Solar calculator, online and free
Tell us what you spend on electricity each month and in 2 minutes you'll have a fully sized solar system: panel kW, battery, inverter, annual output, savings and payback years. No sign-up, no sales calls, no cost.
- 100% free — No cost, no usage limit
- No sign-up — We don't ask for an email to calculate
- 2 minutes — 3 inputs and you have a result
- Real products — Live catalogue prices
What it actually calculates
This isn't a generic "X kW per Y m²" guess. The calculator sizes a specific installation and prices it with material that's in stock.
- System size and panel count — The kWp you need and how many modules that is, using the actual panel wattage we install.
- Battery and hours of autonomy — Capacity in kWh sized for your evening peak — or no battery at all if your profile wouldn't pay it back.
- Inverter and accessories — Single- or three-phase inverter, smart meter, WiFi dongle and optimisers if you flag partial shading.
- Annual and monthly output — The kWh the system will generate given your region's irradiation and your roof orientation.
- Annual and monthly savings — Euros you stop paying, split between direct self-consumption and surplus exported to the grid.
- Payback period — Years and months to break even, calculated on the final VAT-inclusive price of the whole system.
- Itemised quote — Line-by-line bill of materials at catalogue prices, with the option to add professional installation.
- Plain-language explanation — Why this configuration and not another, written by AI on the concrete numbers of your case.
How it works · 4 steps
The only input you really need is your consumption. If you don't know it, the amount on your last electricity bill works fine.
- Your consumption — Enter monthly kWh or, if you don't know them, the bill amount in euros. The calculator converts euros to kWh after stripping out the fixed charges.
- Your roof — Region of Spain, roof type, orientation and whether there is partial shading. That's what tunes the expected output.
- Your priority — Pick partial savings, optimal savings or full independence. It radically changes battery size and total investment.
- Your result — A sized system, a quote with real products, and a side-by-side comparison of all three scenarios.
Three scenarios, three budgets
The same consumption can be solved in three very different ways. The calculator shows all three at once so you can compare.
Partial savings (Lowest investment)
Panels only, no battery. Covers daytime consumption — washing machine, pool pump, air conditioning, working from home. Smallest outlay and very often the fastest payback.
Ideal if you're home during the day or have high daytime loads.
Optimal savings (Best payback)
Panels plus a battery sized for the evening peak. The aim is the point where every stored kWh pays for itself, without oversizing the pack.
The balance most residential customers pick.
Full independence (Maximum autonomy)
Output above 100% of consumption and a large battery. More investment and a longer payback, in exchange for depending on the grid as little as possible.
For those hedging against tariff hikes and outages.
The data behind it
No invented numbers: output comes from average PVGIS yields for grid-tied installations at a performance ratio of 0.80, and savings are computed on the VAT-inclusive tariff you actually pay.
Annual output by region
| Region | kWh per year per kW installed |
|---|---|
| North — Galicia, Asturias, Basque Country, Cantabria | 1100 |
| Northwest — Castilla y León, La Rioja | 1300 |
| Central — Madrid, Castilla-La Mancha, Aragón | 1500 |
| East — Catalonia, Valencia | 1550 |
| Balearic Islands | 1600 |
| Southeast — Murcia, Almería, Alicante | 1650 |
| South — Andalusia, Extremadura | 1700 |
| Canary Islands | 1850 |
Roof orientation correction
| Orientation | Factor vs. optimum |
|---|---|
| South | 1,00 |
| Southeast / Southwest | 0,96 |
| Flat roof (with tilt frames) | 0,92 |
| East / West | 0,88 |
| North | 0,65 |
An east-facing roof produces about 12% less than a south-facing one — yet it's often just as profitable, because it shifts generation into the hours you actually consume.
The formulas it uses
We publish them because a calculator that hides its maths doesn't deserve your trust. These are exactly what runs on our server.
Consumption estimated from the bill
kWh/month ≈ (bill amount × 0.85) ÷ (tariff × 1.21)
The 0.85 is the variable share of the bill; the rest is capacity charges and tolls. The 1.21 is Spanish VAT.
