How to Size a Solar System for Your Property
A free solar sizing calculator that turns your monthly electricity use and peak sun hours into the right solar array size and battery bank capacity for your homestead.
Sizing a solar system is the single most important decision you will make about power on your homestead. Get it right and your panels quietly cover your loads for the next twenty five years. Get it wrong and you are either running a generator every winter or staring at thousands of dollars in panels you never needed.
The good news is the math is simple once you have the right inputs. You need to know how much electricity you use, how much sun your property gets, and whether you plan to live with a battery or stay tied to the grid. The calculator below does the arithmetic for you and the article that follows explains every number it asks for.
Scroll down to the calculator, enter your numbers, then read the guide below to understand what your results actually mean.
Your Energy Needs
Energy Consumption
Check your current utility bill. Average US home uses 850 to 900 kWh per month. A highly efficient homestead might use 300 to 500 kWh.
Average is 4 to 5 hours in most of the US. Use a lower number if you are sizing for winter.
System Configuration
How many days your batteries must power your home without any sun. Most off grid systems plan for 2 or 3.
System Requirements
Daily Energy Need
30.0
kWh / day
Recommended Array Size
8.5
kW
Calculated using 4.5 peak sun hours and an estimated 78% overall system efficiency.
Total Battery Bank
75.0
kWh
Based on 2 days of autonomy and a max Depth of Discharge of 80%.
What Solar Sizing Actually Means for a Homestead
Solar sizing is the process of matching three numbers to each other. The first number is how much electricity your homestead uses every day. The second is how much energy a single kilowatt of solar panels can generate where you live. The third is how much energy you want to store in batteries for nights and cloudy stretches.
When those three numbers line up, your panels make about as much power as you use, your batteries cover the gaps, and you never think about your electrical system. When they do not line up, you either run out of power in February or you waste money on hardware you do not need.
The calculator above does all the math. Your job is to give it honest inputs. The rest of this guide walks through every number it asks for, what good values look like, and the small decisions that change your results in a big way.
How to Use the Solar Sizing Calculator Above
The calculator only asks for five things. Two of them come straight off your utility bill. The other three are choices about how you want to live.
- Enter your monthly electricity usage in kWh. This number is printed on every utility bill in the United States. If you do not have a bill yet, the average American home uses about 850 to 900 kWh per month. A tight, efficient homestead with propane cooking and a wood stove often runs closer to 300 to 500.
- Enter your peak sun hours. This is not the number of daylight hours. It is the equivalent number of hours per day that the sun is strong enough to push a panel to its rated output. The US average is around 4.5 hours. The desert Southwest sees 6 or more. The Pacific Northwest is closer to 3.
- Pick your system type. Off grid means you have batteries and no utility connection, or you want enough battery to run your house when the grid is down. Grid tied means you sell extra power back to the utility and pull from them at night.
- Set your days of autonomy. This only matters for off grid systems. It is how long your batteries must run your home with no sun at all. Two days is the common minimum. Three is safer in the cloudy north.
- Pick your battery chemistry. Lithium iron phosphate (LiFePO4) can be drained to 80 or even 90 percent of its capacity every day. Lead acid should never go below 50 percent if you want it to last. That single difference cuts the size of a lithium battery bank almost in half compared to lead acid.
Once those five values are in, the results panel on the right shows your daily energy need, your recommended solar array size in kilowatts, and your total battery bank in kilowatt hours. Print the estimate and take it to an installer or use it to start shopping panels and batteries yourself.
Understanding Peak Sun Hours
Peak sun hours is the single input that confuses new solar shoppers more than any other. It is worth slowing down on this one because using the wrong number is the most common reason a homestead ends up underpowered.
What peak sun hours actually measure
A peak sun hour is one hour of sunlight at 1,000 watts per square meter. That is the brightness solar panels are rated against in the lab. The sun is rarely that strong, so a long day of weak winter sun might only add up to two peak sun hours, even though the sun was technically up for nine hours. A short June day with overhead sun might give you six peak sun hours from a much shorter window. Peak sun hours are an energy total, not a clock measurement.
Peak sun hours by US region
Annual averages give you a rough starting point.
- Pacific Northwest (Seattle, Portland): roughly 3.0 to 3.5 peak sun hours per day.
- Upper Midwest and New England: roughly 3.5 to 4.0 peak sun hours per day.
- Mid Atlantic and Ohio Valley: roughly 4.0 to 4.5 peak sun hours per day.
