Bilge and House Loads on Solar Overnight

Solar panels do nothing at 2 a.m. That sounds obvious until you realize the whole point of a solar rig at anchor is to survive the hours when the sun is gone and the bilge pump, the anchor light, and the fridge are all still drawing. The panels refill the tank by day; the battery has to carry every load through the dark, and if you sized only for the daylight side of the equation you can wake up to a dead bank and a boat sitting lower than you left it.

Getting your solar overnight loads right is really a battery-and-budget exercise, not a panel exercise. Below is the exact walkthrough I use on a customer’s boat, step by step, with the numbers from a real job I did last spring.

Why the night is the part that bites you

During the day your charge controller (the box that regulates panel voltage into safe battery charging) is feeding the loads directly and topping the battery besides. The bilge pump and fridge barely touch the bank while the sun is up.

Then the sun sets and everything reverses. Now the battery alone feeds the anchor light, the fridge compressor cycling on and off, the occasional bilge pump kick, and whatever phantom draws (small standby currents from gear that is “off” but not really off) you never measured.

Sundown to sunrise in summer is roughly 9 to 11 hours. That is the window the battery has to cover with zero help, and it is where most undersized systems quietly fail.

Tools and materials you will want

You do not need a shop full of gear for this. A shunt-based battery monitor (a meter wired through a shunt so it counts amps in and out accurately) is the one tool that changes everything, because it turns guessing into measuring.

Beyond that: a clamp meter or inline watt-meter for spot-checking individual loads, a notepad, and a couple of hours across an evening and a night. Parts-wise, plan on tinned marine wire, a fuse block, and a bilge pump wired on its own fused circuit. If any of the wiring is new ground for you, keep our 12V Boat Wiring Guide for Beginners open on your phone while you work.

Step 1: List every load that runs after dark

Sit in the cabin one evening and write down everything that draws current while you are asleep at anchor. Not what you use during the day. What runs at night.

The usual list: anchor light, 12V fridge or cooler, bilge pump on its automatic float switch, a bilge high-water alarm, a CO detector, cabin fans, and the phantom draw from a chartplotter or stereo left in standby. Small things add up fast.

The mistake I see most weekends is people counting the fridge and the anchor light, then forgetting the fans and the standby loads that together add another 10 amp-hours by morning.

Step 2: Turn each load into amp-hours per night

Amp-hours (Ah) are just amps multiplied by hours, and they are the honest currency of a battery bank. For each load, multiply its draw in amps by how many hours it actually runs overnight.

A fridge does not run constantly. A typical marine compressor pulls 4 amps but only cycles on maybe 40% of the time, so over a 10-hour night that is 4 amps times 10 hours times 0.4, or 16 amp-hours. The bilge pump might draw 3 amps but run for 90 seconds twice a night, which is a rounding error. The anchor light at 0.5 amps for 10 hours is 5 amp-hours if it is LED, four times that if it is an old incandescent bulb.

Step 3: Add it up and find your real overnight number

Total the column. On the boat I keep referencing, a 30-foot cruiser, the honest overnight tally came out like this.

Thirty amp-hours a night. That number, not the panel wattage, is what the battery has to deliver in the dark without breaking a sweat.

Step 4: Size the battery so the night is the easy part

Now match the bank to that number with headroom. You never want a full night’s draw to be more than about half of your usable capacity, so a rough patch of two cloudy days does not put you on the ropes.

Usable capacity is not the same as the sticker rating. A lead-acid battery should only be drawn to about 50%, so a 100Ah lead-acid battery gives you roughly 50 usable amp-hours. A lithium (LiFePO4) battery of the same 100Ah rating safely gives you about 80 to 90 usable amp-hours, which is why one 100Ah lithium often replaces two lead-acid batteries. For a 30Ah night, a single 100Ah lithium leaves you comfortable with two nights of reserve, and our How to Size a Boat Solar Array guide walks the daytime replacement math that pairs with this.

Step 5: Make sure the panels actually refill it by day

The overnight side sets your battery size. The daytime side has to put those 30 amp-hours back plus cover the daytime loads, and then some for cloudy days.

A 100W panel in real cruising conditions makes maybe 30 to 40 amp-hours across a good summer day, well under its lab rating because of heat, haze, and imperfect angle. So a boat burning 30Ah overnight plus another 20Ah of daytime use wants around 200W of panel to stay ahead comfortably. Getting those amps to the battery instead of losing them as heat is where the controller and wire matter, and our comparison of MPPT vs PWM Solar Controllers explains why the smarter controller usually earns its keep on a boat that runs loads through the night.

A worked example from last spring

The 30-foot cruiser I mentioned came to me because the owner kept waking to a bilge alarm chirping about low voltage after two nights at anchor. His single 100Ah flooded battery was hitting 50% by the second morning and the panel could not catch up.

We measured his real overnight draw at 30 amp-hours, swapped the tired flooded battery for one 100Ah LiFePO4, added a second 100W panel to bring him to 200W total, and split the bilge pump onto its own fused circuit off the battery. Parts came to about $620: roughly $430 for the lithium, $110 for the panel, and $80 for wire, fuses, and the bilge circuit.

Three weeks later he anchored out for four nights in a row and never dropped below 68% state of charge. The panels, mounted clear of shade following the same logic as our guide on Mounting Solar Panels on a Bimini, refilled the bank by late morning every day. The bilge pump, now on its own feed, would keep running even if he flattened the house side completely.

Where people go wrong

The most common error is buying panels first and treating the battery as an afterthought. Panels are the glamorous part, but they are useless after dark, and the battery is what actually decides whether you make it to sunrise.

The second error is ignoring the bilge pump in the power budget entirely. It barely uses any energy, true, but a boat taking on water while you sleep needs that pump to run no matter how flat the house bank gets, and that is a wiring decision, not a capacity one.

The third is forgetting phantom draws. Gear left in standby can quietly burn 5 to 10 amp-hours a night, and it never shows up until you measure with a shunt.

Putting it all together for a quiet night at anchor

Start from the dark side of the day. Measure what your boat actually pulls overnight, size the battery so that draw is the easy half of the job, then set the panels to refill it with margin for a gray morning.

Keep the bilge pump on its own fused, always-live circuit so safety never rides on the same switch as your reading lamp. If you want the full picture from panel to controller to battery to loads, our Marine Solar Power Setup Guide ties every piece into one system.

Do this once, honestly, with a meter instead of a guess, and the night stops being the scary part. You sleep, the pump watches the bilge, and the sun handles the rest in the morning.