An off-grid system is sized in three separate calculations. The array comes from daily energy divided by peak sun hours and system losses: ten kilowatt-hours a day at 3.5 peak sun hours with 25 per cent losses needs a 3.81 kWp array, or nine 430 W panels. The battery comes from consumption times days of autonomy divided by usable depth of discharge. The inverter comes from peak load, not from energy at all.
Confusing the three is the usual way a system ends up unbalanced — a large array charging a battery too small to hold a day of it, or an inverter that trips whenever the fridge starts.
What are peak sun hours?
The equivalent number of hours at full rated irradiance, which is a way of collapsing a whole day of varying sunlight into one number. Four peak sun hours means the array produces as much as it would in four hours at its rating.
The figure varies far more by season than by location, which is what makes annual and winter sizing two different problems. A UK array sized on an annual average of 2.8 peak sun hours produces roughly five times as much in June as in December.
For a grid-tied house that seasonal swing does not matter much, because the grid fills the gap. For an off-grid system it is the whole design problem, and sizing on the annual average guarantees a shortfall for four months of the year.
What losses should you assume?
Around 20 to 25 per cent between the panel rating and the energy that reaches the battery, made up of several small things that multiply rather than add.
- Temperature. Panel output falls as cells heat, typically several per cent on a hot day.
- Soiling and shading. Dust, leaves and a single shaded cell all cost more than their area suggests.
- Wiring and controller. A few per cent, more on a long DC run.
- Orientation and tilt. Anything away from optimum costs output, and the penalty grows quickly past about 30 degrees off south.
- Battery round-trip. Energy stored and retrieved is not energy delivered.
How many days of autonomy?
This is the expensive decision, because each extra day is a linear increase in battery cost and the battery is usually the largest single line in the system.
Five kilowatt-hours a day with two days of autonomy at 80 per cent usable needs a 12.5 kWh bank, which is 260 Ah at 48 V. Going from two days to five nearly triples that spend — and in a northern winter even five days may not bridge a stubborn anticyclone.
The usual resolution is a generator rather than a larger battery. A small generator covering the worst few days a year is far cheaper than the battery that would replace it, and it changes the design from "survive anything" to "survive most things".
Why 48 V rather than 12?
Because current is inversely proportional to voltage and losses go with the square of current. The same 5 kW inverter draws 417 A at 12 V and 104 A at 48 V, and the cable, fuses and busbars for the first are in a different league of expense from the second.
Twelve volts is fine for a van with a few hundred watts of load. Anything with a serious inverter, an electric kettle or a heat pump belongs at 48 V, and the crossover is lower than people expect — usually around 1 to 1.5 kW of continuous load.
How is the inverter sized?
Twice, and the second figure is the one that catches people. The continuous rating is what it can sustain thermally; the surge rating is what it can supply for a second or two before folding back.
A 1,200 W continuous load wants about a 1,440 W continuous rating with headroom, and separately a 3,600 W surge — because motors need three to six times their running power to start. That 1,200 W load also pulls about 111 A from a 12 V bank once inverter efficiency is included.
This is why an inverter that comfortably runs a fridge can trip when the fridge compressor starts. The running load was never the problem; the start-up transient was, and it lasts long enough to matter and not long enough to show up on a meter.
Questions people ask
Should I size for winter or the year? For an off-grid system, winter, with a generator or a deliberate reduction in winter consumption as the alternative. For grid-tied, the annual figure is the right basis.
Should the array match the battery? They are sized from different inputs and the ratio matters: an array that cannot refill the bank on an average day leaves it in a permanent partial state of charge, which shortens lead-acid life sharply.
Do inverters use power when idle? Yes, typically 10 to 30 W. Over a day that is 240 to 720 Wh, which on a small system is a significant fraction of the whole budget.
Does cold weather reduce capacity? Yes, in every chemistry, and lithium additionally should not be charged below freezing. A battery in an unheated outbuilding is not the battery in the specification sheet.
Three components, three calculations, and a generator instead of the last few days of battery. The solar panel size calculator works the array from consumption and sun hours, the solar battery calculator turns autonomy into a bank size, and the inverter size calculator handles continuous and surge separately.