Every solar calculation starts with the same question: how much energy do you use per day? Get this number wrong and everything else — panels, batteries, inverter — will be wrong too. Overestimate and you waste money on equipment you don’t need. Underestimate and your system runs short on the first real day of use.
The process is simple: list your appliances, multiply watts by hours, and split the total into daytime and nighttime. Here’s exactly how to do it.
Why the Day vs Night Split Matters
Most solar guides tell you to calculate total kWh per day and stop there. For off-grid solar, that’s not enough. You need to know how much energy you use during the day and how much you use at night — because the two halves of your consumption are handled by completely different parts of the system.
Solar panels only produce during daylight hours. Everything running at night must come from your battery bank. Your panel array has to generate enough during the day to power daytime loads AND charge the batteries for nighttime use. Your battery bank is sized based on nighttime consumption, not total consumption.
Here’s why this matters in practice. Two homes both use 6 kWh per day. Home A uses 5 kWh during the day and 1 kWh at night. Home B uses 2 kWh during the day and 4 kWh at night. Both need the same number of panels — but Home B needs roughly four times the battery capacity. Skip the day/night split and you’ll either overbuild or underbuild your battery bank.
This is the most common solar system sizing mistake — and it starts right here, before you size a single component.
Step 1 — List Every Appliance
Walk through every room in your house and write down every electrical device you plan to run. Don’t forget rooms like the garage, outdoor areas, and utility closets.
Here are typical wattages for common appliances:
- LED light bulb: 8–12W
- Ceiling fan: 50–75W
- Phone charger: 5–10W
- Laptop: 50–65W
- WiFi router: 10–15W
- TV (LED, 40-55″): 50–100W
- Refrigerator: 100–200W (compressor cycling)
- Microwave: 800–1,200W
- Washing machine: 400–500W
- Electric kettle: 1,500–2,000W
- Hair dryer: 1,000–1,800W
- Air conditioner (window/mini-split): 500–2,000W
- Heat pump: 1,000–4,000W
Tip: Check the nameplate label on each appliance. It shows the wattage directly. If it lists amps and volts instead, multiply them together: watts = volts × amps. A label showing 2.5A at 220V means 550W.
Step 2 — Estimate Hours Per Day
For each appliance, estimate how many hours per day it actually runs. Some are straightforward — you watch TV for 3 hours, you run lights for 5 hours. Others are tricky.
Cycling appliances: A refrigerator is plugged in 24 hours a day, but the compressor only runs about half that time. Don’t use 24 hours — use the effective runtime of 8 to 12 hours. Better yet, check the EnergyGuide label for annual kWh and divide by 365 to get a precise daily number.
Intermittent appliances: A washing machine runs for 1 hour per load. If you do 3 loads per week, that’s 3 hours per week or 0.43 hours per day. A microwave used for 15 minutes a day is 0.25 hours.
Be honest, not aspirational. People consistently underestimate how much they use appliances. If you think you watch TV for 2 hours, it’s probably 3. If you think the AC runs for 4 hours, it’s probably 6. Underestimating usage hours is the single most common error in this process.
Step 3 — Calculate Watt-Hours Per Day
For each appliance, multiply watts by hours to get watt-hours per day. Then assign each appliance’s usage to daytime (while the sun is up) or nighttime (after dark). Some appliances split across both.
Here’s a complete example for a modest off-grid household:
Daytime loads (roughly 7 AM – 6 PM):
- LED lights (2 bulbs): 20W × 1h = 20 Wh
- Refrigerator (daytime compressor): 150W × 6h = 900 Wh
- Laptop: 60W × 4h = 240 Wh
- WiFi router: 12W × 11h = 132 Wh
- Ceiling fan: 60W × 4h = 240 Wh
- Microwave: 1,000W × 0.25h = 250 Wh
Daytime total: 1,782 Wh (1.78 kWh)
Nighttime loads (roughly 6 PM – 7 AM):
- LED lights (6 bulbs): 60W × 4h = 240 Wh
- Refrigerator (nighttime compressor): 150W × 6h = 900 Wh
- WiFi router: 12W × 13h = 156 Wh
- TV: 70W × 3h = 210 Wh
- Ceiling fan: 60W × 4h = 240 Wh
- Phone chargers (2x): 20W × 3h = 60 Wh
Nighttime total: 1,806 Wh (1.81 kWh)
Daily total: 3,588 Wh (3.59 kWh)
This is a modest household. Add an air conditioner running 8 hours a day at 1,500W and the total jumps to 15.59 kWh — more than quadrupling the system size.
Step 4 — Account for Seasonal Variation
Your energy consumption isn’t constant throughout the year. Summer and winter can look very different:
Summer: Air conditioning and fans push consumption up significantly. Longer daylight hours mean more daytime production, but also more cooling demand. A household using 4 kWh/day in spring might use 10 kWh/day in summer with AC running.
Winter: Heating appliances appear — electric heaters, heat pumps, heated blankets. Lighting hours increase as days get shorter. A household that barely touches 4 kWh/day in summer might hit 8 kWh/day in winter with heating.
The worst combination for off-grid solar is winter in a cold climate: your consumption goes up (heating, lighting) while your solar production goes down (fewer sun hours, snow). This double hit is why winter is the critical sizing season for off-grid systems.
Calculate your consumption for both seasons. Size your system for whichever number is larger. Oversizing for the lighter season wastes nothing — the charge controller simply limits the excess. Undersizing for the heavier season means power outages.
Common Mistakes When Estimating Consumption
1. Forgetting “always on” loads. WiFi routers, security cameras, smoke detectors, clock displays — they draw small wattage but run 24/7. A 15W router running all day consumes 360 Wh — more than your laptop used for 4 hours. Walk through your house and look for every device with a glowing light or warm power adapter.
2. Using nameplate watts for cycling appliances. A refrigerator rated at 200W does not use 200W × 24h = 4,800 Wh per day. The compressor cycles on and off. Real consumption for most fridges is 1,000 to 1,500 Wh per day. Use the EnergyGuide annual kWh divided by 365 for accuracy.
3. Ignoring phantom loads. Many appliances draw 1 to 5W even when “off” — TVs in standby, microwave clock displays, game consoles, laptop chargers left plugged in. Individually they’re tiny. Collectively, 5 to 10 phantom loads at 3W each running 24 hours add 360 to 720 Wh per day. That’s a meaningful chunk of a small off-grid system.
4. Forgetting seasonal appliances. Space heaters in winter, pool pumps in summer, holiday lights in December. If these appliances will run on your off-grid system, they need to be in the calculation for the season they’re used.
Let the Calculator Do It for You
Our Solar System Calculator has 18 preset appliances with typical wattages pre-loaded. Select from the dropdown, adjust the hours for daytime and nighttime use, and add custom appliances for anything not on the list. The calculator splits day and night automatically, then uses your consumption numbers to size panels, batteries, inverter, and charge controller.
Once you have your real consumption numbers — not guesses, not estimates from a generic table — the rest of the calculation becomes accurate. The system losses, peak sun hours, and battery chemistry all build on this foundation. Get the consumption number right and everything else falls into place.
