You have solar, the power cuts, and the house is still dark
Plenty of people who have already installed solar discover this at the first outage. Full sun outside, panels perfectly capable of producing, and no electricity indoors at all.
Nothing is broken. It is working as designed. A grid-connected solar system shuts itself down the moment the utility supply fails, because if it kept pushing power into the line a technician is climbing to repair, that technician is the one who pays for it. It is a safety requirement every grid-connected inverter has to meet.
To keep using what the panels produce during an outage you need two more things: an inverter that can run separated from the grid, and somewhere to store energy. That is where the question of whether a home battery is worth it starts.
The short answer depends on what you are buying it for
If the purpose is purely to cut the electricity bill, in Thailand the answer is usually not yet, because what a battery does is move daytime generation into the evening. That pays only if evening power is enough more expensive than daytime, or if the price you get for exporting surplus is low enough that keeping it is better.
If the purpose is that the house does not go dark, the answer changes immediately, because what you are buying is not a discount. It is continuity, and that is worth wildly different amounts to different households. A home with medical equipment, a home where someone works online all day, or a home in an area that loses supply regularly, values it very differently from one that has an outage once a year.
Before deciding, think back over the last twelve months. How many outages, how long each, and what did they actually cost you? If you cannot recall any, that is an answer in itself.
Battery size comes from the load you want to back up, not from the array
The most expensive misunderstanding here is assuming a 10 kW array needs a correspondingly large battery. They answer different questions. The array answers how much you generate. The battery answers how long you want particular things to keep running when there is no supply.
The workable method is to write down what genuinely has to stay on: one refrigerator, some lights, fans, the router, and sockets to charge phones. Together that draws far less than people assume. Air conditioning is the item that changes the whole answer, because one unit uses several times everything else on that list combined.
The question to settle before talking to an installer is how many hours you want that list to run. Four hours and twelve hours are very different systems at very different prices, and most people want four to six rather than a full night, because outages here mostly resolve within a few hours.
Three questions for the installer
Battery quotations are unusually hard to compare because everyone writes them up differently. These three make them comparable.
How much capacity is actually usable? The headline figure and the energy you can draw are not the same, because the system will not run itself completely flat if it wants the battery to last. Ask for usable energy, and ask what the continuous power output is, because a battery that holds a lot but delivers little cannot start an air conditioner compressor or a water pump.
What does the warranty cover and for how long? Battery warranties are written as a number of cycles, a number of years, or a remaining capacity at the end. At minimum, find out what percentage of original capacity the manufacturer guarantees at the end of the term. That number tells you far more about real service life than "ten year warranty" on its own.
During an outage, exactly what stays on? Ask for this by circuit, not in words. Most backup systems do not feed the whole house; they feed a separate sub-board. Agree at the outset which sockets and lights sit on it, because moving circuits later is demolition work.
Battery chemistry in a Thai house
Home batteries on the market are mostly two lithium families, and for a house here the one to choose is lithium iron phosphate, for reasons that match local conditions.
The first is thermal safety, which matters more than people think in a country where a store room routinely passes thirty-five degrees. The second is a longer cycle life, which suits charging and discharging every single day. The trade is more weight and bulk for the same capacity, which is a secondary concern for something bolted to a wall that never moves.
Location deserves a mention. Every battery chemistry ages faster when it is hot. A store room under the roof that hits forty degrees in the afternoon is the worst possible place, even when it looks like the only spare space in the house. What you want is an indoor wall with air movement, out of direct sun, and not in a main circulation route.
Working out payback without fooling yourself
The figure sellers use is the monthly saving divided into the system price, giving a number of years. The problem with that is it omits two things that make the real number longer.
First, batteries have a life. Unlike panels, which run twenty years and more, a battery degrades and gets replaced within the life of the system. An honest calculation therefore includes a replacement, rather than treating this as a one-time purchase.
Second, conversion losses. Charging in and discharging out always loses something. Ten units in gives less than ten units back. It is not alarming, but it makes every optimistic calculation slightly too optimistic.
A straighter approach is to split the question. First: does the solar system alone pay, and over how long? That is calculable and usually yes. Second: what does the battery add to the price, and are you willing to pay that so the house stays lit during an outage? Separating them lets you decide without bending the numbers to look good. The solar side of the payback calculation is in rooftop solar, the tax deduction and payback.
If you are not ready to buy a battery
What returns more than a battery in many households is simply moving consumption into the hours the panels are producing. Run the washing machine at midday, the pump and the dryer in the afternoon, and if there is an electric car, charge it during daylight at weekends. This costs nothing and works immediately.
The other thing to do first is to make sure the tariff matches how the house actually uses power, because a time-of-use tariff changes the entire equation, and if you are on one the case for a battery looks quite different. That is covered in smart meters and the time-of-use decision.
And if the real goal is only that the router and one light stay on, a small uninterruptible power supply costing a few thousand baht does exactly that without touching the house wiring at all. People overlook it because it is unglamorous, and it solves precisely the stated problem.
If you are about to install solar for the first time, run the cabling and leave the space for a battery now even if you are not buying one, because adding it later to a system that made no allowance usually means replacing the inverter entirely. Choosing a properly registered installer is covered in the solar installer registry.
Frequently asked questions
I have solar. Why does my house go dark in an outage?
Because a grid-connected system must disconnect when the utility supply fails, to avoid back-feeding a line someone is repairing. To keep using it you need an inverter that can run islanded from the grid plus storage.
How big should the battery be?
From the list of things that must stay on, multiplied by the hours you want, not from the size of the array. Most households want four to six hours for a fridge, lights, fans and the router. Air conditioning changes the whole answer.
Does it pay if I only count the electricity saving?
In Thailand, usually not yet, once you include a battery replacement and conversion losses. Split the question: does the solar pay, and separately, what is the battery premium buying you?
Which chemistry should I pick?
Lithium iron phosphate suits a Thai house better, being more thermally tolerant with longer cycle life. The trade is size and weight at the same capacity, which does not matter for something on a wall.
Where should it be installed?
An indoor wall with airflow, out of direct sun, away from main circulation. A roof space that hits forty degrees in the afternoon is the worst option, because heat is what ages a battery fastest.
What can I do before buying one?
Shift heavy consumption into daylight hours, match the tariff to how the house uses power, and if the goal is only keeping the router and one light on, a small UPS covers it completely.
In short
- A grid-connected solar system shuts down during an outage by design. That is a safety rule, not a fault.
- Bought purely for bill savings, a battery mostly does not pay here yet. Bought so the house stays lit, it is a different question.
- Size it from the backed-up load times the hours you want, not from the size of the array.
- Ask for usable capacity, continuous output, and which circuits stay live during an outage.
- Install it somewhere shaded with airflow, because heat is what shortens battery life fastest.
- Any payback calculation has to include a replacement battery and conversion losses.