Battery for solar panels: retrofit, price and earnings

A battery for solar panels stores the solar power you don’t use while the sun is shining, so you can use it in the evening instead of selling it cheaply and buying back expensively. In most cases it can be retrofitted to an existing installation, and the price depends on capacity, brand and whether your inverter can be reused. But the thing many people miss: a modern battery can earn money beyond your own solar power — it can simultaneously sell its response capability to Energinet as ancillary services, without hurting your self-consumption.

Can a battery be retrofitted to existing solar panels?

Usually, yes. There are two typical routes:

• Through your current inverter, if it is a hybrid model prepared for a battery. The battery then just needs to be compatible with the inverter’s brand.
• With its own inverter (AC-coupled), if your current inverter does not support a battery. This gives freer battery choice but requires a bit more installation work.

Ask your installer which route fits your installation — and check at the same time whether the solution supports remote control, because that determines whether the battery can later join a virtual power plant. NM1 integrates batteries through the inverter manufacturers, including Solplanet.

What does a battery for solar panels cost?

There is no single price — the cost depends on capacity (kWh), power (kW), brand and the scope of installation, and the market moves quickly. Get specific quotes rather than relying on averages. A more thorough walk-through of price factors, lifetime and warranty is in the article on home battery price and earnings.

What does the battery earn — beyond self-consumption?

Self-consumption is the classic calculation: midday solar power is stored for evening use. But the battery’s greatest technical strength is its response speed — it can switch between charging and discharging in seconds. Energinet buys exactly that capability as FCR-D reserves in daily auctions, because the grid must be kept at 50 Hz at all times. A single battery is too small to participate alone, but through an aggregator it joins a virtual power plant with many others — the background is in the article on grid balancing.

Auction revenue is shared between you and NM1 and paid out quarterly as an ordinary bank transfer from NM1 — see what your battery can earn under calculate your earnings. FCR-D prices vary from auction to auction — past earnings are no guarantee of future earnings.

Does it hurt my solar savings? How the battery is bid in

No — and it is worth understanding why, because this is where serious control systems differ. NM1 reserves only a small share of the battery’s capacity and bids according to the battery’s predicted state of charge:

• After a sunny day, when the battery is full in the late afternoon, we bid only in the direction the battery can actually deliver: sending power to the grid.
• Late in the day, when the battery is nearly empty, we bid the opposite way — it can absorb, not deliver.
• In between, around half charge, the battery can respond in both directions.

We only bid in when doing so is worth more than not doing so. You therefore never risk the battery having to buy power at an inconvenient time — your solar self-consumption continues essentially as before, and the ancillary-service revenue is added on top. You can follow it all in the web app, which shows your devices, activations over a chosen period and your payouts.

Checklist before buying a battery for your solar panels

1. Check your inverter first. A hybrid inverter with battery support makes retrofitting simplest — and determines which batteries fit.
2. Choose a solution that supports remote control. That unlocks the ancillary-services revenue — see which batteries and inverters we can connect.
3. Check your wifi. The battery needs a stable connection, typically 2.4 GHz.
4. Calculate conservatively. Don’t base the purchase on a single year of high market prices.
5. Read the warranty. Serious control stays within the manufacturer’s limits — always check the warranty terms.

If you already own a compatible battery, signing up with NM1 is free — start on the get started page, or see how earnings are calculated on the earnings page.