The short answer
Every downstream decision in a power system traces back to one number: daily amp hours consumed. Get that number honestly, apply your autonomy target and your usable depth of discharge, and the bank size falls out. Charge input must then be sized to refill it. Cable, fusing, bus bars and physical location follow. Reverse that order and you end up with holes in the roof serving an array that cannot keep up.
Start with an amp hour audit, not a battery catalog
Sit down with a spreadsheet before you look at a single product page. List every load that will run on the house system, and for each one record the current draw in amps at twelve volts and the hours per day it actually runs.
A workable starting list:
- LED lighting, small draw but long hours
- Water pump, high draw, very short duty cycle
- Refrigerator, either compressor or absorption on DC, and this is usually the largest single line
- Roof or under-bench fan
- Furnace blower and control board, seasonal but significant when running
- Laptop, phone and camera charging through the inverter
- Induction cooktop or kettle if fitted, brief but very large
- Inverter idle draw, which people forget and which runs all day
- Any diesel or gasoline heater
- Entertainment, starlink style terminal, or networking gear
Multiply amps by hours for each row. Sum the column. That total is your daily amp hour consumption. Be honest rather than optimistic; the audit is only useful if it reflects how you will really live in the unit.
Two notes on accuracy. Convert any load specified in watts by dividing watts by the nominal system voltage. And for anything running through an inverter, add roughly ten to fifteen percent for conversion loss.
Days of autonomy and the depth of discharge you will use
Autonomy is how many days the bank must carry you with no meaningful charge input. Two days is a common target for a unit that sees sun most days. Three to four is appropriate if you park under trees, work in a shaded yard, or expect long overcast stretches.
Depth of discharge is where chemistry changes the math sharply. A flooded or absorbed glass mat lead bank is conventionally worked to about half its nameplate capacity, so a two hundred amp hour lead bank gives you roughly one hundred usable. Lithium iron phosphate banks are routinely worked far deeper, commonly to eighty percent or more of nameplate, so the same nameplate figure delivers substantially more usable energy and does so at a flatter voltage.
The formula:
Bank size equals daily amp hours, multiplied by days of autonomy, divided by the usable fraction.
Run it before you shop. A build consuming one hundred amp hours per day with two days of autonomy needs about four hundred nameplate amp hours in lead, or about two hundred fifty in lithium. Those are very different physical installations, and they lead to different decisions about where the bank lives, how much floor space disappears, and what the charge input has to look like.
One caution on autonomy: it is tempting to buy your way out of the problem with a very large bank. Past a point the extra capacity never gets used, because the array cannot refill it and the deep reserve sits idle. Size for the trip you actually take, not the worst one you can imagine.
Sizing charge input to match the bank
A bank you cannot refill is not capacity, it is ballast. Charge input has to be sized against the same daily consumption figure.
Three sources, each with a real-world derate:
- Solar. Rated array watts times realistic sun hours, then reduced for panel temperature, controller efficiency, shading and panel orientation. Plan on considerably less than the nameplate figure suggests. Our solar installation service covers array layout and controller selection.
- Shore charging. The converter or charger output in amps, times the hours you expect to be plugged in.
- Alternator. Through a DC to DC charger with a stated current limit, times driving hours.
Add them and compare against daily consumption. If solar alone cannot replace a day's use in a day, you are relying on driving or shore power, and that is a legitimate design as long as it is a decision rather than a surprise.
Undersized charge input is the single most common fault we see on a build that was specified backwards. The bank is generous, the array is small, and the owner reports that the batteries never come off the middle of their range.
Where the bank goes, and what that dictates
Physical placement is not an afterthought, and it is the point where planning intersects with cutting.
Considerations:
- Ventilation. Flooded lead requires a vented enclosure. Sealed absorbed glass mat and lithium have different requirements, and the enclosure design follows from the chemistry you chose in the previous step.
- Temperature. Lithium has a charging temperature floor, so an exterior compartment in a cold climate needs either pack heating or a charging lockout.
- Cable run length. Every foot between the bank and the inverter adds voltage drop under load. Short runs are cheaper in copper and better in performance.
- Service access. Terminals, fuses, the shunt and the disconnect all need to be reachable without dismantling furniture.
- Mass placement. A large bank is heavy, and where it sits relative to the axles is a decision to make deliberately with the builder.
Each of these can force a change in where a panel, a vent, a cable pass-through or a compartment door goes. That is precisely why the bank gets sized before anything is cut.
Cable, fusing and the numbers that follow
Once bank size and inverter rating are fixed, the rest is arithmetic rather than judgment.
Inverter continuous rating in watts, divided by the lowest expected DC bus voltage, gives peak DC current. That current plus the run length gives cable gauge, sized for an acceptable voltage drop rather than merely for ampacity. Cable gauge gives lug size and terminal type. Peak current gives the main fuse rating and class, which must have an interrupt rating appropriate to the bank's short circuit capability, and lithium banks can deliver far more fault current than lead.
From there: a battery disconnect rated above peak current, a shunt sized for the same, bus bars rated accordingly, and branch fusing on every conductor leaving a bus. A battery and charging system review before installation catches gauge and fusing mismatches while they are still cheap to change.
What changes if you decide after the cut
Deferring the sizing decision costs money in specific, predictable ways.
A roof already penetrated for four panels cannot accept a fifth without another penetration and another sealed gland. A compartment already trimmed and lined to fit two group thirty-one cases will not take three. A cable pass-through sized for two-gauge cable has to be enlarged and resealed for four-aught. Interior cabinetry built before the inverter location was fixed frequently has to come apart.
None of this is catastrophic, but all of it is rework, billed at the mechanical and electrical rate of $260 per hour, on top of the original labor you already paid. It is far cheaper to spend an evening on the audit. If the inverter itself is already installed and behaving oddly under load, inverter and converter diagnosis is the right starting point before adding capacity. All of this work is performed in shop in Yorba Linda, roughly a twenty mile drive from Eastvale by SR-71 to SR-91.