How to Figure Out Your Daily Energy Usage
- Check your electricity bill. US bills show monthly kWh. Divide by 30 to get daily average. A 900kWh/month home uses 30kWh (30,000Wh) per day. For off-grid builds with no prior bills, skip to step 3.
- Identify your biggest consumers. HVAC, electric water heaters, clothes dryers, and EV chargers dominate most homes. An electric water heater alone uses 4,000-5,000Wh/day. If these are on gas, your solar needs drop significantly.
- Build a load list for off-grid. Write down every device with its wattage and daily run hours. Multiply watts by hours. A 12V fridge (50W average x 10 hours = 500Wh), LED lights (40W x 5 hours = 200Wh), laptop (65W x 4 hours = 260Wh). Add all items for your total. Be honest about run time — underestimating here is the most common sizing mistake.
- Add a buffer. Include 10-15% extra for things you forget or seasonal changes. A calculated 2,000Wh should become 2,200-2,300Wh for planning purposes.
Example: Estimating Panels for an RV
A typical RV with a 12V compressor fridge, LED lights, phone charging, a laptop, and a small fan uses about 1,500-2,000Wh per day. If you add a microwave for 15 minutes daily (1,000W x 0.25h = 250Wh) and a coffee maker for 5 minutes (600W x 0.083h = 50Wh), you are at roughly 2,300Wh.
Camping in the US Southwest with 5.5 peak sun hours and 20% losses (accounting for less-than-ideal panel angle on a roof-mounted setup): 2,300 / (5.5 x 0.80) = 523W. Two 300W panels (600W total) give a healthy 15% margin above minimum.
If you travel to the Pacific Northwest where sun drops to 3.5 peak hours in winter: 2,300 / (3.5 x 0.80) = 821W. Now you would want three 300W panels or need to reduce usage during grey stretches.
Why Oversizing by 25% Is Smart, Not Wasteful
Your solar panels will almost never produce their theoretical daily output for three reasons.
First, peak sun hours are averages. Some days are below average — sometimes far below. A 5-hour average location might see 2 hours on an overcast day. Without extra panel capacity, one bad day means a partly discharged battery bank.
Second, panels degrade over time. Most manufacturers guarantee 80-85% output at 25 years. After 10 years, expect 90-92% of original capacity. Panels you size to the exact minimum today will be undersized in a few years.
Third, your energy needs tend to grow. You add a new device, start working from home, or get an electric cooler for the boat. Extra capacity today handles tomorrow's additions without rewiring.
The cost of adding 25% more panel wattage at installation is far less than the cost of adding it later (when you may need a new charge controller, additional mounting, and more wiring).
Worked Examples
Sizing Panels for an All-Electric Suburban Home
Context
Calculation
Daily usage = 28,000Wh
Minimum solar = 28,000 / (4.7 × 0.85) = 28,000 / 3.995 = 7,009W
With 25% buffer: 7,009 × 1.25 = 8,761W
Panel count (400W panels): 8,761 / 400 = 21.9 → 22 panels (8,800W system)
Interpretation
A 22-panel, 8.8kW system covers the full 28kWh daily consumption in average conditions. In summer with 6+ PSH, it overproduces and sends energy back to the grid. In winter with 3.5 PSH, it underproduces and draws from the grid.
Takeaway
Grid-tied systems can size for the annual average since the grid handles seasonal shortfalls. For energy cost tracking, use the kWh calculator to compare your production against utility rates month by month.
Minimum Viable Solar for a Hunting Cabin
Context
Calculation
Daily usage = (30×4) + (10×2) + (5×6) = 120 + 20 + 30 = 170Wh
Minimum solar = 170 / (3.5 × 0.78) = 170 / 2.73 = 62.3W
Recommended: a single 100W panel (61% buffer over minimum)
Interpretation
At just 170Wh/day, this is a minimal load. A single 100W panel easily covers it even on below-average sun days. A 50W panel would technically work but leaves almost no margin for cloudy stretches.
Takeaway
Small loads need small systems. Pair this 100W panel with a single 50Ah LiFePO4 battery for 2+ days of autonomy. Check whether that battery handles your loads with the 12V battery runtime calculator.
Frequently Asked Questions
Glossary
Load Audit
A detailed inventory of every electrical device, its wattage, and daily run hours. Multiplying watts by hours for each device and summing the results gives total daily energy consumption in watt-hours — the starting point for solar sizing.
Grid-Tied System
A solar system connected to the utility grid that exports excess power and draws from the grid when production is low. Grid-tied systems don't need batteries for basic operation, though batteries add backup capability.
Oversizing Buffer
Extra panel wattage beyond the calculated minimum, typically 20-30%. This margin accounts for below-average sun days, panel degradation over time, and future increases in energy consumption.
Worst-Month Sizing
Designing a solar system based on the month with the fewest peak sun hours. Off-grid systems must meet loads year-round, so the weakest month determines the minimum array size.
Once you know your panel size, figure out how many batteries you need for storage with our batteries for solar system calculator. Try it now →
The right panel size depends on your actual energy use, not your neighbour's system or an internet rule of thumb. Do the load audit, use honest sun hour data for your worst month (if off-grid), and add a buffer. Getting this number right is the single most important step in any solar project.
Last updated:
Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.