TL;DR — Off-Grid Power Calculation
Your off-grid power requirement is your daily watt-hour consumption. Run it as a calculation, not a guess. A modest homestead runs 2,000–4,000 Wh per day. A full residential home with refrigeration, water pump, HVAC, and tools runs 6,000–15,000 Wh per day. That number drives every component decision in your system. Get it wrong and every component is wrong with it.
A rancher in East Texas bought a 6kW solar system based on his contractor's recommendation. The contractor asked how many square feet the house was. He never asked what appliances were running, what the well pump drew, or whether the shop ran three-phase equipment. The system failed every winter because the battery bank was sized for a house half his actual load. The calculation takes thirty minutes. It would have saved him $12,000 in oversizing corrections.
Table of Contents
Why load calculation is the only starting point
The battery bank stores your energy. The battery bank is sized for your daily consumption. The panels charge the battery bank. The inverter converts battery power for your appliances. Every component chains back to the battery bank. The battery bank chains back to the load calculation.
Skipping the load calculation means sizing the battery bank by guesswork. Sizing the battery bank by guesswork means the panels, inverter, and charge controller are all wrong by the same factor. A system undersized by 40% will discharge its batteries into damage every cloudy week until you replace everything.
"The average U.S. home consumed 10,791 kilowatt-hours of electricity in 2022 — approximately 29.6 kWh per day — though consumption varies significantly by region, with homes in the South averaging 40% higher consumption than homes in the Northeast."
— U.S. Energy Information Administration, Residential Energy Consumption Survey 2022
That average matters. A 30 kWh per day home cannot run on an off-grid system sized for 10 kWh per day. And a well-managed off-grid homestead running efficient appliances may land at 4–6 kWh per day — not 30. Your number is your number. Calculate it before looking at any component.
How to calculate your daily power consumption
The calculation has three steps.
Step 1: List every electrical load you plan to run off-grid.
Include everything: refrigerator, freezer, lighting, water pump, washing machine, phone chargers, laptop, television, CO2 detector, sump pump, shop tools. Do not estimate. Find the wattage rating for each appliance — stamped on the back panel or in the owner's manual.
Step 2: Estimate daily hours of use for each appliance.
Refrigerators and freezers cycle, not run continuously. A 150-watt refrigerator running 30–50% of the time draws an effective average of 45–75 watts. Use the compressor cycle estimate, not the nameplate wattage continuously.
Step 3: Multiply wattage by daily hours. Sum all appliances.
| Appliance | Watts | Hours/Day | Daily Wh |
|---|---|---|---|
| Refrigerator (efficient) | 150 | 8 (cycle avg) | 1,200 |
| Chest freezer | 100 | 6 (cycle avg) | 600 |
| LED lighting (10 fixtures) | 100 | 5 | 500 |
| Laptop | 65 | 6 | 390 |
| Phone charging (2) | 20 | 3 | 60 |
| Well pump (shallow) | 750 | 0.5 | 375 |
| Washing machine | 500 | 0.5 | 250 |
| Small TV | 80 | 3 | 240 |
| Misc (clocks, routers, fans) | 100 | 8 | 800 |
| Total | 4,415 Wh/day |
That is a real off-grid homestead at modest consumption. Add electric water heating, HVAC, a shop, or electric cooking and the number climbs. Know before you buy.
Load profiles by homestead type
Minimal cabin / emergency backup: 1,000–2,500 Wh/day LED lighting, laptop, phone charging, small refrigerator, no well pump, no major appliances. This is a weekend cabin or a get-off-the-grid minimal setup.
Modest off-grid homestead: 3,000–6,000 Wh/day Full refrigeration, well pump, washing machine, lighting, laptops, routers, and basic entertainment. Most rural homesteads building for primary residence fall here.
Full residential off-grid home: 6,000–12,000 Wh/day All of the above plus HVAC (mini-split runs 500–2,500 watts depending on size and duty cycle), chest freezer, electric cooking, and shop tools. Serious off-grid homesteaders and rural homes in climate extremes.
