Off-grid solar system: How to plan and size one

Updated 2026-10-25

An off-grid solar system powers a cabin, RV or home with no connection to the utility grid. Everything you use comes from your solar panels and batteries, plus a generator if you have one. With no utility to fall back on, you can’t design for an average day. You design for the worst week of the year: short winter days, a run of cloudy weather, and your heaviest loads.

This guide covers planning an off-grid solar system around that worst case. It explains how to list your loads, size the array for winter, size the battery bank for cloudy days, choose a controller and inverter, and plan backup power. A worked example compares a weekend cabin with a full-time home, and the guide ends with what living with the system is like. For building any DIY solar system step by step, see our guide to planning and building a DIY solar system.

Is off grid right for you?

Off-grid solar makes the most sense where a grid connection is expensive, unreliable or impossible:

• Cabins and remote land, where running a utility line would cost more than a solar system, or where there’s no line nearby at all.
• RVs, vans and boats, which have no grid connection on the move.
• Full-time homes, for people who want energy independence and are prepared to manage their use.

It’s a bigger commitment for a full-time home than a cabin. You become your own utility, responsible for keeping the power on through winter, which usually means a generator as well as panels and batteries. Some areas also have rules about connecting occupied homes to utilities, so check with your local building department before you plan a full-time off-grid home. If you’d rather keep the grid as a backup, a grid-tied system with batteries (a hybrid system) is the usual alternative.

Step 1: List your loads

Start with a load list: every device you’ll power, its watts, and the hours a day it runs. Multiply watts by hours to get watt-hours (Wh) a day for each device, then add them up. Our DIY solar system guide shows a sample load list.

Off grid, two extra details matter:

• Seasonal changes. Your winter loads may differ a lot from summer ones. Lights run longer in winter, and a furnace fan or heat-tape may come on. In summer, fans or air conditioning may run. Make a list for each season, and design to the season where your loads are highest compared with the sun.
• Big loads. Electric heating, electric cooking, electric water heating and clothes dryers use far more power than everything else combined. Most off-grid homes run these on propane, wood or solar thermal instead, which can shrink the system you need several times over.

Step 2: Size for winter, not summer

Your panels make the least power in winter, when days are short and the sun is low, and that’s usually when your loads are highest. An off-grid system has to cover that month, not the yearly average.

To find your winter sun:

1. Open NREL’s free PVWatts calculator and enter your address.
2. Enter your planned tilt and direction for the panels.
3. In the monthly results, find the lowest month’s solar radiation figure, in kWh per square meter per day. That figure is your winter peak sun hours.

In much of the US, winter peak sun hours are well below the yearly average, often around half in northern states. That gap is why off-grid arrays are bigger than grid-tied ones for the same use. Tilting the panels more steeply favors the low winter sun and narrows the gap.

Step 3: Size the array

Use your winter figure in the array formula:

Array size (W) = daily use (Wh) ÷ (winter peak sun hours × loss factor)

The loss factor, about 0.75 to 0.85, covers heat, dust, wiring and conversion losses. Our guide to how much power a solar panel produces explains each term.

Step 4: Size the battery bank for cloudy days

The battery bank carries you through nights and cloudy spells:

Battery capacity (Wh) = daily use (Wh) × days of autonomy ÷ usable depth of discharge

Full-time off-grid homes usually plan for three days of autonomy or more, while a cabin with a generator can get by with fewer. Our guide to sizing and building a solar battery bank covers battery types, system voltage and wiring.

Step 5: Controller and inverter

The charge controller has to handle the array’s current and its cold-weather voltage. With a winter-sized array, the controller may need to be larger than the figure your average-day sizing would suggest. Our charge controller guide has the sizing formula.

The inverter has to cover everything running at once, plus the surge when motors such as a well pump start. In a full-time home with 240 V appliances like a well pump, you’ll need a split-phase (120/240 V) inverter, or two inverters stacked together. Our guide to choosing an off-grid inverter size walks through running and surge watts.

Once every part is sized, our step-by-step guide to mounting and installing solar panels covers putting the system together, from racking and wiring to the inspection.

Backup power

Even a well-sized off-grid system will eventually meet a cloudy stretch longer than its batteries can cover. A backup generator handles those weeks far more cheaply than a battery bank big enough to cover them:

• Size the generator for charging. An inverter-charger charges the batteries from the generator while passing power through to your loads. The generator needs to cover the charger’s input plus whatever loads run at the same time, with a margin. Check the inverter-charger’s manual for its maximum charging input.
• Use auto-start. Many inverter-chargers can start a compatible generator automatically when the batteries fall to a set level, and stop it when they’re charged. That protects the batteries when you’re away or asleep.
• Choose the fuel. Propane stores for years without going stale, which suits a generator that runs only occasionally. Gasoline doesn’t store well without a stabilizer.
• Cut loads during long cloudy spells. Put off laundry and other heavy jobs until the sun returns, and switch off loads you don’t need. Even a few changes can stretch the batteries by a day or more.

Worked example: A weekend cabin vs a full-time home

Here’s how the steps compare for two hypothetical systems in the same northern location, with round numbers for illustration:

The cabin is the easier case. It’s used only on weekends, so the array has the rest of the week to refill the batteries, and a smaller array than the formula suggests can work. The full-time home has no such slack. The array must keep up every day, and the battery bank is about ten times the cabin’s.

Living with an off-grid system

Once the system is running, a few habits keep it healthy and keep the lights on:

• Watch the battery level. A battery monitor or the battery’s BMS app shows the state of charge. Learn how fast it falls on cloudy days, and when to cut back or start the generator.
• Use power when the sun shines. Run the washing machine, pumps and power tools in the middle of sunny days, when the panels can supply them directly and the batteries stay full.
• Keep the panels clear. Brush off snow when it’s safe to do so, and rinse off dust and pollen. Check for new shade as trees grow.
• Look after the batteries. Water flooded lead-acid batteries on schedule. For any battery type, check the connections once a year.
• Service the generator. Change its oil, and run it now and then so it’s ready when you need it.

Frequently asked questions

How much does it cost to go off grid with solar?

It depends almost entirely on how much power you use. To estimate it, size the array, battery bank, controller and inverter with the steps above, then price each part. Add mounting, wiring, a generator if you need one, and any professional installation. Our solar panel cost calculator shows how to estimate the cost of the solar side. For a remote site, compare the total with the cost of bringing in a utility line.

Can you run a whole house off grid?

Yes. Many households live fully off grid, but it takes a large system and careful use. Most off-grid homes run heating, cooking, hot water and clothes drying on propane, wood or solar thermal, so the solar system only has to cover lighting, appliances, electronics and pumps.

How many solar panels do I need to go off grid?

Divide your array size from Step 3 by the wattage of the panels you plan to use. In the full-time home example, 5,000 W of 400 W panels is 12.5, so you’d round up to 13 panels. Off-grid arrays are usually larger than grid-tied ones for the same use, because they’re sized for winter.

Can I go off grid and keep a grid connection?

Not strictly: a system connected to the grid isn’t off grid. A hybrid system, which is a grid-tied system with batteries, gives you some of the same independence. It runs on solar and batteries most of the time, keeps chosen circuits running during an outage, and uses the grid as a backup instead of a generator.