Solar Recharge Time Calculator for Panels and Batteries

How the recharge maths works

Three steps, in order. First, size the hole. Battery capacity times the percentage you are refilling gives the watt-hours needed. A 1,000 Wh station from 20% to full needs 800 Wh. Second, size the tap. Panel watts times the conditions factor give the real average harvest rate. A 200 W panel at a 75% factor delivers about 150 W. Third, divide: 800 Wh ÷ 150 W is roughly 5.3 hours of equivalent full sun. That number is not 'time outside'. It is time at full strength. That is why the fourth input matters. At a seasonal average of five peak sun hours per day, the refill is a single decent day. At two peak sun hours it is a three-day project.

Charging is not perfectly linear at the ends. Most battery systems taper the final stage to protect the cells. Very cold batteries accept power reluctantly. The taper typically costs a little on the last ten to twenty percent. None of this breaks the model. It is simply another reason to plan with a conditions factor slightly lower than your optimism suggests. Treat the result as the middle of a range, not a promise.

Why nameplate watts overstate the harvest

Five separate thieves take a share of a panel's laboratory rating. Angle and orientation come first. A panel lying flat on a caravan roof in winter sun can lose a third or more against a correctly tilted one. Facing the wrong compass direction costs a tenth to a quarter, depending on latitude and sky. Temperature works against instinct. Solar cells produce less as they heat. A baking summer afternoon can underperform a crisp spring day, even though it feels like better sun. Cloud and haze chop irradiance hard; thin cloud still yields something, thick cloud can drop harvest to a tenth. Dust, bird lime and partial shading bite unevenly, and one shaded cell can drag down a whole string. Finally the controller: basic PWM designs lose noticeably more than MPPT when panel voltage sits far above battery voltage.

The conditions factor bundles all five into one number you can defend. A clean, cool, well-angled panel in strong sun justifies 80-90%. The typical mixed reality of caravanning, camping and an occasionally hazy sky is the 65-80% band. Winter, flat mounting, persistent overcast or a cheap PWM controller argue for 40-60%. If you cannot judge, enter 70%. Revisit it after you have logged one real day against the prediction. That single calibration is worth more than any table.

• 80–90%: clean, cool, correctly angled panel in strong sun
• 65–80%: ordinary good day, mixed sky, decent setup
• 40–60%: winter, flat mounting, haze, or PWM controller losses

Worked example: refilling after a night of fridge duty

Take the classic caravan morning. A 1,000 Wh station wakes at 20% after running the fridge through the night. You have a 200 W panel, mild sun with a touch of haze, and five peak sun hours on offer. Needed energy is 800 Wh. The harvest rate is 200 × 0.70, so 140 W. Charge time works out at 800 ÷ 140 ≈ 5.7 peak-sun-hours. That is just over one good day. In reality it finishes mid-morning the next day, because the first hour of weak sunrise sun does not count at full strength. Compare that with the optimistic label maths of 800 ÷ 200 = 4 hours. Now you can see why people conclude their panel is faulty when it is only being honest.

The same arithmetic runs in reverse as a sizing tool. Say your load consumes 500 Wh per day. You are camping in four peak-sun-hour conditions at a 70% factor. The panel you need is 500 ÷ (4 × 0.70) ≈ 180 W. Any smaller array is not a budget decision. It is a slow discharge you have not named yet. Building panels around consumption, rather than around what fits on the roof, is the difference between indefinite solar living and a very quiet battery alarm.

Reading peak sun hours correctly

Peak sun hours confuse almost everyone at first because the name suggests clock time. It does not. It is the total energy in a day, expressed as the hours the sun would need to shine at its lab standard of 1,000 W per square metre. A day with eight hours of gentle spring sun might only bank four peak sun hours; a brutal six-hour summer day might bank six. This is why winter refills feel broken rather than merely slow. At mid-latitudes, the swing between summer and winter peak sun hours is routinely a factor of three or four. The same panel that tops up a station in one summer afternoon needs a week of short grey December days to do the identical job.

Use the reference table as a planning band, not a local forecast. Real values for your site and season come from public solar datasets. They also come from logging your own panels' output for a week. Plan around your winter number, not your annual average. That is the household that still has lights in February. The sizing question got answered on the worst week of the year, not the brochure week.

Accuracy, assumptions and limitations

This model is a steady-state estimate: it assumes your average harvest holds across the day and that everything between panel and battery works. It cannot see clouds moving over. It cannot see the fridge cutting in while you charge, a connector failing, or a battery that refuses high current below five degrees. Taper at the top of charge, cable losses and charging consumption sit in the conditions factor only if you put them there on purpose. Treat outputs as a planning band, and when the difference between one day and two days of harvest matters, size for the pessimistic end.

Two habits make every future estimate better. First, check the prediction against one real day. Log the watt-hours harvested with whatever metering your station provides. Divide by peak sun hours, and you have measured your actual conditions factor. Second, keep a reserve rule in the plan rather than in the maths. Charging is only 'done' when the battery comfortably covers your next night's load without further sun. That framing converts a pleasant calculation into a decision procedure, which is the only job any of these estimates has.