Why an audit beats a bill
A grid bill tells you one number for a whole house, and for a grid-tied array that is enough, because the meter averages everything and the utility absorbs the peaks. Off-grid there is no averaging and no absorbing. You need three separate numbers and they come from three different places in the same list: the energy per day, which sizes the array and the battery; the continuous power, which sizes the inverter; and the instantaneous surge, which decides whether the inverter shuts down when the pump starts.
Those numbers do not scale together. A cabin can use 900 watt-hours a day and still need a 3,000 watt inverter because of one well pump. Another can use 6 kilowatt-hours a day on nothing but lights, a fridge and a laptop and be perfectly happy with 1,200 watts. Sizing either one from the other produces a system that is wrong in an expensive direction.
The two multipliers people leave out
The first is inverter efficiency, which is not the number on the datasheet. That figure is the peak, measured somewhere near half load. At 3 percent of rated load a big inverter can be under 70 percent efficient, which is exactly the condition a 3,000 watt inverter runs in at two in the morning with a router plugged into it.
The second is idle draw, called tare or no-load consumption. A 20 watt idle is 480 watt-hours a day if the inverter is never switched off. On a system whose real loads total 1,500 watt-hours, that is a quarter of the whole budget spent on the privilege of having AC available. This is the reason serious off-grid installations run lighting, refrigeration, water pumping and communications on DC and treat AC as an occasional convenience rather than the default.
| Load | Runs on | Why it lands there |
|---|---|---|
| Lighting | DC, easily | LED strips and fixtures are native DC. Running them through an inverter converts DC to AC to DC again. |
| Refrigeration | DC if the budget allows | Compressor fridges built for 12 or 24 V avoid both the conversion loss and the inverter idle, because nothing else needs the inverter on. |
| Water pump | DC on small systems | Short run time, large surge. A DC pump keeps the surge off the inverter entirely. |
| Router, modem, monitoring | DC with a buck converter | Small, constant, and the reason the inverter never gets to sleep. |
| Microwave, kettle, power tools | AC, unavoidably | Large, brief, and worth switching the inverter on for. |
| Washing machine | AC, with a real surge | Motor start dominates the inverter choice on many small systems. |
Surge, and why nameplate watts mislead
Anything with an induction motor draws several times its running current for a fraction of a second while the rotor comes up to speed. A fridge compressor at 150 running watts can ask for 900 at the moment it kicks in. The inverter does not need to sustain that, but it does need to survive it without shutting down, and it has to do it while everything else in the house is still running. That is why the calculator adds the largest single surge delta on top of the diversified continuous load rather than summing every surge figure: two motors starting in the same half-second is possible but rare, and designing for it doubles the inverter.
The exception is a system where several motors share a control signal, so they genuinely do start together. If that describes yours, enter the combined surge on one line and treat it as a single load.
If your loads are AC only and you are pricing a generator instead of a battery, the same list feeds the generator sizing calculator, which handles the altitude derate and the staggered-start question in more depth.
What the design day is for
The list you write is an average day imagined on a good afternoon. The design day is the one that actually breaks systems: two extra people, a load of washing, a cold snap that runs the fan heater, and the laptop charging twice. Twenty to thirty percent is the usual allowance, and it is not the same thing as days of autonomy, which is about how many consecutive bad days the bank can carry. The design day sets how big each day can get; autonomy sets how many of them in a row you can survive. Both matter and they multiply.
Once you have the design-day figure, it feeds directly into three other pages: the battery bank sizing calculator for amp-hours and series-parallel layout, the solar panel array sizing calculator for the array, and the inverter and charge controller sizing calculator for the electronics between them. If the whole thing is going in a vehicle, the RV and van solar calculator takes the same daily figure and works backwards from the roof you actually have.
Hazards this page does not cover
A PV array is energized whenever light falls on it. There is no switch on the panel that turns it off, and a module lying face up on the grass is a live source. DC arc faults do not self-extinguish the way an AC arc does, because DC current never passes through zero. A battery bank can deliver thousands of amps into a short circuit without any warning noise, and a dropped wrench across two terminals is a serious burn and fire hazard rather than a spark. Lithium cells that have been damaged, punctured, or charged below freezing can fail violently, and the failure is not always immediate. Nothing on this page is a wiring, commissioning or battery-assembly procedure, and none of it should be used as one.
Anything that ties a solar system to a building supply or to the grid is decided by your Authority Having Jurisdiction and your utility, not by a calculator. Interconnection, transfer switching, backfeed protection, disconnect placement, grounding, rapid shutdown and conductor sizing all fall to them, and in most places the work is licensed. Net metering terms, export credit rates and incentive programmes differ by state, by utility and by year, so treat nothing here as a statement of what yours will do.
Questions people ask
How do I find the watts of an appliance I already own?
In descending order of reliability: a plug-in energy meter left on it for a week, the yellow energy label divided out into average watts, the nameplate, and a figure from the internet. The gap between the first and the last is enormous for anything that cycles. A fridge nameplate might say 150 W, but the compressor runs perhaps a third of the time, so the honest entry is either 150 W for 8 hours or 50 W for 24 — both give the same watt-hours, and the second is easier to think about. For anything thermostatic, measuring for a full 24 hours is the only way to know, because duty cycle depends on your kitchen, not on the appliance.
Why does the calculator apply inverter efficiency to AC loads but not DC?
Because that is where the loss physically happens. Energy leaving a 12 volt battery bank and going to a 12 volt LED strip passes through wire and nothing else. The same energy going to a laptop charger passes through the inverter, which turns DC into AC at 88 to 94 percent efficiency, and then through the charger, which turns it back into DC. The double conversion is real and it is the reason the same laptop costs you noticeably more battery on an inverter than on a DC adapter. The DC loads here still lose a little to wiring resistance, which the calculator does not model separately because it is buried in the end-to-end efficiency figure used for the array.
Should I switch the inverter off overnight?
If nothing needs AC while you sleep, yes, and it is usually the largest single saving available on a small system. A 20 watt idle for eight overnight hours is 160 watt-hours, which on a 200 amp-hour 12 volt bank at 50 percent usable depth is over 13 percent of everything you have. The obstacle is never the inverter, it is the loads people forget are AC: the router, the alarm, the thermostat, the doorbell transformer. Moving those to DC first is what makes switching the inverter off practical rather than annoying.
My list says 2 kWh a day. How big does the battery need to be?
That question has at least three more inputs: how many days without sun the bank has to carry, what depth of discharge the chemistry tolerates, and what temperature the bank lives at. As arithmetic, 2 kWh a day for two days at 80 percent usable depth is 5 kWh of nameplate capacity, which at 48 volts is about 104 amp-hours and at 12 volts about 417. At 50 percent usable depth, which is the conventional limit for lead-acid, the same requirement is 8 kWh. Run the actual numbers with your chemistry on the battery bank sizing page rather than doubling in your head, because the depth-of-discharge term moves the answer more than anything else on the list.
Does the load list need to include phantom loads?
Yes, and they are the entries most likely to be missing. Anything with a remote, a clock, a standby light, a wall wart or a network connection draws something continuously, and continuous is what matters in a watt-hour budget. A 3 watt phantom is 72 watt-hours a day, which is more than a 90 watt television watched for 45 minutes. The practical way to catch them is to walk the place with a meter and check everything that stays plugged in, then enter the total as one line called standby at 24 hours.