How Long Do Solar Lights Last? Runtime Calculation Guide

Solar light runtime is calculated by dividing the battery's usable watt-hours by the fixture's average power draw in watts, then dividing by the hours of light required each night. This guide gives buyers a five-step calculation they can apply to any spec sheet, with worked examples using real CMJ Solar street and flood light parameters rather than marketing wattage.

Step 1: Calculate Usable Battery Energy in Watt-Hours

Start from the battery, not the LED. Usable energy equals nominal voltage multiplied by amp-hour capacity multiplied by the allowable depth of discharge. Integrated solar lights store energy in LiFePO4 packs built around 3.2V nominal cells, so a fixture with a Grade A LiFePO4 85Ah battery holds 3.2 times 85, or about 272 watt-hours nominal. A prudent design depth of discharge of 90 percent for LiFePO4 leaves roughly 245 watt-hours actually usable each night.

Do not assume 100 percent. Reserve margin protects the cells, covers the natural efficiency loss of aging packs, and matches the way manufacturers rate autonomy. Smaller fixtures follow the same math: a 15Ah pack holds about 48 watt-hours nominal and 43 watt-hours usable. Why LiFePO4 rather than lead-acid appears in this arithmetic, including its deeper usable discharge and longer cycle life, is explained in our LiFePO4 vs lead-acid comparison.

Step 2: Use Average Power, Not Nameplate Watts

This is the step that produces the most wrong answers. The '200W' or '600W' printed on a solar fixture is a product-class label tied to LED count, not the measured electrical draw of the light engine. A 6500-lumen integrated road fixture typically draws on the order of 12 to 18 watts at full brightness, not 200 watts. Always request the actual power curve, meaning measured watts at full output and in dimmed standby, and treat printed watts as marketing unless a measured figure is supplied.

Motion-controlled lights spend most of the night in a low-power standby state and only reach full output when radar or PIR detects movement. Average power therefore comes from weighting the schedule. For example, four hours at 12 watts plus eight hours at 1.8 watts gives 48 plus 14.4, or about 62 watt-hours consumed per night. Average draw over the twelve-hour night is roughly 5.2 watts. Choosing profiles for a real road is covered in our solar street light wattage and road-width guide.

How PIR and Radar Sensors Change the Real Energy Budget

Motion sensing is the single biggest lever in the runtime equation, and buyers should model it explicitly. On a quiet village road, a fixture may trigger full brightness fewer than ten times per hour for twenty seconds each, which means it spends more than 95 percent of the night in standby. In that pattern the average draw collapses toward the standby wattage and autonomy can nearly double relative to a steady-on schedule. On a busy urban road the same fixture triggers almost continuously during evening traffic, so the energy budget approaches the full-brightness figure and a standby assumption becomes dangerously optimistic.

The correct procedure is to ask the site three questions: how many hours of traffic, what fraction of minutes contain movement, and whether a constant low base level is required by the lighting class. A common compromise profile is 100 percent output for the first two or three hours after dusk, then 20 to 30 percent base brightness with a boost to full output on detection until dawn. This keeps walkways visibly lit for safety and CCTV while reserving most of the battery. Sensors on CMJ Solar integrated fixtures are microwave radar with a 6 to 8 meter detection distance on the Navigator II range, chosen because radar tolerates the dust, rain and fouled lenses that silence passive infrared sensors. The security angle is covered further in our solar security light guide.

Step 3: Divide for Nightly Runtime and Autonomy Days

Runtime hours equal usable watt-hours divided by average watts. Autonomy days, the figure datasheets usually quote as 'rainy days', equals runtime divided by the required lighting hours per night. Using the numbers above, 245 usable watt-hours divided by about 62 watt-hours per night returns just under four nights of light with no solar input at all, which is exactly the 4 to 5 rainy-day rating published for the MJ-SSO200 with its 85Ah battery and radar control profile.

Run the same check on a smaller fixture to validate the method. The Navigator II 100W stores about 48 watt-hours nominal in a 15Ah pack, or 43 watt-hours usable. A typical profile of three hours around 4 watts followed by nine hours of radar standby near 0.4 watts consumes roughly 16 watt-hours per night, giving about 2.8 nights, consistent with the published three-rainy-day rating.

