Batteries for photovoltaic systems: are they really worth it? (simple calculation in 2 minutes)

“They cost twice as much... the normal ones are already not cheap, let alone these.”

It is a comment one often reads. And it is understandable: a battery is a major expense, and if you only look at the “shelf” price it always seems too much.

The point, however, is another: the convenience of a battery is not measured by its price, but from how much you can turn “low-paying” energy into “high-saving” energy”.

Let's see it in simple numbers (and a formula you can use on your own system).

1) The real comparison is not “battery yes / battery no”.”

The real comparison is:

• Energy fed into the grid: you are paid (approximately) ~0.08 €/kWh
• Self-consumed energysaves you from buying it on the bill (indicative) ~0.30 €/kWh

So every kWh you manage to “shift” from the grid to self-consumption is worth about:

0.30 - 0.08 = 0.22 €/kWh

? In practice: the battery pays off when it makes you self-consume many more kWh (especially in the evening/night), because those kWh are worth much more than to sell them.

Quick note: The values 0.08 and 0.30 are “order of magnitude” numbers and vary according to contract, zone, period and conditions. But the logic remains the same.

2) The easy formula to understand whether it pays off on your system

You need two numbers:

1. Installed battery cost (or quote) → let's call it Cost
2. kWh per year of additional self-consumption thanks to the battery → let's call them kWh_shifted

✅ Estimated annual savings

Savings_year = kWh_spent × (0.30 - 0.08)

Savings_year = kWh_spent × 0.22

✅ Break-even years

Return_years = Cost / Savings_year

Return_years = Cost / (kWh_spent × 0.22)

The end. That is really all.

3) Concrete examples: why the return is often 6-8 years

Case A (very common): retrofit with return ~7-8 years

• Installed battery cost (typical retrofit): 7.000 €
• kWh shifted/year4,000 kWh
• Value per kWh shifted: 0,22 €

Savings_year = 4,000 × 0.22 = 880 €/year

Return = 7,000 / 880 ≈ 7.95 years

? This is a “typical” case for many homes: an existing photovoltaic system, a lot of daytime energy fed into the grid and significant evening consumption. Under these conditions, the return is often around 8 years.

Case B (energy house / heat pump / EV): payback ~6-7 years

• Installed battery cost (typical retrofit): 7.000 €
• kWh shifted/year5,000 kWh

Savings_year = 5,000 × 0.22 = 1,100 €/year

Return = 7,000 / 1,100 ≈ 6.36 years

? The more you consume in the evening/night (heat pump, EV, hotplates, tumble dryer...), the more the battery works “well” and the return goes down towards 6 years.

4) What about the 50% tax deduction?

If you can access a deduction at 50%, the “effective cost” is halved (in a simplified way), and the re-entry time also tends to be halved.

Example of Case A:

• Cost: 7,000 €
• Actual“ cost after 50%: ~€3,500

Return = 3,500 / 880 ≈ 3.98 years

Example of Case B:

Return = 3,500 / 1,100 ≈ 3.18 years

? That is why, when there is a deduction, the battery often goes from “I think about it” to “it makes sense”.

5) How to estimate your kWh_split (without being an engineer)

Method 1 (the best): look at your inverter/app data

If you have monitoring, you often find:

• Self-consumption (%)
• Feed-in (kWh)
• Grid withdrawal (kWh)

In a simple way, the kWh_shifted are a part of:

• how much energy today enter in sunny hours
• and how much energy today pick up evening/night

The battery “shifts” energy from day to night. So the more daytime input and more evening collection, the more potential you have.

Method 2 (fast): realistic estimate for a domestic retrofit

On many homes with photovoltaics already installed, the battery is often able to move indicatively:

4,000 - 5,000 kWh/year (depends on season, actual surplus, battery size, habits and presence of evening loads such as heat pump or electric car).

? If you want to remain cautious, try 4,000 kWh and do the calculation again. If you have significant evening consumption, try 5,000 kWh.

6) When it is NOT convenient (and it must be clearly stated)

A battery does not apply to all installations. Here are the most common cases in which it may make little sense:

• Small plant and little surplus production
• Almost all daytime consumption (you are already very much in self-consumption without a battery)
• Low difference between how much they pay you for the energy you feed in and how much you pay for it in your bill
• Few evening kWh (the battery remains “stationary”)
• Lots of shade/irregular production (you have no energy to store)

In such cases, it is often better to optimise loads first (boiler, pumps, washing machine in the solar range, etc.) and then re-evaluate.

7) “They cost twice as much”: why looking only at the price is deceptive

Saying “it costs twice as much” only makes sense if performance and durability were identical. But in reality the correct question is:

How much does each kWh shifted over the system's lifetime cost me?

I.e. the total cost / useful energy actually stored and used (TCO).

This is where factors such as:

• security
• thermal stability
• degradation over time
• real utilisation (how many useful cycles you make)
• reliability and maintenance

8) What about Heiwit batteries in particular?

We start from the same principle: the battery has to make ends meet, not just “be there”.

Our retrofit solutions aim to make accumulation make sense especially in the real world (everyday use), with a strong focus on:

• safety and stability of sodium technology
• domestic use: evening/night self-consumption, peaks, intelligent load management
• transparent approachwe help you do the maths on your system, without “selling” you the battery regardless

9) Do the calculation on your plant (and if you want we will do it for you)

If you want to do it yourself, that's all you need:

Return_years = Cost / (kWh_spent × 0.22)

Where:

• Cost = installed battery cost
• kWh_shifted = annual kWh that the battery makes you self-consume extra
• 0,22 = (0.30 - 0.08) in our example

If, on the other hand, you want us to calculate it precisely, send us:

• plant power (kW)
• annual production (kWh) or screenshot inverter app
• annual consumption (kWh) (a utility bill will do)
• if you already have an estimate of self-consumption/input

Honest conclusion: not on every installation, but in most domestic retrofits with daytime surplus and evening consumption yes, and often the actual return is between 6 and 8 years (and with deductions it can go down a lot).