In This Guide
Why a Power Audit Comes First
Buying solar before knowing your load is one of the single most common and expensive mistakes in off-grid setups. Without a power audit, you’re guessing — and the consequences of guessing wrong in either direction are painful. Undersize and you’ll be rationing power within weeks. Oversize and you’ve spent thousands of dollars on capacity you’ll never use.
A power audit takes about an hour with a pen and paper (or a spreadsheet). It gives you the single most important number in off-grid solar design: your daily watt-hour (Wh) consumption. Everything downstream — panel array size, battery bank, inverter — is calculated from that one figure.
Step 1: List Every Electrical Load
Go through your home or cabin — physically room by room — and write down every device that uses electricity. Include things you might overlook:
- All lighting (count each bulb separately)
- Refrigerator and freezer
- Phone and laptop chargers
- Water pump
- Fans and ventilation
- Router and networking equipment
- Power tools (even if used occasionally)
- Entertainment (TV, radio, speakers)
- Small kitchen appliances (blender, coffee maker, etc.)
Do not leave anything off the list. You can deprioritize high-draw items later, but you need to see the full picture first.
Step 2: Find the Wattage of Each Device
The wattage of any device is either printed on a label (look on the back or bottom of the unit) or available in the product manual. You can also look it up by model number online. Common label formats include “120V 60Hz 1.5A” — in that case, multiply volts × amps (120 × 1.5 = 180W).
For the most accurate reading, use a plug-in power meter (also called a Kill-A-Watt meter). These cost $15–$30 and give you real-time wattage readings for any appliance you plug in. They’re especially useful for refrigerators, which cycle on and off — running them for 24 hours gives you an actual average consumption figure that beats any label estimate.
Continuous Watts vs. Startup Surge Watts
Every motor-driven appliance (refrigerators, pumps, fans, power tools) draws significantly more power at startup than while running. This is called the startup surge or inrush current. A refrigerator that runs at 150W may surge to 600–900W for a fraction of a second each time the compressor kicks on.
For the purposes of calculating daily Wh consumption, use the running (continuous) wattage. But when sizing your inverter, you must account for surge — your inverter needs a surge rating high enough to handle the momentary spike without shutting down. Most quality inverters specify both a continuous and a surge (peak) wattage rating.
Step 3: Estimate Daily Hours of Use
For each device, honestly estimate how many hours per day it runs. Be realistic rather than optimistic. If your lights are on for 4 hours in the evening, write 4 hours — not 2. If your refrigerator runs roughly half the time (typical for most units), write 12 hours or use a Kill-A-Watt reading.
Some devices are easy: a laptop running 6 hours a day at 45W is straightforward. Others are trickier. A water pump might only run for a few minutes per day total, but when it runs it draws 500W — that still contributes to your daily Wh total.
Step 4: Calculate Wh/Day for Each Device
The formula could not be simpler:
Wh/day = Watts × Hours per day
Calculate this for every device on your list, then sum the column. That total is your daily watt-hour consumption.
Common Appliance Wattage Reference
| Appliance | Typical Running Watts | Startup Surge | Typical Wh/Day |
|---|---|---|---|
| LED bulb (per bulb) | 7–12W | None | 30–50 Wh (4 hrs) |
| 12V energy-efficient fridge | 40–60W | 100–150W | 300–600 Wh |
| Standard 120V refrigerator | 100–200W | 400–900W | 800–1,500 Wh |
| Laptop computer | 30–60W | None significant | 150–360 Wh (6 hrs) |
| Smartphone charging | 5–20W | None | 15–40 Wh |
| Router / networking | 5–20W | None | 120–480 Wh (24 hrs) |
| Box fan | 25–75W | 50–100W | 200–600 Wh (8 hrs) |
| Water pump (1/2 HP) | 375W | 750–1,500W | 50–150 Wh (short cycles) |
| 32” LED TV | 30–55W | None significant | 120–220 Wh (4 hrs) |
| Coffee maker | 800–1,200W | None significant | 100–200 Wh (brew cycle) |
| Blender | 300–600W | 600–1,000W | 10–30 Wh (short use) |
| Electric toaster | 800–1,500W | None | 25–50 Wh (short use) |
| Electric water heater | 3,000–5,500W | None | 3,000–8,000 Wh |
| Electric range / oven | 1,000–5,000W | None | 1,000–3,000 Wh |
| Central air conditioning | 2,000–5,000W | 4,000–10,000W | 10,000–40,000 Wh |
The Big Three to Avoid Off-Grid
Electric water heaters, electric ranges, and central air conditioning are the three appliances that make off-grid solar prohibitively expensive. A single electric water heater can consume more energy in a day than all of a cabin’s other loads combined. Most successful off-grid homes replace these with propane cooking, wood-fired or propane water heating, and passive cooling design or a small propane/mini-split AC unit.
Step 5: Total It Up and Interpret the Result
Add up all your daily Wh figures. This is your baseline daily load. Common ranges:
- Under 500 Wh/day: Very efficient small cabin or van. A modest solar system ($800–$1,500) handles this easily.
- 500–1,500 Wh/day: Typical small off-grid cabin. A well-designed system in the $2,000–$6,000 range works well.
- 1,500–3,000 Wh/day: Small family or homestead. Expect to invest $5,000–$15,000 for a robust system.
- Over 3,000 Wh/day: Full-home load. Make sure you’ve eliminated every unnecessary high-draw appliance before sizing. A system in this range costs $15,000–$40,000+.
If your total surprises you on the high side, look for opportunities to reduce load before buying hardware. Switching from a standard 120V fridge to an energy-efficient 12V unit alone can save 500–1,000 Wh/day. Replacing incandescent or CFL bulbs with LEDs cuts lighting load by 75%.
Where to Go Next
Once you have your daily Wh number, you’re ready to size your system:
- How to Size Your Solar System — Use your Wh number to calculate panels, batteries, and inverter
- LiFePO4 vs Lead-Acid — Choose the right battery chemistry for your budget
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