at one winter bill and one summer bill, because they will differ. If your utility offers online usage data, a full year of it is even better.
Cut the load before you size the system
Every watt you do not use is a watt you do not have to generate and store. Before sizing, look at easy wins:
- LED lighting instead of incandescent or halogen.
- A high-efficiency refrigerator or freezer.
- Propane, wood or other fuels for heating and cooking in off-grid settings, because electric heating and cooking are usually the largest loads.
- Switching things off at the source. Standby loads add up over 24 hours.
Peak watts matter too
Daily energy (kWh) tells you how much to generate and store. Peak power (kW) tells you how big the inverter must be. Motors, pumps and compressors draw a surge when they start, so list the surge rating of anything with a motor.
What to do with your number
Once you have your kWh per day, you can start choosing a system type. Our guide to off-grid, on-grid and hybrid systems explains the differences. When you are ready to compare complete options, the complete solar systems collection is the place to start.
This article is general information, not electrical or engineering advice. Your installer or designer should confirm final sizing for your site.