Step 1: how do you calculate daily load in watt-hours?
Start with the energy the equipment uses in a full day. For each device, multiply its average power draw in watts by the hours it runs: watt-hours per day equals watts times hours. Add every device together. Use average draw, not the label maximum, and remember that some loads vary: infrared illuminators draw more at night, a motorized camera draws more while moving, and a router draws more while transmitting. Heaters add load in the cold.
Illustrative example, not a VDS spec: two cameras at 10 watts each running 24 hours use 480 watt-hours. A router at 15 watts uses 360. A network switch and recorder at 10 watts use 240. Averaged heater and other loads add 120. The daily total is 1,200 watt-hours. Measure real draw wherever possible, because estimates from spec sheets are often optimistic, and add a small safety margin to cover conversion losses inside the power system.
Step 2: how many days of autonomy do you need?
Autonomy is the number of days the system should keep running with no solar input at all. It is a design choice that depends on how critical continuous coverage is and how long cloudy or snowy spells last at the site. Battery energy needed equals daily load times autonomy days, divided by the usable fraction of the battery. The usable fraction reflects the reserve you keep so the battery never reaches its cutoff.
Illustrative example, not a VDS spec: with a daily load of 1,200 watt-hours and 3 days of autonomy, the system needs 3,600 watt-hours. If the usable fraction of the battery is 0.8, divide: 3,600 divided by 0.8 equals 4,500 watt-hours of nominal capacity. At a nominal 24 volts that is 187.5 amp-hours, which rounds up to a standard 200 amp-hour pack. More autonomy buys resilience but adds cost, weight and a longer recharge.
Step 3: what are sun hours and why does season matter?
Peak sun hours express a day's solar energy as an equivalent number of hours of full, standard-strength sunlight. A site with a total daily energy equal to two hours of full sun has two peak sun hours, no matter how many hours the sun is actually up. The figure varies by location, season, weather and panel tilt. Public solar resource databases and mapping tools provide monthly values for most regions, and a local installer or engineer can confirm them.
Season is the key. Summer values are far higher than winter values, and a system sized for the average or the summer will run short when the days are shortest. Design for the worst month in which the site must stay reliable. Illustrative example, not a VDS spec: assume two peak sun hours in the design winter month and five in summer. Sizing to the winter figure means the array has surplus in summer, which is the price of year-round reliability.
Step 4: how do you derate and size the solar array?
Panel wattage is a laboratory rating, so real output is lower. The derating factor, a number below one, combines losses from heat, dirt, wiring, charge controller conversion, battery round-trip efficiency and imperfect panel angle. The array formula is: array watts equals daily watt-hours divided by the product of peak sun hours and the derating factor. A derating factor around three quarters is a common planning assumption, but you should justify it for your site.
Illustrative example, not a VDS spec: daily load 1,200 watt-hours, winter peak sun hours 2, derating factor 0.75. Multiply 2 by 0.75 to get 1.5. Then 1,200 divided by 1.5 equals 800 watts of solar. Notice what this means: at 800 watts the array produces exactly the daily load in the design winter month, with nothing left over to refill a battery that has been drawn down. That is a break-even design, not a safe one, so the next step adds margin.
Step 5: will the battery recharge after a cloudy spell?
Autonomy is useless if the battery cannot refill. Recharge time equals the energy used from the battery divided by the daily surplus, where surplus is array yield minus daily load. Array yield is array watts times peak sun hours times the derating factor. If surplus is small, recovery takes a long time, and a second cloudy spell may arrive before the battery is back. Recharge is often the hidden constraint in winter.
Illustrative example, not a VDS spec: raise the array to 1,000 watts. Winter yield is 1,000 times 2 times 0.75, which is 1,500 watt-hours, so daily surplus is 300 after the 1,200 load. Refilling 3,600 watt-hours would take 12 winter days. Options are more panel, a lower load, a better tilt, an AC charger as backup, or accepting less autonomy. Also check the charge controller: array watts divided by battery volts, here about 42 amps at 24 volts, sets the minimum current rating.
Which sizing mistakes cause winter failures?
The most common error is sizing to an average day or summer sun. The second is using label wattage for loads when real draw is higher, especially for infrared illumination at night, motors, heaters and cellular transmission. The third is forgetting aging: batteries lose capacity over the years, panels collect dirt, and cold reduces usable battery energy. The fourth is shade from the mast, buildings and trees at the lowest sun angle, which can cut harvest sharply.
Treat sizing as a model that real data refines. Monitor battery voltage, solar harvest and load once the unit is deployed, then compare to the plan. If winter harvest is short, you have options: reduce load with scheduling, adjust the tilt, add panel, or shorten autonomy expectations. Do not rely on a single number, and be skeptical of any supplier who quotes how many cameras a panel can run without stating the load, the season and the assumptions.
Key terms
- Watt-hour
- A unit of energy equal to one watt of power used for one hour, the basic currency for sizing battery and solar.
- Autonomy days
- The number of days a system should keep running from battery alone with no solar input.
- Peak sun hours
- A day's solar energy expressed as the equivalent number of hours of full-strength standard sunlight.
- Derating factor
- A multiplier below one covering real losses such as heat, dirt, wiring and conversion when estimating usable solar energy.
- Usable fraction
- The share of a battery's nominal capacity that the design allows to be used, leaving a reserve at the bottom.
- Daily surplus
- Solar energy produced in a day minus the energy the load consumes, which is what refills the battery.
- Break-even design
- A system whose solar yield only equals the load in the design month, leaving nothing to recover from a dark spell.
The worked example above is an illustrative example, not a VDS spec. For reference, Gen 8 and Gen 9 trailers combine 900W of bifacial solar with a 25.6V nominal LiFePO4 battery that has built-in heaters and a battery management system, and the lighter unit pairs one 450W panel with a 300Ah battery for two cameras. Heater load and cellular router draw belong in any winter budget.
Common questions
How many days of battery autonomy should an off-grid camera have?
It depends on how critical continuous coverage is and how long cloudy or snowy spells last at the site. Many designers choose several days, then verify the array can refill the battery afterward. More autonomy adds weight and cost, and needs a larger surplus to recharge.
What is the formula for solar array size?
Array watts equals daily watt-hours divided by the product of the design-season peak sun hours and the derating factor. Use the worst month the site must stay reliable, then add margin so the array produces a surplus for recharging the battery.
Why does winter decide the design?
Winter has the shortest days, the lowest sun and often more cloud, so solar yield is lowest just when heaters and night lighting may add load. If the system works in the design winter month, it usually has plenty of margin in the rest of the year.