Reduced Solar Charging Outputs During Autumn
As I type this at 10am on a Christchurch autumn morning, the sun is trying to burn through a layer of fog and low cloud on what is forecast to be a fine/sunny day. We’ve nearly reached the solar equinox, so are about halfway between theoretical solar charging maximum (December) and solar minimum (June).
Even without the morning fog, daily maximum sunlight hours have reduced from over 15 in December, to less than 13 in March, and peak solar output from a flat mounted panel (e.g. on a boat or van roof) has reduced by around 27%. According to NIWA data, the average daily solar output from a flat mounted panel (van or boat) has reduced by around 40% from December levels. As we move through Easter we’re on our way to June’s grim low output, which is approximately 80% less charging than December!
To make matters worse, power consumption increases as the nights get longer, cooler, as we spend more time indoors. While LED lighting doesn’t use a lot of power, general loads and screen time tend to increase, plus running diesel/gas heaters suddenly introduce an extra load onto the battery bank. Net result? Less battery charging, and more battery load.
What problems show up?
Lack of monitoring: Many people don’t have proper battery monitors that measure volts, amps and calculate battery capacity. A battery monitor will help highlight system performance and highlight issues before they destroy your batteries, thus allowing you to manage charging more effectively.
For example: if you notice your batteries aren't reaching full charge, you might decide to drive to a powered campsite for the night to top them up to 100%. People with battery monitors tend to replace their batteries less frequently because they are able to care for them better.
Worn-out or insufficiently sized batteries: If your batteries can’t store enough power to last through the night, your equipment will gradually switch off as voltage drops. This is especially critical for Lead Acid batteries, which should be kept above 50% charge.
Aged Lead Acid batteries hold significantly less power than when new. In summer, lower loads may hide this issue, but colder temperatures and higher usage make reduced capacity obvious. They may seem to charge normally, but voltage can quickly sag under load.
Fully draining Lithium batteries and powerstations should also be avoided to preserve lifespan.
Insufficient charging: When your system fully charges each day, the batteries only need to last a single night. As days shorten and cloudy weather reduces solar output, batteries may not reach full charge, leaving less capacity for the next night.
Even with adequate battery size, an undersized charging system can cause the whole setup to fail prematurely.
What to do if your system is underperforming?
- Test whether your batteries have enough storage to comfortably last through the night. Fully charge them using a battery charger for around 24 hours until ~4 pm, then run your normal loads overnight. A Lead Acid battery should still show a resting voltage above 12.2 V the following morning, before solar starts charging again. If not, either replace old batteries or increase your battery bank size.
- If you have an older PWM solar regulator, consider upgrading to a modern MPPT regulator. MPPT units are much more efficient, extracting 10–40% more charging from the same solar panel. Victron regulators, for example, offer excellent performance, competitive pricing, and a 5-year warranty.
- Assuming your batteries are in good shape, you can add more solar panels. Solar panels are relatively low cost and long lasting so investing in good solar output can save money by reducing battery replacements. Flat roof-mounted panels are far less effective in winter, so consider temporary panels or solar blankets that can be angled directly toward the sun. In mid-winter, a panel facing north and hung vertically can produce nearly twice the power of a flat panel.
- You can also add charging from other sources, such as your vehicle or boat engine, or a generator. For faster generator charging, ensure your 230V battery charger has enough capacity to minimize run time and fuel use. Multiple charging sources can run simultaneously, just make sure the total charge rate matches your batteries. As a rule of thumb, Lead Acid batteries shouldn’t be charged above 30% of their capacity (0.3C = 30Amps for a 100Ah battery), while Lithium Iron Phosphate batteries can usually accept up to 50% (0.5C), though you should always check the battery label.
- Finally, add a proper battery monitor that measures amps in and out and calculates the battery’s state of charge. This helps you manage power use and protect your batteries.
As the days get shorter and the weather cools, reduced solar output is a reality for van and off-grid setups, but it doesn’t have to leave you stranded with flat batteries. By checking your battery capacity, upgrading to efficient MPPT regulators, adding extra solar panels or alternative charging sources, and keeping an eye on power use with a proper battery monitor, you can stay powered up all autumn.
Planning ahead and understanding how seasonal changes affect your system will save you money, extend battery life, and keep your adventures running smoothly. With a few simple adjustments, you can make sure shorter days don’t mean less power, just smarter, more reliable solar charging.
