Spain · solar credit products
Virtual batteries: useful solar credit or expensive add-on?
A virtual battery is normally a supplier-managed financial balance, not equipment storing electricity at your home. Its value depends on the tariff, fees and rules for using the credit.
Simplified surplus compensation applies value within the billing-period economic balance. A supplier may separately offer to record value that could not be used on that bill and apply it later under its own terms. That recorded value is not an electrical kWh waiting in the grid.
1. Separate physical storage from financial credit
A physical battery charges with electricity, has usable capacity in kWh, power limits in kW, efficiency losses and a warranty. A virtual balance is an accounting entry. It may be denominated in euros, linked to surplus value and usable only against defined future charges.
Because the grid is not storing the household’s original electricity for later return, compare the product as a financial service: what value is credited, which charges can it pay, how long does it last and what happens when the contract ends?
2. Establish the order of the calculation
Ask the supplier to show a sample bill. Determine how exported kWh are valued, how the simplified compensation limit is applied, when excess value enters the virtual balance and when that balance is used. Confirm whether taxes, power charges, services or another supply address can be offset.
The order matters. A product may advertise that it reduces a bill to zero but exclude taxes or services. Another may permit credit across multiple contracts but charge a monthly fee for each.
Illustrative accounting only
Credit value is not stored energy
- Export valued this month
- €36
- Usable on current bill
- €24
- Potential virtual balance
- €12
- Future usable amount
- Depends on contract
This example shows the questions to ask. It is not a description of any supplier’s current product.
3. Test every fee
Add the monthly virtual-battery charge, higher import rate, required maintenance, membership fee or lost discount. A product that recovers €60 of otherwise unusable annual credit but costs €72 a year is not creating value.
Compare the complete supply contract against an alternative with a lower import cost and no virtual product. Do not compare only the amount carried forward.
4. Check expiry and cancellation
- Is the balance kept in euros or another unit?
- Does it expire monthly, annually or never while the contract remains active?
- Is there a maximum balance or earning period?
- Can it offset energy, power, taxes and services?
- Can it be shared with another supply contract?
- Is the balance lost when switching supplier or tariff?
- Can the supplier change the terms or compensation formula?
5. Model seasonality
Virtual credit is most likely to matter when sunny months produce more export value than the current bill can use, followed by months with greater grid imports. Use a full year of measured or conservative monthly data. A household with high year-round demand may already use most compensation on each bill and gain little from carry-forward.
If export is small, prioritise import price and fixed costs. If export is large, investigate whether changing consumption timing or adding physical storage produces more value than the financial service.
6. Compare three options
| Option | Include |
|---|---|
| Standard compensation | Import cost minus usable monthly credit |
| Virtual balance | Same costs, carried credit and all fees |
| Physical battery | Reduced imports, losses and ownership cost |
These options are not mutually exclusive, but modelling them separately shows which element creates value. A physical battery may provide backup or other benefits not captured by bill savings; state those benefits separately.
7. Keep monthly evidence
Download bills and balance statements. Reconcile exported kWh, compensation rate, amount used, amount carried and opening and closing balances. If the supplier changes the tariff, check whether the balance rules also change.
Official framework sources
- BOE — Royal Decree 244/2019 and simplified surplus compensation
- IDAE — practical self-consumption guide
- CNMC energy-offer comparator
Reviewed 28 August 2026. Virtual-balance rules are commercial; verify the supplier’s current contract and sample bill.