- Mountain West, Texas, Florida: roughly 5.0 to 5.5 peak sun hours per day.
- Desert Southwest (Phoenix, Las Vegas): roughly 6.0 to 6.5 peak sun hours per day.
For a more precise number, look up your zip code on the NREL PVWatts calculator. It is free and uses thirty years of weather data to give you a monthly breakdown.
Why you must size for your worst month
Annual averages are useful for grid tied systems but dangerous for off grid ones. If your batteries run dry every December because you sized for a yearly average, you will end up running a generator constantly during the months you can least afford to. For off grid systems, look up your peak sun hours for the worst month at your latitude (usually December or January) and size against that number. You will spend more on panels but you will not be sitting in the dark on the shortest day of the year.
Daily Energy Use: Reading Your Utility Bill
Your utility bill is the single best source of truth about how much electricity you actually use. It is not a guess. It is metered down to the kilowatt hour. The trick is reading it correctly and adjusting for the future homestead, not the current apartment or rental.
The average US household
The Energy Information Administration pegs the average American home at about 10,500 kWh per year, or roughly 875 kWh per month. That assumes electric cooking, electric water heating, electric clothes drying, and central air conditioning. If your house uses gas or propane for any of those, your number drops fast. If you run electric heat or have a hot tub, it climbs fast.
A typical efficient homestead
Most of the homesteaders I talk to land somewhere between 300 and 600 kWh per month once they get serious about efficiency. The big swings come from heat (wood stoves are free, electric baseboard is brutal), water (a deep well pump can pull 1,500 watts on every cycle), and laundry (a heat pump dryer or a clothesline saves hundreds of kWh per year compared to an electric dryer). Start with your real bill. Then subtract anything you plan to switch to propane, wood, or a clothesline, and add anything new like a chest freezer or a well pump.
Phantom loads and well pumps
Two things tend to surprise people. The first is phantom loads. Every device that has a clock, a remote, or a standby light is drawing power around the clock. Add them up and a typical home loses 50 to 100 kWh per month to devices nobody is using. The second is well pumps. A standard submersible pump might only run twenty minutes a day but the startup surge can be three to five times its running wattage. Both matter when you are off grid because they decide how big your inverter has to be, even though they barely move your monthly total.
Off Grid vs Grid Tied Sizing
The single biggest factor in how much your solar system costs is whether you stay connected to the utility. The panels are roughly the same either way. The batteries are what change the math.
Grid tied basics
A grid tied system uses the utility as your battery. Your panels make power during the day, your house uses what it needs, and any extra runs backward through your meter. At night you pull from the grid like any other customer. You only need to size your panels, not your storage. Many grid tied homesteads only cover 50 to 80 percent of their annual usage because adding the last few panels has the worst payback.
Off grid basics
An off grid system has no utility connection. Every watt your house uses has to come from the panels or the battery, including the watts you use at midnight. That changes the math in two ways. First, you have to size the panels to make enough power on the shortest day of the year, not the average day. Second, you have to add a battery bank big enough to ride out the bad weather. Off grid systems typically cost two to three times more than grid tied systems for the same daily use because of the battery hardware.
Hybrid systems
A hybrid system is a grid tied system with a smaller battery bank for backup power during outages. It is the fastest growing category in residential solar and a popular choice for homesteads that want the resilience of off grid without the cost of a full battery bank. If you pick this approach, you can size the batteries for one or two days of essentials (fridge, well, lights, a few outlets) rather than your entire load.
Battery Bank Sizing and Days of Autonomy
Your battery bank carries you through the night and through bad weather. It is also where most of the money goes in an off grid system. Getting this number right matters.
Depth of discharge by chemistry
Batteries can rarely be drained all the way down without damage. Depth of discharge (DoD) is how much of the capacity you can actually use. Lithium iron phosphate batteries (LiFePO4) tolerate an 80 percent DoD as a daily routine and many manufacturers warranty them to 90 percent. Lead acid batteries (whether flooded, AGM, or gel) should be limited to 50 percent if you want them to last a decade. The calculator above adjusts your battery bank size based on the chemistry you pick.
Lithium vs lead acid in practice
Lithium costs more per kilowatt hour up front but lasts two to four times longer than lead acid. Most lithium banks come with a ten year warranty and a cycle count north of 6,000. Lead acid is cheap to buy but loses capacity quickly if it is not maintained perfectly, and a deep discharge once or twice can ruin a brand new bank. If you are doing the math over twenty years, lithium almost always wins on cost per kilowatt hour delivered. If you have a tight budget today, lead acid is still a workable starter option.
