Solar battery calculator
Bank size is daily consumption times days of autonomy, divided by the usable depth of discharge. Five kilowatt-hours a day with two days of autonomy at 80% usable needs a 12.5 kWh bank, or 260 Ah at 48 V.
How to size a battery bank
Days of autonomy is the expensive decision. Each extra day is a linear increase in battery cost, and batteries are usually the largest single line in an off-grid system. Going from two days to five multiplies the battery by two and a half, and in a northern winter even five days may not bridge a stubborn anticyclone. Most off-grid systems settle on two or three days of storage plus a generator, because the generator covers the rare deep gap far more cheaply than the batteries that would otherwise be needed.
The array figure needs reading carefully, because it answers a narrower question than the label suggests. It is the array that generates one day’s consumption in one day’s sun, allowing for the losses you entered, and no more: 1,905 W on the defaults, which is 5 kWh over 3.5 peak sun hours at 75 per cent. That keeps a full bank full on an average day. It does not recover a bank that has already been drained, because an array in exact balance with the load has nothing spare to put back. After the two days of autonomy have been used, 10 kWh is missing; replacing it over the following five days needs 2 kWh a day of surplus, which is another 762 W of array. Entering 7 kWh a day instead of 5 shows the whole answer at once: 2,667 W and seven panels rather than five.
The panel count assumes 400 W panels, which is a reasonable residential module today and is not what you will necessarily buy. Divide the watts figure by your own module rating and round up. The watts figure is the one to carry into a quote; the panel count is a sanity check on roof area.
Peak sun hours are where an off-grid design is usually lost. The number is a daily average, and averaging over a year hides exactly the season that decides whether the system works. In northern Europe the December figure is a small fraction of the June one, often under a quarter, and PVGIS will give you the monthly numbers for your own coordinates. Try it here: drop 3.5 to 1.0 and the array jumps from 1,905 W to 6,667 W, from five panels to seventeen. Most off-grid systems are not built to that number. They are built nearer the annual average and paired with a generator for the weeks when it is wrong, and deciding that deliberately is the difference between a design and a surprise.
What people use it for
- Sizing storage for an off-grid cabin
- Adding a home battery to an existing array
- Planning a van or boat electrical system
- Comparing what a lithium and a lead-acid bank each have to be worth
- Seeing what a winter sun-hour figure does to the array size
Questions
Daily use times days of autonomy, divided by usable depth of discharge. Five kWh a day for two days at 80% needs 12.5 kWh.
One for grid backup, two to three off-grid with a generator, five or more only if there is no generator and no tolerance for going dark.
Current. A 5 kW load is 104 A at 48 V but 417 A at 12 V, which needs enormous cable and loses far more to resistance. The amp-hour figure on the panel shows the same thing: 260 Ah at 48 V becomes 1,042 Ah at 12 V for the identical bank.
No. It sizes the array to generate one day’s consumption in one day’s sun, so it holds a full bank full. An array in exact balance has no surplus, and a drained bank stays drained.
Enough surplus to replace the missing energy in the time you are prepared to wait. Putting the defaults’ 10 kWh back over five days is 2 kWh a day, or another 762 W. Entering 7 kWh a day instead of 5 gives the combined answer: 2,667 W.
400 W a module. Divide the watts figure by your own module rating instead if you have already chosen one.
Around 80 to 90 per cent for lithium iron phosphate, and 50 per cent for lead-acid if you want a reasonable cycle life out of it. The 80 per cent default is a lithium figure.
You buy a bank 37.5 per cent smaller than the one you need, and shorten its life on top. The same 5 kWh a day over two days wants 20 kWh and 417 Ah at a 50 per cent depth of discharge, against the 12.5 kWh and 260 Ah the 80 per cent default gives — which is to say sizing it honestly costs 60 per cent more battery than the lithium figure suggests.
The figure for your location and, for an off-grid system, for your worst month rather than your average one. PVGIS publishes monthly irradiation for any coordinates in most of the world.
Only for a grid-tied system, where the grid covers the shortfall. Off-grid, an annual average describes a system that works from March to September and runs out in December.
Yes, markedly. Lead-acid delivers noticeably less below about 10 °C and substantially less near freezing, and neither chemistry is at its nameplate in an unheated shed in January. Size for the temperature the bank will actually sit at. The formal temperature correction for lead-acid in stand-alone PV is set out in IEEE 1013, which is a paid IEEE publication rather than a free one; your battery manufacturer’s own capacity-against-temperature curve says the same thing at no cost.
Not below freezing without a heater. Lithium iron phosphate will discharge happily well below zero but plates lithium metal if charged there, which permanently damages the cell. Most quality packs refuse the charge themselves; cheap ones do not.
The array side: soiling, panel temperature, wiring, the charge controller and the inverter. Twenty-five per cent is a conservative off-grid figure and a bit above what a grid-tied model would use.
No. The bank is sized on the energy you take out, and storing that energy costs more than it returns: roughly 5 per cent for lithium iron phosphate and 15 to 20 for lead-acid. Add that to the array, not to the bank.
Size for the battery in year seven. Every chemistry loses capacity with cycling, and a bank that exactly met the requirement when new will not meet it later. Twenty per cent on top is a common allowance.
Total. It is the nameplate capacity you have to buy, already grossed up by the depth of discharge you entered, so do not divide it again.
Only at the rate it was rated at, usually a twenty-hour discharge. Pull it harder and you get measurably less, an effect lithium iron phosphate barely shows.
No. An inverter is sized by the largest simultaneous load in watts, not by daily energy. A cabin using 5 kWh a day might still need a 3 kW inverter for one kettle.
Off-grid at any northern latitude, almost certainly. It covers the rare deep gap far more cheaply than the two or three extra days of battery that would otherwise be required.
PVGIS, published by the European Commission’s Joint Research Centre, gives monthly and hourly irradiation for a set of coordinates and covers most of the world rather than only Europe.
It is enough to know roughly what you are buying and to check a quote for sense. Cable sizing, fusing, controller selection and the local wiring rules are all outside it, and off-grid DC currents are large enough that those are not details.