High-consumption homestead with shop: 12,000–20,000 Wh/day Welders, air compressors, large HVAC, electric water heating, multiple refrigerators, full shop operation. These require large 48V systems with substantial battery banks — typically 30–60kWh of storage.
What contractors use instead — and why it fails
Most solar installers use one of three shortcuts:
Square footage estimates. They assume a 2,000 square foot home uses a certain amount of power. It does not. A 2,000 square foot home with a heat pump, electric range, electric water heater, and chest freezer uses three to four times the power of a 2,000 square foot home with propane heating, propane range, propane water heating, and a small refrigerator.
Utility bill averaging. They take your last twelve utility bills and average them. This is closer but still wrong for off-grid. Your utility bill includes power you wasted, power you used inefficiently, and power you pulled from the grid at hours when off-grid systems would have been running on batteries. It does not account for efficiency upgrades you will make when you go off-grid.
Panel count recommendation first. They tell you how many panels you need before calculating the load. Panels are the output of the sizing calculation, not the input. A contractor who starts with panels is working backward.
The free Solar Power Estimator uses the correct sequence. Enter your appliances and usage. It calculates your load, factors in your location's peak sun hours, and outputs your battery bank size, panel count, and inverter spec. Use it before any contractor conversation.
Seasonal load variation — the number most people ignore
Your load is not the same in January as it is in July. Neither is your solar harvest.
Summer: Longer days, higher sun angle, maximum panel output. Air conditioning load peaks if you run it. Refrigerators work harder in heat. Outdoor lighting needs are lower.
Winter: Shorter days, lower sun angle, 30–50% less panel output in northern climates. Heating loads increase if you use electric heat. Demand for stored power is higher while supply is lower.
Size your system for the worst case — winter in your climate zone. A system that runs fine in July but fails in December is undersized. NREL provides peak sun hour data by location and season. The Solar Estimator incorporates this automatically.
A battery bank sized for winter autonomy requirements will be more than sufficient in summer. That excess summer production can charge secondary battery banks, run water pumps at off-peak times, or be managed by your charge controller without damage.
🦍 WATTSON ON MEASURING: "I run a Victron monitoring system on my 48V setup. I know my daily average, my monthly trend, my winter low, and my summer peak — to the watt-hour. When something changes — a new appliance, a different usage pattern — I see it in the data the same day. You cannot size a system you have not measured. And you cannot protect a system you are not watching. Monitoring is not optional. It is the operating system."
Calculate Your Exact Power Requirement
The Solar Power Estimator guides you through the full load calculation and outputs your battery bank size, panel count, and inverter spec. Free. Takes ten minutes.
What to do with your load number
Once you have your daily watt-hour total, the system sizing math is straightforward.
Battery bank size: Daily load (Wh) × days of autonomy ÷ usable depth of discharge.
Example: 4,415 Wh × 2 days ÷ 0.80 (LiFePO4 usable depth) = 11,037 Wh battery bank. Round up to 12–15kWh for margin.
Panel array size: Daily load ÷ peak sun hours ÷ system efficiency.
Example: 4,415 Wh ÷ 5 hours ÷ 0.85 efficiency = 1,039 watts of panels. Round up to 1,200–1,500W for winter margin.
Inverter size: Peak simultaneous load × 1.25 buffer.
Example: If the well pump (750W), refrigerator (150W), and lighting (100W) run simultaneously, peak load is 1,000W. Size the inverter for 1,250–2,000W minimum.
Bring these numbers to every vendor conversation. Any contractor who cannot justify a deviation from your calculated specs with specific data is guessing. Guesses are your money.
The next step is understanding how to size your battery bank in detail — the most expensive component in any off-grid system and the most common place for undersizing to happen. See Battery Sizing for Off-Grid Solar: What Most Charts Get Wrong.
Frequently Asked Questions
How many kWh per day does an off-grid home use?
It varies significantly by usage pattern and efficiency. A minimal cabin runs 1,000–2,500 Wh per day. A modest primary residence runs 3,000–6,000 Wh per day. A full residential home with HVAC runs 6,000–12,000 Wh per day. Calculate your specific load before assuming any average applies to your situation.What appliances use the most power off-grid?