Sizing for storms and cloudy stretches
Days of autonomy is the buffer between you and a multi day storm. Two days covers a typical weather event in most of the country. Three or four days is wise in the cloudy north or in mountain regions that can sit under thick overcast for a week. Going beyond four days starts to get expensive fast because the battery bank scales linearly. Most off grid homesteads pair two or three days of autonomy with a small backup generator that can carry them through the rare ten day stretch of bad weather. It is far cheaper than buying a battery bank that can survive any imaginable scenario.
System Efficiency: Why Panels Never Hit Their Nameplate Wattage
A 400 watt solar panel is not actually a 400 watt solar panel in real life. That number is what it produces in a lab at 25 degrees Celsius, with perfectly clean glass and no wiring losses. In your backyard, every panel loses some of its rated output to heat, dust, wire resistance, and the inverter that turns DC into AC. The total of those losses is called the derate factor, and a typical residential system delivers about 78 percent of nameplate.
Temperature is the biggest single factor. Solar panels make less power as they heat up, and the cells on your roof in July can hit 140 degrees Fahrenheit. Dirt, pollen, and bird droppings cost another two to five percent unless you wash the panels. Wiring resistance and inverter conversion knock off another five to ten percent depending on the run length and the inverter quality. The calculator above bakes a 78 percent system efficiency into your array size so you do not have to do that math yourself.
If you live somewhere very hot or have a long wire run from a ground mount, you might want to bump up your array size another five to ten percent. If you live somewhere cool and your panels are close to the house, you can probably trust the 78 percent default.
The Six Most Common Solar Sizing Mistakes
- Sizing for the annual average instead of the worst month. This is the single most common mistake for off grid systems. It looks fine on paper and then leaves you running a generator every December.
- Forgetting to add the well pump and the shop. A homestead usually adds loads after the panels go up. Pump houses, outbuildings, electric fence chargers, and chest freezers all add up.
- Confusing kW and kWh. Kilowatts (kW) measure power, like the size of a faucet. Kilowatt hours (kWh) measure energy, like the total water that comes out of the faucet. Panels are sized in kW. Batteries and bills are measured in kWh.
- Skipping efficiency upgrades before sizing. Every dollar spent on better insulation, LED lights, or a heat pump water heater saves three to five dollars in solar and battery hardware. Always look for the easy wins before sizing the array.
- Buying lead acid to save money up front. Lead acid is cheap to buy and expensive to own. A single deep discharge can kill a young bank. For most homesteads building today, lithium is the better long term choice even at higher up front cost.
- Sizing the inverter to the average load, not the peak. Your inverter has to handle the moment your well pump kicks on while the microwave is running and the fridge starts a cycle. That peak is often three or four times your average load. Undersize the inverter and you trip breakers all winter.
How Solar Sizing Fits Into Your Homestead Infrastructure Plan
Solar is one piece of your homestead infrastructure, not the whole picture. Water, heat, food storage, and shelter all draw power, and they all change the size of the system you need. If you can heat with wood, dry clothes on a line, and run a propane stove, your solar bill drops by half. If you insist on electric everything, you will pay for it in panels and batteries.
The smart sequence is to plan your other systems first, then size your solar to match. That way you are not paying for power you decided to stop using a year later. The calculator above is built for that order. Once your loads are settled and your peak sun hours are looked up, the math takes seconds.
If you are new to off grid power and want a deeper walkthrough of how the whole system fits together, read the beginners guide to off grid solar power. It covers the components, the wiring layout, and the questions to ask any installer before you sign a contract.
Frequently Asked Questions
How many solar panels do I need to make 1 kW?+
Does this calculator size my inverter?+
What is the difference between kW and kWh?+
How can I lower the cost of my solar system?+
Do I need batteries if I am grid tied?+
How long do solar panels last?+
How long do solar batteries last?+
Can I size my solar system for just the essentials instead of my whole house?+
Winter Planning is Crucial
For off grid homesteads, size your system for the worst month of the year, not the annual average. A system sized to average sun hours will leave you in the dark during December and January. Look up your location's peak sun hours for the winter solstice and rerun the calculator above with that number to see what your winter array size really needs to be.