CONSIDERATIONS WHEN BUYING AND INSTALLING AN INVERTER
Inverters are handy when you are offgrid and need to use appliances that run on normal grid power (120v AC in the USA, 230v-240v AC in most of the world).
After all, just because you’re offgrid it doesn’t mean you should be without good coffee!
Basically, an inverter converts 12v/24v/36v/48v DC to 120v or 240v AC. For most of this article we discuss 12v because they are the most common, but please see the chapter at the end on Input Voltage.
Two types of Inverters (plus a few more)
Broadly speaking there are two types of inverters (with some in-betweens called “Modified Sine Wave”). To all intents and purposes an inverter is either:
Pure Sine Wave
Not Pure Sine Wave (Square wave, modified sine wave, etc)
Of the two, the former (Pure Sine Wave) is the closest to national grid power. Actually, sometimes better than grid power. Square wave are cheaper to buy (about half the price) but have a few disadvantages:
Less efficient, using up your battery much faster
Some equipment heats up more, and damage may even occur. Some appliances don’t work at all under square wave or modified sine wave
More noise, particularly a humming or buzzing sound
The in-betweens (modified sine wave) vary in effectiveness depending on how close they come to a pure sine wave, but even reasonably good designs can cause buzzing and heat buildup.
Therefore, for the remainder of this article, we’ll only be talking about the Sine Wave (or “Pure Sine Wave” as is often mentioned). Our STRONG recommendation is that you do NOT consider purchasing anything but a Pure Sine Wave Inverter! Perhaps we’re being unfair on the good modified sine wave, but unless you have a really tight budget we’d recommend you purchase a Pure Sine Wave inverter.
How Big
So now that you’ve decided on a Pure Sine Wave Inverter, the next question is: how big?
To answer this question, a little bit of waffle:
The size of the inverter is determined by both the input and the output. If you need 1000w it’s no use buying a 600w inverter (basic common sense). You need a 1000w or even better a 1200w, essentially sizing about 20% above your maximum requirements. Bear in mind the startup surge for some appliances.
However …. It’s no use buying a 3000w inverter if you only have one single 12v 100ah lead-acid battery. The inverter size therefore also depends on what current you can feed it and for how long! If you need 30 minutes to brew your coffee drawing 120A off your effective 50ah battery (you only get 50ah of usable power from a 100ah lead-acid battery), won’t be enough.
So we’re going to assume that you have a big enough battery bank, or that you have a project to increase battery storage to meet demand. That is covered in detail in another article.
Typical Power Requirements
The following are some typical power requirements for appliances that are used for a short duration:
Nespresso M100 coffee machine 1200w
Russell Hobbs 20680 Buckingham Filter Coffee Machine 1000w
Most coffee machines range from 600w to 1200w
Milk frothers are usually around 500w
Kettles range from around 1000w to 2000w
Hairdryers range from 800w to 1800w
Hair straighteners range from 80w to 120w
Medical and healthcare appliances - huge variation in terms of power requirements
Appliances used for a longer duration:
Laptop chargers, dependent on SOC of the laptop battery but usually maximum 100w
Mobile phone chargers can be run from an inverter but it’s far more efficient to run directly from 12v USB adaptors (cigarette lighters)
Some medical or healthcare appliances
Why not buy an inverter with a huge capacity?
Most, if not all, inverters are less efficient when drawing small amounts of power compared with capacity. Installing a 3000w inverter when you only need 300w would result in an inefficient setup and in turn you’d use up your batteries faster. You'd be better off installing a 600w inverter. Drawing only 50w off a 3000w inverter would be even worse! It’s better to size an inverter according to your actual needs (and cheaper too).
Efficiency
What about efficiency?
Inverters vary from around 60% (quite horrific really) up to 95% (great but unusually high). Most good pure Sine Wave inverters are around 80% to 90% efficient.
As a general rule, in our calculations, we assume an 80% efficiency.
How does efficiency affect my decision?
Inverters are rated by their output. Most ads state two figures:
Continuous power
Peak power or Surge power
Continuous power is what you really want to pay attention to. Peak or Surge power generally applies to the few seconds when you turn the appliance on.
Efficiency affects how much the inverter will draw from the battery, but not how much power it will provide.
We’re going to be using the age-old equation of P=IV (power = amperage * voltage).
When you size your inverter you will need to take into account efficiency.
Let’s assume you require 1200w of power for your Nespresso machine. If your inverter was 100% efficient that would mean you require 1200w from your 12v, or I=P/V, 1200/12 = 100amps.
To cater for the loss of efficiency simply increase your power requirements by that factor (yes I know there are more precise calculations but this will do).
So therefore in order to drive your 1200w you actually draw around 1400w (or 1440w), and in amps based on 12v that would be 120amps.
As a rule of thumb: simply divide your wattage requirements by 10, so 1200w requires 120amps from your 12v battery using an inverter of 80% efficiency.
Note: this does not build any margin for error, it’s pretty much a case of knowing you will draw 120amps or more (but probably not less than 120amps).
How do I know if an advert is for a Pure Sine Wave Inverter?
As a general rule of thumb, if it doesn’t specifically say it’s a pure sine wave then it isn’t.
