How Big a Solar and Battery System Does an Off-Grid Site Need?

Undersize an off-grid solar system and the site dies on the fourth cloudy day in January. Oversize it and you just spent the budget on panels that do nothing most of the year. The right answer sits in between, and it comes from a short chain of arithmetic that anyone can follow. So how big a solar and battery system does an off-grid site need?

An off-grid site needs a solar array that covers its daily energy use in the worst month of the year, plus a battery that carries the load through several sunless days without discharging too deeply. For off-grid solar sizing, the array is daily watt-hours divided by worst-month sun hours and a derate (often about 0.75).

The battery is daily watt-hours times days of autonomy, divided by depth of discharge. Then add margin for recharging. For example, a 100 W average load in a location with 3.5 winter sun hours needs roughly a 1 to 1.2 kW array and about 9 kWh of lithium battery for three days of autonomy.

Step 1: How Much Energy Does the Site Use Per Day?

Everything starts with a load budget in watt-hours per day. List each device, its power draw in watts and how many hours a day it runs, then multiply and add.

Illustrative example for a small edge site:

• LTE router: 10 W × 24 h = 240 Wh
• RTU and instruments: 8 W × 24 h = 192 Wh
• Small edge computer: 40 W × 24 h = 960 Wh
• Camera with infrared at night: 12 W × 24 h = 288 Wh
• Enclosure fan and heater, averaged: 30 W × 24 h = 720 Wh
• Total: 2,400 Wh per day, or 100 W average

Use measured or datasheet numbers, not nameplate maximums for everything, but do include things that cycle on and off, like heaters, fans and radios that transmit in bursts. If any loads run on AC through an inverter, add the inverter's losses and its own idle draw, which can be surprisingly large on a small system.

Step 2: What Are Peak Sun Hours, and Which Month Counts?

Peak sun hours are the day's total sunlight expressed as hours of full-strength sun (1,000 watts per square meter). A location that gets 4 kWh per square meter on a tilted panel in a day has 4 peak sun hours, even though the sun is up much longer.

For off-grid work, use the worst month, usually December or January in the Northern Hemisphere. Annual averages hide the problem. NREL's free PVWatts calculator gives monthly production for any US location and tilt, which makes it easy to find your design month. Tilting panels steeply (around latitude plus 10 to 15 degrees) raises winter sun hours at the cost of some summer output, which is the right trade for an off-grid site.

Step 3: How Big Should the Solar Array Be?

Divide daily energy by sun hours, then by a derate factor for real-world losses. Those losses include hot panels producing less than their rating, dust, wiring, the charge controller and the energy lost going in and out of the battery.

Worked example, continuing from Step 1:

1. Daily load: 2,400 Wh
2. Design-month sun: 3.5 peak sun hours (example input)
3. Raw array: 2,400 ÷ 3.5 = 686 W
4. With a 0.75 derate: 686 ÷ 0.75 = 914 W

That 914 W is the minimum to break even on a typical winter day. As Step 5 shows, you will want more.

Step 4: How Big Should the Battery Be?

Multiply daily energy by the days of autonomy you want, then divide by the depth of discharge (DoD) the battery can tolerate regularly. Autonomy is how many days the site can run with little or no sun. Critical sites in cloudy climates commonly plan for five days or more. Less critical sites with good winter sun may get by with two or three.

Battery chemistry changes the answer a lot. Lithium iron phosphate (LFP) can routinely use about 80% of its rated capacity. Lead-acid batteries last much longer if you keep regular discharge to about 50%. The chart below shows how battery size grows with autonomy for the 2,400 Wh example site.

For three days of autonomy, the math is 2,400 Wh × 3 ÷ 0.8 = 9,000 Wh, or 9 kWh of LFP. The same three days on lead-acid needs 2,400 × 3 ÷ 0.5 = 14.4 kWh. Cold weather reduces usable capacity further, so many designers add a temperature factor on top for sites that see hard freezes.

