How to Size Your Off-Grid Solar System

Getting the size right is the single most important decision in any off-grid solar project. An undersized system leaves you in the dark on cloudy days. An oversized system wastes hundreds or thousands of dollars on panels and batteries you will never use. Unlike grid-tied solar, there is no utility company to bail you out when your off-grid system falls short, so sizing needs to account for your worst-case scenario, not your best.

The good news is that sizing an off-grid solar system is not complicated. It is a series of straightforward calculations that anyone can follow. This guide walks you through every step, from counting your appliance loads to choosing the right inverter, using a real cabin example so you can see exactly how the numbers work.

Step 1: Calculate Your Daily Energy Needs

Everything starts with knowing how much energy you actually use each day. This number, measured in watt-hours (Wh), drives every other decision in your system.

How to Do an Appliance Audit

Grab a notebook or spreadsheet and list every device that will run on your off-grid system. For each one, you need two numbers: how many watts it uses and how many hours per day you run it. Multiply those together and you get daily watt-hours for that appliance.

Most appliances list their wattage on a label or in the manual. For devices that cycle on and off, like refrigerators, use the average running wattage rather than the peak. A modern efficient refrigerator might be rated at 150 watts but only runs its compressor about 40 percent of the time, averaging around 60 to 80 watts continuously.

Real Example: A Small Off-Grid Cabin

Let us walk through a realistic scenario. You have a small cabin in the Colorado mountains that you use year-round. Here is what you want to power:

| Appliance | Watts | Hours/Day | Daily Wh | |---|---|---|---| | LED lights (5 bulbs) | 50 | 5 | 250 | | Refrigerator (efficient) | 70 avg | 24 | 1,200 | | Laptop | 50 | 6 | 300 | | Phone charging (2 phones) | 20 | 3 | 60 | | WiFi router | 12 | 24 | 288 | | Small TV | 80 | 3 | 240 | | Coffee maker | 900 | 0.15 | 135 | | Water pump | 250 | 1 | 250 | | Total | | | 2,723 Wh |

But you are not done yet. Every off-grid system has losses. Energy is lost in the charge controller, the inverter, the wiring, and the battery's own charge-discharge cycle. A safe rule of thumb is to add 25 percent to your total to account for these losses.

2,723 Wh x 1.25 = 3,404 Wh effective daily demand

That is the number you will use for all your sizing calculations going forward.

Common Appliances That Surprise People

A few appliances use far more energy than most people expect. Electric space heaters draw 1,000 to 1,500 watts and are extremely difficult to power off-grid for extended periods. Hair dryers pull 1,500 to 1,800 watts. Electric water heaters are essentially impossible to run on a small off-grid system. If you are planning an off-grid cabin, consider propane for heating and hot water, and save your solar and battery capacity for everything else.

On the other hand, some things use very little. LED lights are incredibly efficient. Modern laptops sip power. A ceiling fan on low uses less electricity in a day than running a microwave for five minutes.

Step 2: Size Your Solar Panels

Now that you know your daily energy demand, you can figure out how many watts of solar panels you need.

The Formula

Required panel watts = Daily Wh demand / Peak sun hours x 1.25

The 1.25 multiplier is a derating factor that accounts for real-world losses. Panels do not produce their rated wattage consistently. Temperature, dust, wiring resistance, and panel aging all reduce output. This buffer keeps your system reliable.

Peak Sun Hours by Region

Peak sun hours are not simply the number of daylight hours. One peak sun hour equals one hour of 1,000 watts per square meter of sunlight intensity. A location might have 14 hours of daylight in summer but only 5 to 6 peak sun hours because the sun is weak in early morning and late afternoon.

Here are annual averages for different US regions:

| Region | Peak Sun Hours | |---|---| | Southwest (Arizona, New Mexico) | 6.0 - 7.5 | | Mountain West (Colorado, Utah) | 5.0 - 6.0 | | Southeast (Florida, Texas) | 4.5 - 5.5 | | Midwest (Illinois, Ohio) | 3.5 - 4.5 | | Pacific Northwest (Washington, Oregon) | 3.0 - 4.0 | | Northeast (New York, Massachusetts) | 3.5 - 4.5 |

Critical Rule: Size for Your Worst Month

If you are using your off-grid system year-round, you must size your panels for winter sun hours, not the annual average. Winter has fewer hours of daylight, the sun sits lower in the sky, and clouds are more common in many regions.

Our Colorado cabin gets about 5.5 peak sun hours in summer but only 3.5 in winter. We size for winter.

3,404 Wh / 3.5 hours x 1.25 = 1,214W of solar panels

Round up to the nearest practical panel configuration. Four 400-watt panels give us 1,600W, which provides a comfortable margin above the minimum. That extra headroom means faster battery charging and better performance on partly cloudy days.

Panel Tilt and Orientation

Tilt angle matters more than most people realize. For year-round use, set your panel tilt equal to your latitude. For winter optimization, increase the tilt by 15 degrees to capture the low winter sun more effectively.

An adjustable ground mount is ideal for off-grid systems because you can change the angle seasonally and clear snow easily. Roof mounts work but are harder to adjust and clean.

Panels should face true south in the Northern Hemisphere. An east-west split is sometimes necessary due to roof orientation but costs you 15 to 20 percent of potential production.

Step 3: Size Your Battery Bank

Your battery bank is the most expensive component and the one that determines whether your system can handle cloudy days without running out of power.

