Solar · off-grid

Off-grid solar sizing

An off-grid system is sized by its worst month, not its average one — and the difference between those two is far larger than grid-tied intuition suggests.

What do kWp and kWh mean?

kWh — kilowatt-hour: A unit of energy, and the thing your electricity bill charges you for. A 1,000-watt heater running for one hour uses one kWh. A typical home uses somewhere between 2,000 and 10,000 kWh a year depending on where it is and how it heats.
kWp — kilowatt-peak: A unit of capacity — how big the solar array is, not how much it makes. It is the output the panels would produce under standard test conditions: bright, cold and perfectly aimed. Real roofs rarely see those conditions, which is why a 4 kWp array does not generate 4 kW for most of the day. One modern panel is roughly 0.4 kWp, so 4 kWp is about ten panels.
Putting them together: kWp is the size of the system; kWh is what it produces over time. The ratio between them — kWh generated per kWp installed, per year — is the honest way to compare locations, because it strips out how big the system happens to be. It runs from roughly 700 in cloudy high latitudes to over 1,800 in sunny deserts.

Location

Where is the site?

Searches use OpenStreetMap. Nothing is stored.

Or pick an example location

Or enter coordinates directly

Latitude

Longitude

Searching or using your location fills these in, so you can always see exactly which point the figures are for.

Climate comes from PVGIS, the European Commission's reference dataset — the same source this model is validated against, for examples and searches alike.

What you need to run

Add up what everything uses in a day. A fridge is around 1 kWh; a laptop about 0.3.

Energy used per day

Kilowatt-hours a day, averaged. Everything else on this page follows from it.

Days without sun

How long the bank alone must carry the load. Three days is a common design figure.

The battery bank

What kind of battery?

Sets how much of the bank you may actually use before damaging it.

System voltage

Higher voltage means less current for the same power, which matters for cabling.

Where does the bank live?

Cold batteries hold less than their rating. Routinely forgotten in sizing.

Round-trip efficiency %

Energy out as a share of energy in. Off-grid, almost everything goes through here.

The array

Which way does the roof face?

The direction you would face standing on the roof looking down the slope.

Charge controller

The largest efficiency decision in a small system, and the least discussed.

Panel tilt

Degrees from flat. Off-grid wants a steeper angle than a grid-tied roof — see below.

4.12 kWparray, sized on Dec

11.8 kWhbattery bank, nominal

245 Ahat 48 V

86 Athrough the charge controller

Sized on Dec, this site needs 4.12 kWp. Sized on the annual average it would need only 1.22 kWp — 3.4 times less, and dark all winter.

A kilowatt-peak here returns 4.05 kWh on an average day in the best month and 0.79 kWh in the worst — a swing of 5.1 to one. A grid-tied system never has to care, because the grid quietly covers December. Off grid, December is the entire design problem.

What the system comes to

Every figure follows from the daily load and the leanest month.
Usable energy needed	9.0 kWh	3 kWh a day for 3 days without sun
Nominal bank	11.8 kWh	after depth of discharge and the temperature the bank lives at
Bank in amp-hours	245 Ah	at 48 V — the number batteries are sold by
Array	4.12 kWp	to refill that from Dec sun, through the controller and the battery
Controller current	86 A	array power divided by system voltage, before any margin
Runtime from full	3.0 days	with no generation at all

What a kilowatt-peak returns, month by month

Average kilowatt-hours a day per kWp installed. The shortest bar is the one that sizes the system; every other bar is surplus you will spill once the bank is full.

Daily yield per kWp, by month
Month	kWh
Jan	1
Feb	2
Mar	3
Apr	4
May	4
Jun	4
Jul	4
Aug	4
Sep	3
Oct	2
Nov	1
Dec	1
Total	32

Off-grid wants a steeper tilt than a roof

At this site the tilt that maximises annual output is 30°, and the tilt that maximises the worst month is 55°. Moving from the first to the second raises the December floor by 7.2% and costs 5.6% of the annual total.

For a grid-tied system that trade is a straight loss: annual kilowatt-hours are what you are paid for, and the shape of the year is the grid's problem. Off grid it is the opposite. The annual total is largely spilled — once the bank is full on a June afternoon, the array has nowhere to put anything more — while every percent added to the worst month comes straight off the size of the array and the bank you have to buy. Steeper panels also shed snow, which the model does not credit them for.

The charge controller is not a detail

A PWM controller connects the array almost directly to the battery, pulling the modules down from the voltage where they make most power to whatever the battery happens to be sitting at. The difference is not stored anywhere; it is simply never collected. Switching this system from MPPT to PWM does not make it slightly worse — it makes the array it needs substantially bigger, because the shortfall has to be made up in panels.

Where this tool stops

This sizes energy and capacity. It does not design an electrical installation. It will not tell you the conductor size, the fuse or breaker rating, the earthing arrangement, or how many modules belong in a string. Those are safety-critical calculations governed by standards that differ by jurisdiction and revise on their own schedule, and getting one wrong can destroy equipment or start a fire. This section does not publish them, because publishing them responsibly requires review by an engineer credentialled where you are and independent of whoever wrote the tool. A disclaimer is not that review, so the tools are absent rather than hedged.

What the model does not represent

An average day in the worst month, not the worst day. A run of five overcast December days is worse than the December average, which is exactly what the autonomy figure is there to cover — so choose it deliberately rather than accepting three.
All load is treated as passing through the battery. True for most off-grid sites and conservative where it is not: energy used while the sun is on the panels avoids the round-trip loss.
No generator, and no seasonal load. Many real off-grid systems are sized against a generator for the worst weeks instead of against panels, which is a different and often much cheaper design. A holiday cabin used only in summer is a different problem again.
Charge acceptance is not modelled. A lead-acid bank cannot absorb charge arbitrarily fast, so a large array may not refill it as quickly as the energy arithmetic suggests.
No snow, no shading, no ageing. Batteries lose capacity with cycles, and a bank sized exactly at year one is undersized by year five.