Sellers of pure sine wave almost always include that in the title of their product. If there is any doubt either ask the vendor or assume it’s not pure sine wave.
Here are some links to a good inverter:
BMS
Does my BMS limit my choice of inverter?
Yes, it certainly does.
A BMS is a battery management system for certain batteries, such as lithium iron (e.g. LifePO4). Most BMS’s have limitations regarding how much can be drawn off the batteries (maximum amperage) or how fast they may be charged.
If you have a limited BMS, allowing for example only 100A to be drawn off your battery, you cannot run a load of 120A continuously. There are a number of ways around this (including bypassing the BMS for your inverter altogether), replacing your BMS, or even adding more batteries thereby running through more than one BMS in parallel.
Below is a table of maximum Inverter loads and BMS output requirement, based on 12v and 80% efficiency.
Battery Size
Does battery size affect inverter decision?
In general yes
If you have a single 100ah lead-acid battery rated at C3 (that’s another discussion) you have an effective 50ah of usable power and can draw a maximum current of 300A.
Basically, 50ah means that you can draw 50amps from that battery for one hour.
Let’s say you went all-out and bought a 3000w inverter. If you attached something drawing all 3000w you would be drawing 300amps from your battery.
300 / 50 = 6
So you would be able to use the battery for 1 sixth of an hour, or ten minutes.
The more power you need, and the longer you need to run your inverter the more it makes sense to consider upgrading from lead-acid or AGM to Lithium (LifePO4). Our practical experience running off a 400ah lead-acid battery bank to power a microwave is that results are disappointing. Keeping everything else the same but changing to a single 280ah LifePO4 battery resulted in much better microwave operation. The assumption that usable capacity from a lead-acid battery is actually 30-40% of stated capacity instead of the 50% often quoted we find is actually true.
However …. if you actually need the power, as in your power requirements are non-negotiable, then you simply need to buy an inverter big enough and increase your battery bank. Simples.
Charging Rate
What about charging rate?
That’s another topic altogether, but it’s no use planning to draw more power than you are capable of replenishing on time for the next occasion you require it. This is especially true where you have limited charging capacity (solar in winter, small vehicle alternator, inefficient charging system, small B2B charger etc). You will have to do your calculations to determine how much you can put back into your batteries and base your inverter size on something that fits within those constraints. Again, if you really need the power then upgrade your charging capability.
Battery Type
What about battery type?
Lead-acid vs LifePO4 vs other: this doesn’t really matter.
In general, you only get 50% of a lead-acid battery capacity so do your calculations accordingly.
If you run LifePO4 batteries you’re probably going to set your system to using 80% or 90% of the capacity (if you want your battery to last a long time don’t charge over 95% and don’t discharge to less than 5%). Again, do your calculations accordingly.
Remote Switch
Do I need a remote control (Inverter on/off)?
Not absolutely 100% but you’d be crazy not to. The inverter will use battery power by just being on standby. Generally the more powerful the more it uses just to be on standby. You should really go for a model with a remote on/off.
Actual Voltages
Do I really base my calculation on 12v when my battery is 13v?
OK so you’ve measured your voltage and it’s actually 13v. Do you, therefore, base your calcs on 13v?
No. If anything you need to go more conservative. Many inverters have an auto shut-off at 11v, so factor this in. In addition to this, voltage drops when you are drawing huge amounts of power anyway. This is why we’re comfortable with building a safety margin in.
Remember that your inverter will provide a constant amount of power to your appliance and if your battery starts at 13v and drops slowly (or quickly!) to 11v the amps drawn from the battery will increase as the voltage drops.
OTHER INPUT Voltages (12v, 24v, 36v, etc)
This should be an article in it’s own right … but let’s cover it anyway.
Inverters, at least common consumer units, handle several different input voltages, mostly they are:
12v
24v
36v
48v
Using the well-known equation of Power = Amperage * Voltage it stands to reason that a battery of higher voltage drops the amperage draw proportionally. The power (wattage) will remain the same regardless of the input voltage, so while 1000w = 83.33amps * 12v it is equally true that 1000w = 41.66amps * 24v. So by doubling the voltage you halve the amperage. Doubling that again from 24v to 48v once again halves the amps.
Higher voltages mean you can use thinner cables. Low voltage, high amperage setups need thick cables. However - the vast majority of alternators in RVs, campers, vans and boats are 12v. Some vehicles have enough room in the engine bay to add a second alternator, but not many. So if you can’t change the alternator the only way to charge the battery from the alternator is a step-up DC-DC charger. While 12v-to-24v DC-DC chargers are plentiful the field narrows considerably for 12v-to-36v chargers. Of course, if you don’t charge the battery from your engine this issue falls away.
The vast majority of vehicle and boat leisure electrics are 12v, but fortunately, they are low current. It’s quite feasible therefore to use 24v-to-12v converters to drive the RV, Van, Motorhome and boat electrics.
What we recommend is that if you require up to 2kw (2000w) appliances then stick with 12v and keep it simple. Mount the inverter very close to the battery and use a thick cable (1AWG or 35mm) If you absolutely need more than 2kw (but less than 4kw) then we’d suggest you have a 24v battery system - probably 8 LifePO4 cells running with an 8s 24v BMS.
