What size inverter battery does my house need?
You have probably been quoted a battery size already. Fast, confident, and based on nothing
but the shape of your house.
That guess costs you either way. Too big and you pay for capacity you will never touch. Too
small and the price sounds sweet, you say yes, and three weeks later the lights go off at 2am
and you are reaching for the generator you were promised you had left behind.
You do not need an engineer to find your real number. You already own the tool that has been
measuring your home this whole time. Your prepaid meter.
Why nobody can quote you a battery size honestly yet
Think about how odd the normal way is. If you walked into a hospital and the doctor prescribed
drugs before checking anything, you would walk back out. Yet with power, which is a real spend
of real money, we accept a size handed over by a stranger who has not measured a thing.
The reason is simple. Measuring takes a day. Guessing takes a minute. Most sellers guess
because guessing closes the sale faster, and because the customer usually cannot tell the
difference until months later.
A battery is sized around one thing above all: how much energy your home uses in a day. Not
how many rooms you have. Not how big the house looks. Two identical houses on the same street
can differ by several units a day, because one runs two ACs and a big freezer and the other
runs fans and a small fridge. Same house, very different battery.
So when someone sizes you off the shape of your house instead of your usage, they are
decorating, not engineering.
Your prepaid meter already did the maths
Your prepaid meter shows a running count of units used, in kWh. One unit is one kWh. That is
the number we want.
You read it once, wait a full day, and read it again. The difference is what your home used in
that day. No app, no clamp meter, no electrician.
Pick an ordinary day, not the weekend you have visitors and not the week the AC was broken.
Read it at a fixed time, say nine at night, and read it again at nine the following night.
Subtract the first from the second. Do it over three normal days if you can, and average them.
That figure is your daily usage, and it is the honest starting point every proper quote should
be built on.
How to turn your meter reading into a battery size
Now the part everyone gets wrong. Your daily usage is not your battery size.
A battery is never drained fully to zero, and a little energy is always lost turning stored
power into the power your sockets use. So the battery has to be bigger than a day’s usage to
actually deliver a day’s usage. How much bigger depends on the battery type, and this is where
lithium and the old tubular batteries part ways hard.
Old tubular and gel batteries do not like being emptied. Drain them deep, often, and they die
young. To be kind to them you only really use about half of what you paid for, so you have to
buy roughly double. The rule of thumb for tubular is daily units times 3.5.
Lithium is a different animal. You can safely use almost all of it, day after day, for years.
The rule of thumb for lithium is daily units times 2.2.
Take a home using five units a day. Tubular: five times 3.5 is 17.5 kWh. Lithium: five times
2.2 is 11 kWh. Same house, same day of power, and the lithium answer is smaller because you
are allowed to actually use it. That gap is why one lithium battery does the work of two,
sometimes three, of the old tubular units it replaces.
So take your own daily number and multiply it by 2.2. That is your rough lithium battery size
in kWh, and you now know more about sizing your home than the person who quoted you in two
minutes.
What your number cannot tell you
Here is where we have to be straight with you, because certainty in the wrong place is
dangerous. Your daily number sizes the battery. It does not size the whole system. Three
things sit outside it.
The first is surge. Some appliances draw a big gulp of power at the instant they switch on,
far more than they use while running. An AC compressor and a big deep freezer are the usual
culprits. Your daily units say nothing about that gulp. Get the inverter too small for it and
the AC trips it every time it kicks in, even with a full battery. That is why homes with a
“big enough” battery still fail. The battery was fine. The inverter could not take the punch.
The second is sunlight. If you want panels charging that battery every day, how much sun your
roof actually catches matters. In Port Harcourt that is its own conversation: softer sun than
the far north, a long grey rainy season, and soot that films over panels and quietly steals
output unless they are cleaned. A meter reading cannot see your roof, your shade or your sky.
The third is you, tomorrow. People buy solar, finally stop worrying about fuel, and start
using more. The new freezer. The extra AC for the baby’s room. A good design leaves room for
the life you are about to live, not just the one you measured.
One more thing worth knowing. A prepaid meter only counts while NEPA is on, so on a poor
supply street the reading undercounts. Treat it as a floor. A plug-in power meter fixes that,
because it measures whatever flows through the socket whether the power came from NEPA or your
generator, and it will tell you exactly what your freezer or your AC really draws.
None of this is meant to scare you off your number. Your number is real and it is the right
place to start. It shows you where self sizing ends and proper design begins.
Get the free guide and the worksheet
It costs you nothing but one day of paying attention. Tell us where to send it and the guide
and the printable worksheet come straight to you.
- The 24 hour meter method, step by step, in plain language
- The simple rule that turns your reading into a lithium battery size
- Why lithium replaces two or three of the old tubular batteries
- The three things your number cannot tell you (so you are never caught out)
- A printable worksheet to record your reading
When you have your number, message us on WhatsApp and tell us what you want to run. We will
design around your actual home rather than a guess. That conversation is free, and you get a
straight answer whether you buy from us or not.