The Formula

Battery capacity (Wh) = Daily Wh demand x Days of autonomy / Max depth of discharge

Days of autonomy is how many consecutive low-production days your battery should carry you through. For most locations, 2 days is a reasonable starting point. Areas with long cloudy stretches in winter, like the Pacific Northwest, might want 3 days. Sunny climates like Arizona can get away with 1 to 2 days.

Depth of discharge (DoD) is how much of the battery's capacity you can actually use. Different battery types have different safe limits:

| Battery Type | Max DoD | Cycle Life | |---|---|---| | LiFePO4 (lithium iron phosphate) | 80-90% | 3,000 - 5,000 cycles | | Li-NMC (lithium nickel manganese cobalt) | 80% | 500 - 1,000 cycles | | Lead-acid (flooded) | 50% | 300 - 500 cycles | | Lead-acid (AGM) | 50% | 400 - 600 cycles |

LiFePO4 is the clear winner for off-grid systems. It costs more upfront but lasts 5 to 10 times longer than lead-acid, making it cheaper over the system's lifetime. For a deeper look at battery options, see our home battery storage guide. Lead-acid batteries still have a place in very tight budgets, but plan to replace them every 2 to 4 years with heavy use.

Our Cabin Example

3,404 Wh x 2 days / 0.80 (LiFePO4 DoD) = 8,510 Wh or roughly 8.5 kWh

In practical terms, that means two 5.12 kWh server-rack style LiFePO4 batteries (a popular and cost-effective format), which would cost approximately $1,500 to $3,000 total. Alternatively, you could use two all-in-one solar generators with at least 4,000 Wh each, though that option typically costs more.

Battery Voltage: 12V, 24V, or 48V?

Choosing the right system voltage affects efficiency and wiring costs.

• 12V systems work for small setups under 2,000 watts but require thick, expensive cables for higher loads
• 24V systems are a good middle ground for 2,000 to 5,000 watt systems
• 48V systems are the most efficient choice for larger installations and are required by many higher-quality inverters

Our cabin example would work well at 48V, which keeps cable sizes manageable and pairs with a wide selection of quality inverters.

Step 4: Size Your Inverter

The inverter converts DC battery power to AC household power. Sizing is simpler than panels and batteries but getting it wrong means tripped breakers or damaged equipment.

Two Numbers That Matter

Continuous wattage is the maximum sustained load the inverter can handle. Add up the wattage of every appliance you might run at the same time. For our cabin, a worst case might be: microwave (1,000W) plus refrigerator (70W) plus lights (50W) plus laptop (50W) equals 1,170 watts.

Surge wattage handles the brief spike when motors start up. Refrigerator compressors pull 2 to 3 times their running wattage for a few seconds at startup. Water pumps do the same. Your inverter's surge rating needs to handle these spikes.

For our cabin: add refrigerator startup surge (about 400 watts extra) and water pump startup surge (about 500 watts extra) to the continuous load estimate.

Recommended inverter: 3,000W continuous / 6,000W surge. This gives comfortable headroom above our calculated needs and allows for future load additions.

Always choose a pure sine wave inverter. Modified sine wave inverters are cheaper but can damage sensitive electronics and make some appliances buzz or run inefficiently. Our best solar inverters guide covers the different inverter types in detail.

Putting It All Together: Our Cabin System

Here is the complete system for our Colorado off-grid cabin:

| Component | Specification | Estimated Cost | |---|---|---| | Solar panels | 1,600W (4x 400W) | $800 - $1,200 | | Charge controller | 60A MPPT | $200 - $400 | | Battery bank | 10 kWh LiFePO4 (48V) | $2,000 - $4,000 | | Inverter | 3,000W pure sine wave | $400 - $800 | | Wiring and hardware | Cables, breakers, combiner box, mounts | $500 - $1,000 | | Total | | $3,900 - $7,400 |

That is a complete, year-round off-grid power system for under $7,500 at the high end. Compare that to the cost of running a utility power line to a remote property, which can easily cost $20,000 to $50,000 or more depending on distance.

Common Sizing Mistakes to Avoid

After walking through dozens of off-grid builds, these are the mistakes that trip people up most often.

Sizing for summer instead of winter. Summer sun hours can be double winter hours in some regions. A system that works great in June will leave you powerless in December if you did not size for the lean months.

Forgetting system losses. That 25 percent buffer is not optional. Inverters alone lose 10 to 15 percent of energy as heat. Add wiring losses and charge controller inefficiency, and the total hits 20 to 25 percent easily.

Ignoring surge loads. A 2,000W inverter cannot start a refrigerator and a water pump simultaneously if their combined surge exceeds the inverter's surge rating. Always check surge specs, not just continuous ratings.

Skipping the generator backup. Even a well-sized off-grid system can face unusual situations: a week of heavy clouds, unexpected guests doubling your load, or a panel covered in snow for days. A small propane or gasoline generator ($500 to $1,500) that you run for a few hours a year is cheap insurance against running out of power when it matters.

Start With the Math, Then Shop

If you are still deciding between off-grid and grid-tied solar, work through these sizing calculations first — seeing the real numbers often makes the choice clear.

The most expensive mistake in off-grid solar is buying before calculating. Resist the urge to shop first. Run your numbers using the steps above, write down the panel watts, battery capacity, and inverter size you need, and then go looking for equipment that matches. You will spend less money and end up with a system that actually meets your needs every day of the year.

If you are ready to shop, check out our guide to the best off-grid solar kits in 2026 for specific product recommendations at every budget level.