A road-class example rounds out the picture. The MJ-SSO100 carries a 60Ah pack, about 192 watt-hours nominal and 173 usable, and its 4500-lumen engine draws roughly 8 watts at full output and 1.5 watts in standby. A five-hour full-brightness plus seven-hour standby night consumes about 50 watt-hours, so the fixture clears three nights of reserve with healthy margin, matching its published 4 to 5 rainy-day rating once the controller's deeper economy logic is included. If a calculation based on declared parameters returns far fewer nights than the supplier claims, either the assumed dimming schedule is too aggressive or the published reserve is optimistic; ask the factory to show the duty profile behind the number.

Step 4: Check That the Panel Can Actually Refill the Pack

Autonomy covers cloudy stretches, but the panel must refill the battery under normal sun. Daily harvest equals panel watts multiplied by effective sun hours multiplied by charge-path efficiency, for which a conservative 85 percent factor covers the controller, wiring and battery losses. A 75-watt panel in a 4.5-sun-hour climate delivers roughly 287 watt-hours per day, several times the 62 watt-hours consumed each night, which is why the MJ-SSO200 recharges comfortably even after heavy discharge. In a weak-sun month with only two effective hours it still harvests about 127 watt-hours, comfortably above nightly use.

The Navigator II 100W's 13-watt panel in four effective sun hours yields about 44 watt-hours, matching its 15Ah pack and the published 6 to 8 hour charge time in strong sun. Flood lights use the same arithmetic with a separate panel: the Alien 800W solar flood light pairs a 54Ah LiFePO4 pack, about 173 watt-hours nominal and 156 usable, with a 5V/60W monocrystalline panel, and its rated 18-hour discharge time implies an economy-profile average around 8.6 watts, which 156 divided by 18 confirms at roughly 8.7 watts. Sizing for yards, warehouses and perimeters is detailed in our solar flood light buying guide, and the full fixture families sit in the solar flood light category.

Step 5: Derate for Battery Aging and Temperature

Runtime is not constant for ten years. LiFePO4 packs in compact solar fixtures are commonly rated for 2000 cycles on value series and 4000 cycles on premium packs. At one discharge cycle per night, 2000 cycles is about five and a half years and 4000 cycles approaches eleven years, by which point capacity typically reaches the 80 percent threshold. At 80 percent health the 245 usable watt-hours in the worked example falls toward 196 watt-hours, trimming four-night autonomy to roughly three nights. Buyers tendering multi-year contracts should size for end-of-life reserve rather than day-one reserve, specifying one extra night of capacity where service obligations are strict.

Temperature deserves a small derating too. Cold raises internal resistance and trims immediate discharge capacity, although it recovers on warming, while extreme heat accelerates calendar aging. In markets with freezing nights, apply the cold-climate guidance from our LiFePO4 maintenance article and favor the 4000-cycle MJ-SSO packs. Routine inspection of panel cleanliness and controller settings preserves the original harvest assumption, since dust or shading silently breaks the recharge balance the calculation depends on.

A Five-Minute Buyer Checklist

When comparing quotes, run the full chain on every shortlisted fixture. Write down nominal voltage and amp-hours, convert to usable watt-hours at 90 percent, obtain measured full and standby watts rather than printed class watts, build the nightly duty profile in watt-hours, divide for autonomy days, verify the panel's worst-month harvest against nightly consumption, and finally derate for cycle life and local temperature. Fixtures whose numbers reconcile with their published rainy-day ratings are engineered products; fixtures whose numbers do not reconcile are usually relying on buyer inattention. For project quotes and custom dimming profiles, browse the all-in-one solar street light category or contact the CMJ Solar team for measured power curves on any model.

Frequently Asked Questions

Why does a light labeled 200W run for several nights on a small battery? Because the 200W label is a series classification based on LED count, while actual full-brightness draw is typically 12 to 18 watts and average draw with radar dimming is far lower. Runtime calculations must always use measured or declared system watts.

How many rainy days of autonomy should I specify? Three nights is adequate for many tropical and subtropical projects with daily recharge, while monsoon and temperate winter markets should specify 4 to 5 nights, and critical security or road applications add one more night to cover end-of-life capacity fade.

Is it better to buy a bigger battery or a bigger panel? They solve different problems. Battery capacity funds consecutive cloudy nights; panel size guarantees daily recharge in the weakest sun month. Size the panel first against worst-month harvest, then add battery for the required autonomy, rather than over-sizing only one side.