Try the Off-Grid Solar Calculator

Plug in your own average watts, worst-month sun hours, autonomy days and depth of discharge below. It returns the array watts and battery kWh using the same method described here.

If you want to check your work against a published method, the Louisiana Department of Natural Resources has a stand-alone PV sizing worksheet. Its worked example uses a 7.46 kWh daily load, 3.8 design-month sun hours, seven days of autonomy and 0.8 depth of discharge, and lands at about a 68.8 kWh battery and a 2.9 kW array once its correction factors are applied.

Step 5: Can the Array Refill the Battery After Bad Weather?

This is the step most quick estimates skip, and it is why many minimum-sized systems fail in winter. An array sized to exactly cover the daily load has nothing left over. After three cloudy days drain the battery, it can run the site but never catch up until spring.

Here is the recharge check for the example site:

• Energy drained after three cloudy days: about 7,200 Wh
• Array oversized by 30%: 914 W × 1.3 = 1,188 W
• Winter production: 1,188 W × 3.5 h × 0.75 = 3,119 Wh per day
• Surplus after the 2,400 Wh load: 719 Wh per day
• Time to refill: 7,200 ÷ 719 = about 10 days of normal winter sun

Ten days may be acceptable if long cloudy spells are rare. If not, increase the array further, add a small generator for winter backup, or cut the load. Running this check is what separates a design that survives January from one that only works on paper.

How Does Temperature Change the Math?

Heat hurts panels and batteries, and cold hurts batteries and can trip up controllers. A good design checks both ends of the thermometer.

Hot Panels Make Less Power

Crystalline silicon panels lose roughly 0.3% to 0.5% of their output for every degree Celsius above 25 °C (77 °F) cell temperature, and cells on a sunny roof or rack often run 20 to 30 °C hotter than the air. That loss is part of why the derate factor sits well below 1.0. Leaving an air gap behind the panels helps them run cooler.

Cold Batteries Hold Less

Both lead-acid and lithium batteries deliver less usable energy when cold, and most LFP batteries must not be charged below freezing without a heater. Put batteries in an insulated or buried enclosure where you can, and size with a cold-weather allowance if the site sees hard freezes.

Cold Panels Raise Voltage

Panel voltage climbs on cold, bright mornings. A string that looks safe on a summer datasheet can exceed a charge controller's maximum input voltage in January. Check the coldest expected temperature against the controller's limit before you wire panels in series.

Off-Grid Solar Sizing Mistakes That Kill Sites

The same handful of errors show up again and again:

• Sizing for annual average sun instead of the worst month.
• Forgetting heaters, fans and inverter idle draw in the load budget.
• Planning for 80% DoD on lead-acid batteries, which shortens their life sharply.
• Skipping the recharge check, so the battery never recovers after a storm.
• Letting shade from new equipment fall on the array.
• Adding loads later (a second camera, a bigger computer) without resizing anything.

When Does Solar Alone Stop Making Sense?

When the load gets big or the winter gets dark. A few hundred watts of continuous load is comfortable for solar and batteries. Several kilowatts around the clock, in a place with short winter days, can need an array and battery bank so large that a hybrid system with a generator makes more sense.

Telecom operators face this choice constantly, since a cell site runs all day and night. Our guide to off-grid cell tower power shows how they mix diesel, batteries and solar to keep towers on the air.

Off-Grid Solar Sizing Comes Down to the Worst Week, Not the Average Day

So how big a system does an off-grid site need? Big enough to cover its daily load in the darkest month, carry it through several sunless days, and recover afterward. That means a solid load budget, real winter sun data, a sensible derate, a battery sized for autonomy and chemistry, and enough extra array to refill the battery.

Run the math, then check it against the calculator and a published worksheet. If the numbers look huge, the answer may be a smaller load or a backup generator rather than more panels.

Two numbers get you most of the way: the site's daily watt-hours and its winter sun hours from PVWatts. Gather those before you call a supplier, and you can size the whole system in an afternoon and check their quote line by line.

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