Electric water heaters (3,000–5,500W), electric ranges (1,000–5,000W), HVAC systems (500–3,500W), well pumps (750–1,500W), and chest freezers (100–200W cycling). Identifying these high-draw appliances and their duty cycles is the most important part of the load calculation. Replace electric water heating with propane and the daily load drops dramatically.Can I run a well pump on solar off-grid?
Yes. A shallow well pump at 750W running thirty minutes per day adds 375 Wh to your daily load. A deep well pump at 1,500W changes the math significantly. Size your inverter for the pump's surge startup current — typically two to three times the running wattage — or the inverter will trip on startup.How do I calculate a refrigerator's actual power draw?
A refrigerator's nameplate wattage is its peak draw. The compressor cycles on and off, typically running 30–50% of the time in moderate temperatures. Estimate your refrigerator's effective daily draw as nameplate wattage multiplied by eight to twelve hours of effective running time, not twenty-four. A Fluke clamp meter measuring your actual fridge draw for twenty-four hours gives you the exact number.How many solar panels do I need to run an off-grid home?
Calculate your daily load first. Divide by your location's peak sun hours. Divide by your system efficiency (typically 80–85%). The result is your panel array wattage. Divide by your panel wattage to get panel count. A 4,400 Wh per day home in a 5-hour peak sun location needs roughly 1,000–1,300W of panels — three to four 300W panels — as a baseline.What is peak sun hours and how does it affect my system?
Peak sun hours is the number of hours per day when solar irradiance averages 1,000 watts per square meter — the standard for panel output ratings. Phoenix, Arizona averages 6.5 peak sun hours. Seattle, Washington averages 3.5. A solar system in Seattle needs roughly twice the panel capacity as the same system in Phoenix to deliver the same daily output. NREL maps peak sun hours by location for free at nrel.gov.Should I include electric cooking in my off-grid load calculation?
Calculate it and then seriously consider replacing it with propane or wood. Electric cooking is a massive load — an electric range runs 1,000–5,000 watts. Running an electric oven for two hours adds 4,000–8,000 Wh to a single day's load. Most serious off-grid homesteaders use propane ranges and propane ovens to keep their electrical load manageable. Factor the appliance choice into your system design before buying the stove.How do I account for future load growth in my off-grid system design?
Add 20–30% to your current calculated load when sizing the battery bank. Choose a charge controller rated for a larger panel array than you install initially. Choose an inverter with headroom above your current peak load. A system designed to grow is dramatically cheaper to expand than one designed to your exact current load with no margin.Do I need to calculate my load differently for winter?
Yes. Calculate summer load and winter load separately. Size your battery bank and panel array for the winter case — higher load, lower production. A system that meets summer needs only will fail in December across most of the continental US. MPPT charge controllers maximize winter harvest by optimizing panel output at lower irradiance and steeper sun angles.What tools do I need to measure my actual power consumption?
A plug-in energy monitor (Kill A Watt or equivalent) measures individual appliance consumption precisely — plug it between the appliance and the outlet, run it for twenty-four hours, and read the exact kWh. A clamp meter measures hard-wired loads. For a whole-home baseline, read your utility meter for thirty days before going off-grid and track your daily kilowatt-hour consumption. That is your most accurate starting load number.The calculation is thirty minutes. The mistake is ten years.
Every off-grid system failure traces back to a load number that was guessed instead of calculated. The calculation is not hard. It is a list of appliances and a multiplication problem. Thirty minutes of work prevents years of undersized system problems.
Run the load calculation before you call the first installer. Before you price the first panel. Before you look at battery options. The number is the foundation. Everything else is built on it.
The $12,000 retrofit is what happens when you skip the calculation. The rancher in East Texas paid it. The veteran in Montana nearly paid it — he ran the estimator first and caught the undersizing before the purchase. Thirty minutes of math. The grid does not get to decide your family's safety window. Neither does a contractor who never asked what you actually run. Calculate it yourself. Then verify it. Use the Solar Power Estimator and arrive at the table with your number in hand.