Mutau Ranch

Reserve is the friendly number: 100% means the batteries are full, 0% means you've used everything you should — time to stop and let the sun refill them.

• The batteries are rated 675 Ah but are ~9 years old, so we count on roughly 500 Ah actually being usable. Reserve is based on that smaller, honest number.
• Lead-acid batteries shouldn't be drained past half — so 0% reserve is set at the 50% mark, not truly empty.
• The power doesn't shut off at 0% — the inverter cuts out when battery voltage falls to about 43 V, somewhat past the floor. But past 0% the voltage drops fast and the shutoff can happen with little warning.
• Big motors are special: starting the water pump can briefly yank the voltage down to the cutoff even with healthy reserve — that's a surge-power limit of old batteries, not empty batteries.

House rule of thumb: run big appliances while the sun is up — the goal is to end each day with a full battery. The banner up top does the math for you, using today's solar forecast: a dishwasher run costs about 1.5 kWh, a load of laundry about 0.5.

These are estimates, tuned conservatively. When in doubt, the status banner at the top tells you what to do.

There's no meter on the house itself, so this number is worked out rather than measured — at every hour, day and night. Two meters do the work: one on the solar, one on the batteries.

Whatever the sun makes has to go somewhere. If it isn't going into the batteries, the house is using it:

house = solar − battery

• Charging on a sunny afternoon? Solar is bigger than what the batteries are taking, and the gap is the house.
• After dark solar is zero, so the house is simply whatever is coming out of the batteries.

One wrinkle we correct for. The solar controller reports what the panels made, but some of that is lost as heat inside the controller before it reaches anything. Left alone, that loss gets blamed on the house — and because it grows with sunshine, it painted a mountain on the chart every morning at an empty cabin.

So we subtract it first, using a standard piece of solar engineering called the Schmidt–Sauer loss model. It says a converter wastes a fixed amount just being switched on, plus a share of whatever is flowing through it:

loss = 16.7 W + 6.1% of solar

We measured those two figures here rather than trusting a spec sheet: over 12 days with nobody at the ranch the real load held steady near 226 W, which let us see exactly how much the sun was inflating it. Tested against days left out of the maths, it lands within a few watts.

It's a good estimate, not a meter. Treat single readings as approximate; the shape over a day is what to trust.