There is so much wrong with your planning it is hard to know where to start. So let's start with batteries.
A group 27 has maybe a 90 ah capacity, of which only half or 45 ah are useable. With 3 batteries you have 135 ah to play with. Lead acid batteries are notoriously inefficient at accepting a charge. If you are lucky it will be 85% efficient, that means in order put back 1 ah it will take about 1.2 ah of charging. Discharging your batteries to 50% will require about 160 ah to bring them back to 100% SOC and it will take about 5-6 hours.
Nigel Calder estimates that a solar panel output will average about 3 times the nominal panel capacity. On a bright sunny day in June, it may be higher, on an overcast day in December it will be much lower. My experience with 2 years of living off solar is consistent with Calder's assertion. The 400 watts you are planning will yield on average 1200 wh (100 ah @12vdc). It will take a day and a half of bright sunny weather with no shading and no other load to bring your batteries back to 100% SOC from 50% SOC.
AGM batteries must be kept at 100% SOC as much as possible to prevent sulfating and an early death. This should happen at least every week or two. If not, the AGMs will be useless in as little as a year. And, the capacity will decline over that time.
The 55a Hitachi will never put out 55a for more than a few minutes and will rapidly decrease its output regardless of battery SOC. If you get 25 a at cruising speed for any length of time consider yourself lucky. Remember, an AGM battery's charge acceptance rate decreases as the SOC increases, and decreases sharply after the battery reaches 80-90% SOC. It will take 5 or more hours at cruising speed with no other loads to recharge the battery at 50% SOC.
Refrigeration, electronics and navigation instruments, lighting, and an autopilot consume a lot of energy. Average daily consumption in the range of 75-100 ah a day without a watermaker would not be out of the ordinary. In other words, what you propose to install is simply inadequate for what you intend to do.
Thanks — this is exactly the kind of teardown I was hoping for when I published it, and I've checked your numbers rather than argued with them. They hold up. You're Amazing. If you are ever in the neighborhood, I owe you a cold beverage or two.
Three AGM Group 27s at 90 Ah is 270 nominal, 135 usable at 50% DoD. At 85% charge efficiency that's ~160 Ah to put back. Calder's 3× rule gives 400 W × 3 = 1,200 Wh/day. All correct.
One correction, and it goes against me: 1,200 Wh ÷ 12 V = 100 Ah assumes charging happens at 12 volts. It doesn't — it happens at 13.8 to 14.4. The real delivered figure is closer to
87 Ah/day. Your estimate was generous.
I've also now costed my actual loads instead of hand-waving them. Compressor fridge 40 Ah, Raspberry Pi nav stack 6–12, autopilot 8 underway, LED lighting 8, VHF and instruments 3–4, pump and charging 7.
64 Ah at anchor, 79 underway. So your 75–100 figure was fair, and against 87 Ah of harvest I'm running a ~20 Ah/day surplus at anchor and roughly break-even underway. Refilling 160 Ah from that takes over a week, and the last stretch is the slowest because acceptance is collapsing. Your absorption point is the one that actually matters and I had nothing on the page about it.
Where I'd push back is on scope rather than physics. I'm coastal — weekends and short trips, back on shore power every week or two, where a proper charger takes the bank to 100% and holds it. That's the medicine AGMs need, and in that pattern the array offsets daily draw rather than being the sole charge source. Your critique is correct for the boat you were picturing: anchored out a week or more with the fridge running, there is no path back to 100% and the bank dies young. That's a real boundary on the design and it's now stated on the page rather than left implied.
The honest conclusion your post pushed me to is that the limit isn't the array, it's the chemistry under it. LiFePO4 takes full current to ~95% SOC so there's no tail to lose, it's ~99% efficient instead of 85%, gives 80–100% DoD instead of 50%, and doesn't care about partial state of charge. A single 200 Ah lithium gives more usable capacity than my three AGMs, weighs 150 lb less, and banks every amp-hour the array makes. That's where this goes next.
The alternator point is well taken too — I'd been treating the 55 A Hitachi as a charging strategy rather than a top-up, and 25 A at cruise with a taper is the number I should have been planning around.
Page is updated with the full budget and the absorption section, with credit to this thread. Appreciate you taking the time. Like I said above I owe you a mug of Grog.
I posted the updates to the design at
skipperscott.com/solar.html, and I'll post progress at
skipperscott.substack.com as I update/build things on our little boat.
