The short answer: To calculate your daily energy use for off-grid solar, multiply the running wattage of each appliance by the number of hours you use it per day to find its Watt-hours (Wh). For example, a 50W laptop run for 4 hours uses 200 Wh. Add up the Watt-hours for every device in your home, add 15% to account for system inefficiencies, and you have your total daily energy requirement.
Why Watt-Hours (Wh) Are the Magic Number
When building an off-grid solar system, people often focus on the wattage of their solar panels or the peak output of their inverter. But the most critical metric is your daily energy consumption, measured in Watt-hours (Wh) or Kilowatt-hours (kWh).
Energy is power × time.
- Watts (W) tell you how fast energy is being used right now.
- Watt-hours (Wh) tell you the total volume of energy used over time.
Solar panels generate Watt-hours over the course of a sunny day, and batteries store Watt-hours for the night. If you don't know exactly how many Watt-hours you consume in a 24-hour period, you are entirely guessing at how many panels and batteries you need to buy.
The Basic Calculation Formula
To perform a load audit, you need to evaluate every single electrical device you plan to power.
Daily energy (Wh) = Watts × Hours used per day
- Find the Watts: Look at the appliance's nameplate sticker (usually on the back or bottom). If it only lists Amps and Volts, multiply them (Amps × Volts = Watts). For devices that cycle on and off (like refrigerators), it is highly recommended to use a plug-in energy meter (like a Kill A Watt) for 24 hours to get an accurate reading.
- Estimate the Hours: Be realistic. You might only watch TV for 2 hours, but your internet router runs for 24 hours.
What Most Sizing Guides Forget to Tell You
Many basic calculators will have you add up your Watt-hours and stop there. But a real-world off-grid system loses energy in several ways. If you don't account for these factors, you will end up with dead batteries.
- Inverter Inefficiency: Inverters take DC power from your batteries and turn it into AC power for your wall outlets. This process is not perfect. Most inverters are about 85% to 90% efficient. This means you need to pull roughly 10% to 15% more energy from the battery than your AC appliances actually consume.
- Phantom Loads (Standby Power): Modern appliances consume power even when turned off. A microwave waiting to be used, a TV on standby, and the idle consumption of the inverter itself can easily add 300 to 500 Wh to your daily total without you ever turning a device on.
- Seasonal Variations: A ceiling fan might run 10 hours a day in July and 0 hours in December. When calculating your daily use, you must size your system for your highest consumption season, or build two separate load profiles (Summer vs. Winter).
Illustrative Example: A Weekend Cabin Load Audit
Let's look at an illustrative calculation for a small off-grid weekend cabin to see how a load list comes together.
1. List the appliances and calculate raw Wh:
- LED Lights: 4 bulbs × 10W = 40W. Used 5 hours/day. (40W × 5h = 200 Wh)
- Phone Chargers: 2 phones × 15W = 30W. Used 2 hours/day. (30W × 2h = 60 Wh)
- Small Fridge: Cycles on and off. A plug-in meter shows it uses 1,200 Wh per 24 hours.
- Water Pump: 400W. Runs for 15 minutes (0.25 hours) total per day. (400W × 0.25h = 100 Wh)
- TV: 80W. Used 3 hours/day. (80W × 3h = 240 Wh)
2. Sum the raw Watt-hours: 200 + 60 + 1,200 + 100 + 240 = 1,800 Wh/day
3. Adjust for inverter inefficiency and phantom loads: We multiply the total by 1.15 (adding 15%) to account for the inverter's conversion losses. 1,800 Wh × 1.15 = 2,070 Wh/day
This cabin needs a solar and battery system capable of reliably delivering 2,070 Wh (or 2.07 kWh) every single day.
Frequently Asked Questions
How do I calculate Watt-hours for a refrigerator? You cannot simply multiply a fridge's rated wattage by 24 hours, because the compressor cycles on and off. A 500W fridge might only run for 6 hours total in a day. The only accurate way to measure a fridge is to plug it into an energy monitor (like a Kill A Watt meter) for 24 to 48 hours and read the total kWh consumed.
Should I use starting watts or running watts for my calculation? For daily energy (Wh) calculations, always use running watts. Starting watts (the brief surge of power needed to start a motor) only last for a fraction of a second. They do not meaningfully add to your daily energy total. However, you must use starting watts when sizing your inverter to ensure it doesn't trip.
What if an appliance lists Amps instead of Watts? Use the formula Watts = Amps × Volts. If a 120V appliance is rated at 5 Amps, it consumes 600 Watts (120 × 5 = 600).
How do I account for heating and air conditioning? Heating and cooling are massive energy hogs that can easily triple your daily Wh. If you plan to run a mini-split AC, you must look at the unit's Seasonal Energy Efficiency Ratio (SEER) or use an energy monitor. Heating with electricity (like space heaters) off-grid is generally discouraged; most off-grid homes use wood or propane for heat to save their batteries.
