Sizing a solar system is arithmetic, not guesswork. You add up what you use, work out how much sun you get, and size the panels, batteries and inverter to match, with margin for losses. The steps below work for a grid-tied house and for a cabin with no utility connection at all. Where the two differ, mostly on batteries, the text says so.
An off-grid system generates and stores its own electricity with panels, batteries and supporting equipment, with no connection to the main grid. That makes it the practical choice for remote land where running power lines is impractical or costs too much. It is also immune to grid outages, because there is no grid to fail. The price of that independence is that you have to design it correctly, since nobody else’s capacity is there to cover your mistakes.
Work out what you use #
Start with your electricity bills for the past year. Find the kWh figure on each, add the twelve months and divide by 365 for your average day. Then look at the individual months, because heating in winter and air conditioning in summer can push use well above the average, and the system must cope with your worst stretch.
For an off-grid build there are no bills to read, so build the number from the appliances. For each device, take the wattage on its label and multiply by the hours it runs per day. A 10-watt appliance running four hours uses 40 watt-hours. Do that for every load and add the results. Because no system is 100% efficient, add about 30% on top for conversion and wiring losses. The total is your daily load in watt-hours.
Before you finalize it, ask whether you can lower the load. LED lighting, efficient appliances and better insulation are cheaper per watt than panels and batteries. Decide at this stage what you want out of the system too: a lower bill, a smaller footprint, a backup for outages, or full independence. A goal of offsetting part of your use leads to a very different design than going completely off-grid. An online off-grid load calculator will do the multiplication for you, but do it by hand once so you understand the result.
Estimate your sun #
What matters is peak sun hours, meaning the number of hours per day when sunlight is strong enough to hit the rated output. You can get the average for your area from meteorological sources or a local installer. It varies by season, so use the dim months for an off-grid system.
Then walk the site. Trees, neighboring buildings and chimneys can cast shadows that cut output more than you’d expect, so check the shade at different times of day and across the year. Cloudy climates need a bigger array. If you live where winters bring long stretches of little sun, you may want a second source such as wind or hydro to supplement the panels.
Size the panels #
Divide your daily load by your effective sun hours to get the panel wattage you need. For example, with a daily load of 585 Wh and 5 hours of effective sunlight, you need 585 ÷ 5 = 117 watts of panel. Round up, and a 120-watt, 12-volt panel fits.
Real panels rarely deliver their full rating. A common allowance is that panels produce about 75% of rated capacity in practice, so divide your target by 0.75 as well. Battery charging and discharging lose some energy too, which is another reason to size with margin rather than to the exact number.
There are two main panel types. Monocrystalline panels are cut from a single silicon crystal and are more efficient but more expensive. Polycrystalline panels use multiple crystals and cost less per panel, though you need more roof or ground area for the same output. If space is tight, buy the efficient panels; if space is plentiful, the cheaper ones may win.
Size the battery bank #
Batteries matter most off-grid, and they matter for any grid-tied system where you want to ride out outages. Start with the number of cloudy days you want to cover. Two to three days with no sun is a sensible rule of thumb, adjusted for your climate. Multiply your daily load by that number of days to get the minimum storage.
Then account for depth of discharge, the share of a battery’s capacity you can draw out before recharging. Discharging too deeply shortens battery life, so aim for 50% or less and size the bank to match. That means a bank roughly double the usable storage you calculated. Charging and discharging losses eat a little more, so allow for them.
The two main chemistries are lead-acid and lithium-ion. Lead-acid costs less but needs more maintenance and has a shorter lifespan. Lithium-ion costs more up front, needs less maintenance and lasts longer. For a basic starter system, the original design guide recommended a lead-acid tubular battery of around 80 Ah. Batteries also smooth out the power: many electronics dislike the surges panels produce as clouds pass, and a battery gives them a steady supply.
Add a charge controller #
A charge controller sits between the panels and the battery. It regulates current and voltage so the batteries charge properly and never overcharge. Two types are common: a PWM controller is the affordable option, and an MPPT controller costs more but gets more out of your panels, particularly in cold weather. Size it to the panel array’s voltage and current.
Choose system voltage #
Small systems usually run at 12 or 24 volts. They are simpler and cheaper to set up, but they limit how much power you can deliver and how far you can move it, because low-voltage current in long wires loses more power. Larger systems typically run at 48 volts or higher. They carry more power over longer distances with thinner wire, but they ask for more safety precautions and can be harder to install and maintain. If you’re unsure, a solar professional can tell you which is appropriate for your load.
Pick the inverter #
The inverter converts the DC electricity from the panels and batteries into the AC your appliances use. Three kinds exist: square wave, modified sine wave and pure sine wave. Square wave inverters are the cheapest but work with only a narrow range of appliances. A pure sine wave inverter performs best, runs sensitive electronics safely and is highly efficient, so it is usually the right choice.
Rate the inverter above the most load you’ll run at the same time. Say you run a TV (50 W), a fan (80 W) and a CFL lamp (11 W) together: that’s a 141 W maximum load, so you’d choose a 200 W inverter to leave margin. For a whole house, add up the power ratings of the biggest appliances to estimate the maximum AC load, then add a safety margin for future upgrades. Also compare efficiency ratings, warranty and price.
Find room for the array #
Measure your roof and subtract the space taken by chimneys, vents and dormers. The orientation matters as much as the area. If the roof doesn’t work, a ground-mounted rack or a solar canopy built as a standalone structure can give you better exposure. Ground mounts are also easier to clean and to tilt for the season.
For mounting, the array should receive direct sun for most of the day. On a roof, attach the panels securely, keep them clear of windows, and route wiring so it doesn’t become a tripping hazard. On a ground rack, anchor the rack firmly and secure the panels to it. Think about maintenance access before you build, because a panel you can’t reach is a panel you won’t clean.
Wire it in the right order #
Details vary with the size of the system, but the sequence is consistent:
- Connect the panels to each other, in series or parallel depending on your configuration.
- Connect the panels to the charge controller. Check polarity, because reversing it can damage the controller.
- Connect the charge controller to the battery bank, again watching polarity.
- Wire the inverter directly to the battery bank.
Beyond the major components you’ll need copper wire, MC4 connectors (which link the panels to the charge controller), a breaker and fuses to protect against overloads and short circuits, and a meter to monitor performance. If you are not comfortable working with electricity, hire a licensed electrician for this stage.
What it costs #
A small system that can run a few lights and appliances may cost around $1,000. A system large enough to power a whole home or business can cost upward of $20,000. The up-front price is high, but the operating costs afterward are very low, which is why the long-term math favors solar.
Get a second opinion, then maintain it #
Once you have a design, ask a solar installer to review it. They can assess your home, spot site problems you missed and make recommendations. Online solar calculators are also useful for a cross-check: enter your location, usage, roof area and shading, and compare the recommended system size to your own numbers. Treat their output as estimates and let a professional validate it.
After installation, keep the panels clean, check the batteries regularly to make sure they are charged and working, and think about a backup generator for the days when the weather beats your margin. Planning the system carefully, sizing it with honest numbers and respecting the electrical hazards will get you a system that runs for decades.
For the wider picture, see the self-reliance hub, which ties power, shelter and off-grid living together. Related reading: How To Design A Solar Power System For Long-Term Reliability and Beginner’s Guide To Off-Grid Solar Power Setup.