Annual system output
kWh/year = installed kWp × regional yield × orientation factor
The 0.80 performance ratio (temperature, cabling, inverter and soiling losses) is already baked into the regional yield.
Annual savings
€/year = self-consumed kWh × VAT-inclusive tariff + exported kWh × €0.06
Surplus fed back to the grid is compensated far worse than energy used on the spot. That's why oversizing without a battery yields little.
Payback period
years = total VAT-inclusive investment ÷ annual savings
On the final price of the complete system, installation included if you tick it. No "payback excluding VAT" tricks.
Indicative examples
Three typical cases at a reference tariff of €0.22/kWh plus VAT. These are orders of magnitude — the calculator does the fine maths with your data and today's prices.
| Case | Consumption | System | Output | Estimated savings |
|---|---|---|---|---|
| Flat or small house · Madrid, south roof · no battery | 250 kWh/mo ≈ €82/mo | ≈ 3 kWp · 6-7 panels | ≈ 4,500 kWh/yr | ≈ €45-60/mo |
| Family home · Valencia, southeast roof · with battery | 450 kWh/mo ≈ €145/mo | ≈ 5.5 kWp + 10 kWh | ≈ 8,200 kWh/yr | ≈ €95-120/mo |
| Villa with heat pump and EV · Andalusia, south | 900 kWh/mo ≈ €285/mo | ≈ 10 kWp + 15 kWh | ≈ 17,000 kWh/yr | ≈ €190-240/mo |
Real savings depend on how much energy you consume at the very moment it's produced. Getting that split right is exactly what the calculator optimises when sizing the battery.
Why is it free?
Because we sell the hardware, not the calculation. We're Bilotex Smart Technologies, a solar distributor and installer in Spain, and it suits us that you arrive at the conversation already knowing what you need and what it costs.
Use it as many times as you like, without registering and without leaving a phone number. We only ask for contact details if you choose to save the calculation, request a formal quote or ask for professional installation.
And if you'd rather buy the material and install it yourself, that works too: the quote it generates is a list of products from our shop that you can send straight to the cart.
Frequently asked questions
Is the solar calculator really free?
Yes, completely. There's no paid tier, no cap on calculations and no locked features. It's an open online tool by Bilotex Smart Technologies.
Do I need to sign up to size my solar system?
No. You go in, enter your consumption and see the full result including the quote. Registration is only needed if you want to save the calculation for later or share it.
What data do I need at hand?
One figure is enough: your monthly kWh or the amount of your last electricity bill. Everything else — region, orientation, roof type — is picked from dropdowns and has sensible defaults.
I don't know my kWh, can I use the bill amount?
Yes, that's the most common route. The calculator strips out the fixed part of the bill (capacity charges and tolls, around 15%) and converts the rest to kWh at the current tariff.
Is it accurate or just a rough estimate?
The sizing is serious: PVGIS regional yields, orientation correction, a 0.80 performance ratio and real catalogue prices with VAT. Even so, a final installation needs a look at the roof — specific shading, roof condition and usable space can only be validated on site.
Does the price include installation?
You can tick the professional installation box and the quote includes it. Leave it unticked and you get material only, in case you prefer to install it yourself or with your own fitter.
Does it size the battery too?
Yes. Depending on the profile you choose it proposes a capacity in kWh, or tells you explicitly that a battery wouldn't pay off in your case. It also computes hours of autonomy, and you can force a specific target.
Does it work for three-phase installations or only houses?
Both: you can specify single- or three-phase supply and the system picks a suitable inverter. For industrial buildings and very large loads we'd also recommend contacting us to review the case.
What if my roof has shading?
Tick the partial-shading option and the calculator selects an optimiser-capable inverter and adds the optimisers to the quote, so one shaded panel doesn't drag down the whole string.
Can I buy what it recommends directly?
Yes. The quote is a list of shop products with current stock and price, and you can move it to the cart or request a formal PDF quote.
Related guides
- Partial shading: which optimiser to choose
- Solar inverter guide
- Inverter brand comparison
- Solar battery guide
Size your solar system now
Free, no sign-up, instant result. Two minutes and you'll know what you need and what you save.